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Isolating mechanisms and speciation in Gasterosteus aculeatus L. Hagen, Don Warren 1966

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ISOLATING MECHANISMS AND SPECIATION IN GASTEROSTEUS ACULEATUS L .  by DON WARREN HAGEN B.A.,  U n i v e r s i t y of  Texas,  1957  M„A  University of  Texas,  1963  0/  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS  FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  the  Department of  Zoology We a c c e p t required  this  thesis  as  conforming to  standard  THE UNIVERSITY  OF B R I T I S H COLUMBIA  May,  1966  the  In  presenting  requirements Columbia, for  the  understood cial  and  copying Head  gain  Department  an  agree  reference  tensive by  I  for  of that  shall  thesis  in  partial  advanced  degree  at  the  that  the I  this  thesis  further  Department  copying not  be  or  or  for by  shall  without  of Columbia  it  that  scholarly his  fulfilment  University  make  agree  publication  allowed  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada Date  Library  study.  of my  this  of  of  freely  British available  permission  purposes  may  representatives. of  this  thesis  my  written  for  ex-  be It  for  the  granted is  finan-  permission.  The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FLNAL ORAL EXAMINATION FOR THE  DEGREE OF  DOCTOR OF PHILOSOPHY of DON WARREN HAGEN  B.A.,  U n i v e r s i t y of Texas, 1957  M.A.,  U n i v e r s i t y of Texas, 1963  TUESDAY, JULY 5, 1966, AT 3:30 P.M. IN ROOM 3332, BIOLOGICAL SCIENCES BUILDING COMMITTEE IN CHARGE Chairman:  D..H. Copp  J . F. Bendell, Do H. C h i t t y W. S. Hoar  G. C. Hughes C. C. Lindsey J . D. McPhail  External Examiner:  P. M. Sheppard  The U n i v e r s i t y , L i v e r p o o l England Supervisor:  C. C. Lindsey  ISOLATING MECHANISMS AND SPECIATION IN GASTEROSTEUS ACULEATUS L. ABSTRACT Intensive f i e l d observations were conducted on a small B r i t i s h Columbia coastal stream f o r two years? supplemented by laboratory r e a r i n g and experiment. Stress was placed upon p o s s i b l e i s o l a t i n g mechanisms . as outlined by Mayr, that might serve to keep the marine s t i c k l e b a c k (trachurus) and the freshwater form ( l e i u r u s ) d i s t i n c t . Lelurus permanently occupies the upper reaches of the stream; trachurus i s anadromous, but. enters the lower reaches t o breed i n "pure" freshwater. Between the breeding ground of the two, where some coexistence occurs, h y b r i d i z a t i o n between l e i u r u s and trachurus i s extensive, but it: i s r e s t r i c t e d to very narrow zones both i n L i t t l e Campbell River near Vancouver and i n c e r t a i n Vancouver Island streams. Morphological a n a l y s i s provides f i r m c i r c u m s t a n t i a l evidence that hybrids are p l e n t i f u l and that backcrossing occurs. H y b r i d i z a t i o n i s confirmed by rearing experiments i n the laboratory with crosses i n a l l comb i n a t i o n s . Most backcrossing i s t o the freshwater race and t h i s i s considered i n d e t a i l . s  Behavioural experiments demonstrated the absence of mate preference; random mating occurs w i t h respect to e t h o l o g i c a l i s o l a t i o n and hybrids perform courtship and parental care a c t i v i t i e s normally. Rearing hybrids uniformly and i n a l l conbinations demonstrated the absence of genetic i n c o m p a t i b i l i t y , and vigorous o f f s p r i n g were produced. F e r t i l e hybrids are abundant i n the w i l d , but the sexes are seasonally i s o l a t e d from one another. Behavoiral and genetic blocks to h y b r i d i z a t i o n are absent, and thus there i s no means t o prevent h y b r i d i z a t i o n where l e i u r u s and trachurus come together. But evidence.from the f i e l d and from preference tests demonstrated that e c o l o g i c a l i s o l a t i n g mechanisms are very powerful b a r r i e r s to h y b r i d i z a t i o n . This i s enforced to some extent: by p a r t i a l seasonal i s o l a t i o n , but early spawning migrants make a major contr .bution to h y b r i d i z a t i o n i n the L i t t l e Campbell River. Several f a c t o r s probably operate t o cause very narrow hybrid zones in. s t i c k l e - b a c k s , including, a sedentary h a b i t , h a b i t a t preference, and hybrid i n f e r i o r i t y outside the hybrid zone. The l a t t e r could not be detected i n s i d e the hybrid zone.  The complex of I s o l a t i n g mechanisms found may well, produce s t a b l e hybrid zones over long periods of time, and i s discussed o Reinforcement of ecological. 1 sol at ing mechanisms probably occurs, and Moore's c r i t i c i s m of the spread of r e i n f o r c e d genotypes would not: apply i n t h i s instance. Adaptation and consequent, e c o l o g i c a l i s o l a t i n g mechanisms are the cause of hybrid i n f e r i o r i t y . In t h i s instance pre and postmating mechanisms cannot: be'separated, for they go hand i n hand. 1  Geographic speci.atlon. i s p o s s i b l e but problematical, and sympatric s p e c i a t i o n cannot be excluded as a p l a u s i b l e explanation of divergence between s t i c k l e b a c k s as a r e s u l t of d i s r u p t i v e s e l e c t i o n . D i s t r i b u t i o n a l patterns of l e i u r u s can best be explained by repeated and independent o r i g i n s from trachurus. The two forms f u l f i l l the species d e f i n i t i o n by Mayr and remain reproductively i s o l a t e d , have w e l l developed i s o l a t i n g mechanisms, and show considerable genet i c divergence, much of i t polygenic. There i s no e v i dence that, i n t r o g r e s s i o n occurs, but a reverse c l i n e i n l e i u r u s as w e l l as very narrow hybrid zones demonstrates that s e l e c t i o n severely r e s t r i c t s gene flow. Reproduct i v e i s o l a t i o n between marine and freshwater forms seems to be widespread around the hemisphere.  GRADUATE STUDIES F i e l d of Study:  Zoology  Advanced Ichthyology C e l l Biology Seminar Advanced Ecology  C. C. Lindsey N. J . Wilimovsky Co V. Finnegan P. Ford D, Suzuki D. C h i t t y  PUBLICATIONS Hagen, D.W. Evidence of adaptation to environmental temperatures i n three species of Gambusia. S. W. Nat. 9(1):6 Reproductive behavior of Gambusia heterochi Copeia, 1957:289  i Chairman:  Dr, C. C. Lindsey  ABSTRACT  Intensive  f i e l d observations  small B r i t i s h Columbia c o a s t a l by l a b o r a t o r y  rearing  i s o l a t i n g mechanisms,  serve  keep the marine  freshwater occupies  form  the  anadromous,  (leiurus)  but enters Between  coexistence occurs, is  extensive,  in  Little  but  the  the  lower  (trachurus) Leiurus  stream? reaches  restricted  that  and  permanently  the  is "pure"  two,  leiurus  to v e r y  where  and  narrow  provides  zones  firm  stantial  evidence  t h a t h y b r i d s are  p l e n t i f u l and t h a t  crossing  occurs.  Hybridization is  confirmed by  experiments  i n the  Most b a c k c r o s s i n g considered  in  laboratory is  to  the  with crosses  freshwater  mate p r e f e r e n c e ;  parental  both Vancouver  circumback-  rearing  in a l l  combinations.  r a c e and t h i s  is  detail.  Behavioural experiments  ethological  some  trachurus  Vancouver and i n c e r t a i n  Morphological analysis  might  the  to b r e e d i n  h y b r i d i z a t i o n between is  p l a c e d upon  trachurus  the b r e e d i n g ground o f  it  supplemented  o u t l i n e d by Mayr,  distinct.  Campbell R i v e r near  Island streams.  as  two y e a r s ,  S t r e s s was  stickleback  upper reaches of  freshwater.  stream f o r  and e x p e r i m e n t .  possible to  were c o n d u c t e d on a  demonstrated  the  random m a t i n g o c c u r s w i t h r e s p e c t  i s o l a t i o n and h y b r i d s p e r f o r m c o u r t s h i p  care a c t i v i t i e s  and i n a l l c o m b i n a t i o n s incompatibility,  normally. demonstrates  and v i g o r o u s  absence  of  to and  Rearing hybrids uniformly the  absence of  genetic  o f f s p r i n g were p r o d u c e d , ,  Fertile  h y b r i d s are  seasonally  a b u n d a n t i n the w i l d ,  isolated  f r o m one  Behavioural absent, where the  and thus  is  come  and from p r e f e r e n c e  i s o l a t i n g mechanisms enforced  but. e a r l y  to  are  probably operate  to  Little  together.  tests  i n f e r i o r i t y outside  habit,  f o u n d may w e l l  over  time,  in  long periods of  this  and i s  i s o l a t i n g mechanisms the  spread of  isolation,  Several  factors  in  stickle-  The l a t t e r  The' c o m p l e x  produce  to  preference,  stable  discussed.  and could  of  hybrid  zones  Reinforcement  probably occurs,  of  and M o o r e ' s  r e i n f o r c e d genotypes  would not  apply  instance. A d a p t a t i o n and c o n s e q u e n t  mechanisms  are  the  cause of h y b r i d  instance  pre  for  go h a n d i n h a n d .  they  habitat  i n s i d e the h y b r i d zone.  ecological  contribution  River.  from  to h y b r i d i z a t i o n .  seasonal  the h y b r i d z o n e .  i s o l a t i n g mechanisms  c r i t i c i s m of  that  cause v e r y narrow h y b r i d zones  hybrid  ecological  But e v i d e n c e  demonstrates  Campbell  i n c l u d i n g a sedentary  are  prevent h y b r i d i z a t i o n  make a m a j o r  backs,  n o t be d e t e c t e d  to  some e x t e n t b y p a r t i a l  i n the  sexes  to h y b r i d i z a t i o n a r e  very powerful b a r r i e r s  spawning m i g r a n t s  hybridization  blocks  no means  l e i u r u s and t r a c h u r u s  field  This  is  the  another.  and g e n e t i c  there  but  and s y m p a t r i c  disruptive  s p e c i a t i o n cannot  of divergence selection.  is  cannot  In  be  possible but  be e x c l u d e d as  between  isolating  inferiority.  and p o s t m a t i n g mechanisms  Geographic s p e c i a t i o n  explanation  ecological  this  separated,  problematical, a plausible  sticklebacks  as  a result  D i s t r i b u t i o n a l patterns  of  leiurus  of can  b e s t be e x p l a i n e d by r e p e a t e d and independent o r i g i n s from trachurus. The two forms f u l f i l l  the s p e c i e s d e f i n i t i o n by Mayr  and remain r e p r o d u c t i v e l y i s o l a t e d , have w e l l d e v e l o p e d i s o l a t i n g mechanisms, and show c o n s i d e r a b l e g e n e t i c d i v e r g e n c e , much o f i t polygenic.  There i s no e v i d e n c e t h a t i n t r o g r e s s i o n o c c u r s , b u t  a r e v e r s e c l i n e i n l e i u r u s as w e l l as v e r y narrow h y b r i d zonesdemonstrates t h a t s e l e c t i o n s e v e r e l y r e s t r i c t s gene f l o w . R e p r o d u c t i v e i s o l a t i o n between marine and f r e s h w a t e r forms seems to be w i d e s p r e a d around the hemisphere.  iv  TABLE OF CONTENTS INTRODUCTION THE STUDY AREA MATERIALS AND METHODS The H a b i t a t The H y b r i d Zone MORPHOLOGICAL ANALYSIS MATERIALS AND METHODS A l l o p a t r i c Populations The H y b r i d Zone The H y b r i d I n d e x S c o r i n g and W e i g h t i n g L a b o r a t o r y Reared F i s h Variation Within Leiurus SEASONAL ISOLATION MATERIALS AND METHODS RESULTS Hybrids E a r l y Spawning T r a c h u r u s Vancouver I s l a n d P o p u l a t i o n s ETHOLOGICAL ISOLATION MATERIALS AND METHODS RESULTS Mate P r e f e r e n c e T e s t s Female C h o i c e T e s t s Hybrid Reproductive A c t i v i t i e s ECOLOGICAL ISOLATION DISPERSAL AND TRANSFER EXPERIMENTS M a t e r i a l s a n d Methods Results D i s p e r s a l Experiments Transfer Experiments PREFERENCE TESTS M a t e r i a l s a n d Methods Substrate Preference Tests Vegetation Preference Tests Current Preference Tests Water Q u a l i t y P r e f e r e n c e T e s t s Results Substrate Preference Tests Vegetation Preference Tests Current Preference Tests Water Q u a l i t y P r e f e r e n c e T e s t s ECOLOGICAL OBSERVATIONS ON VANCOUVER ISLAND NESTING S I T E S AND MATERIALS TROPHIC ADAPTATIONS  PAGE  -  1 5 5 6 13 15 15 18 20 24 27 29 36 38 38 39 41 43 45 48 48 51 51 53 54 56 56 56 57 57 58 66 67 67 68 68 69 70 70 71 72 72 74 76 78  V  PAGE POSTMATING ISOLATING MECHANISMS MATERIALS AND METHODS RESULTS S u r v i v a l to H a t c h i n g Survival Fry Hybrid Fecundity Hybrid I n f e r i o r i t y DISCUSSION ISOLATING MECHANISMS AND HYBRIDIZATION I s o l a t i n g Mechanisms The H y b r i d Zone SPECIATION TAXONOMIC STATUS OF LEIURUS AND TRACHURUS CONCLUSIONS LITERATURE CITED  81 81 83 83 85 88 90 91 91 91 97 105 114 117 119  vi L I S T OF TABLES TABLE I  II  III IV V  VI  VII  VIII IX  PAGE A s y n o p s i s o f h a b i t a t , c h a r a c t e r i s t i c s i n the L i t t l e Campbell R i v e r . P h y s i c a l and c h e m i c a l d a t a r e p r e s e n t a v e r a g e s f r o m May t h r o u g h A u g u s t ,  9  D e n s i t y e s t i m a t e s f o r l e i u r u s and t r a c h u r u s from s e v e r a l h a b i t a t s i n the L i t t l e C a m p b e l l R i v e r , Each f i g u r e r e p r e s e n t s the average c a t c h from t h r e e a d j a c e n t 15 f o o t s e i n e h a u l s . Consecutive f i g u r e s f o r a given date are s i m i l a r estimates made a t 200 f o o t i n t e r v a l s o f s t r e a m . Densities o f l e i u r u s i n 1965, n o t g i v e n , were l i k e t h o s e f o r 1964,  11  Numbers o f b r e e d i n g h y b r i d s indicated,  42  taken  P r o p o r t i o n of each race b r e e d i n g I s l a n d s t r e a m s , 1965,  on d a t e s in  Vancouver  46  R e s u l t s o f mate p r e f e r e n c e t e s t s (above) a n d f e m a l e p r e f e r e n c e t e s t s (below) t a k e n f r o m Cole's closed sequential test design,  52  Preference t e s t s with stream water. Numbers o f f i s h i n each compartment (tea coloured, c l e a r ) at i n d i c a t e d times f o l l o w i n g i n t r o d u c t i o n to a p p a r a t u s .  73  Hatching success of h y b r i d c r o s s e s . Numbers i n parenthesis represent p l a t e counts of parents. C o n t r o l s p r e s e n t e d i n l e f t h a n d c o l u m n s and c o r r e s p o n d i n g h y b r i d c r o s s e s i n r i g h t hand column,  84 a - g  S u r v i v a l of laboratory reared hybrids with c o n t r o l s presented i n l e f t hand columns.  86 a - b  Numbers o f m a t u r e e g g s f o r h y b r i d s a n d each r a c e . Sample s i z e o f 40 f o r e a c h and f o r h y b r i d s .  for race  89  Vll  L I S T OF FIGURES FIGURE  ,  1.  Location of c o l l e c t i n g Stations Campbell R i v e r .  2.  D i s t r i b u t i o n o f r a c e s and h y b r i d s o f s t i c k l e b a c k a t S t a t i o n s on t h e L i t t l e C a m p b e l l R i v e r . D a t a b a s e d on more t h a n 6000 f i s h . Minimal s a m p l e s i z e f o r S t a t i o n was 2 0 0 .  10  L a t e r a l p l a t e counts f o r o f l e i u r u s and t r a c h u r u s  19  3. 4. 5.  6.  7„  8.  on t h e  PAGE Little  a l l o p a t r i c populations and o f h y b r i d s ,  7  G i l l raker counts f o r a l l o p a t r i c populations o f l e i u r u s and t r a c h u r u s and o f h y b r i d s „  21  E l e c t r o p h o r e t i c banding patterns for a l l o p a t r i c p o p u l a t i o n s o f l e i u r u s and t r a c h u r u s and o f hybrids. D i a g n o s t i c bands of each r a c e are i n d i c a t e d by arrows. Sample s i z e s a r e 60 l e i u r u s ; 60 t r a c h u r u s ? 120 h y b r i d s . W i t h i n the h y b r i d c l a s s i n d i v i d u a l s h a d e i t h e r b o t h bands d i a g n o s t i c f o r e a c h r a c e p r e s e n t o r one b a n d d i a g n o s t i c f o r one o r t h e o t h e r p r e s e n t . These may be i n t e r p r e t e d as e i t h e r F i h y b r i d s o r e i t h e r backcross c l a s s . P e r c e n t a g e s f o r the t h r e e c l a s s e s weret 11.8% F ^ ; 80.2% b a c k c r o s s e s t o l e i u r u s ? 8% b a c k c r o s s e s t o t r a c h u r u s .  22  Morphological characteristics for a l l o p a t r i c p o p u l a t i o n s o f l e i u r u s and t r a c h u r u s and o f hybrids.  23  Frequency d i s t r i b u t i o n o f the h y b r i d i n d e x . Inverted histograms represent a l l o p a t r i c populations; upright histograms represent samples from the h y b r i d z o n e .  30  L a t e r a l p l a t e counts f o r l a b o r a t o r y reared ( b l a c k h i s t o g r a m s ) and o f w i l d caught f i s h (hatched h i s t o g r a m s ) . L e i u r u s u s e d as p a r e n t s to o b t a i n o f f s p r i n g had p l a t e counts from 3-5; trachurus counts 3 2 - 3 4 „ Data b a s e d on 6 i n t r a r a c i a l crosses with leiurus parents, 6 i n t r a r a c i a l crosses with trachurus parents, 7 i n t e r r a c i a l c r o s s e s f o r F \ h y b r i d s , and 7 backcrosses. A p p r o x i m a t e l y e q u a l numbers o f r e c i p r o c a l c r o s s e s made.  31  VXX1  FIGURE 9.  PAGE G i l l raker counts f o r l a b o r a t o r y reared (black histograms) and o f w i l d caught (hatched histograms) f i s h presented f o r comparisons. See c a p t i o n , F i g u r e 8, f o r o t h e r d e t a i l s .  33  Body d e p t h i n t o s t a n d a r d l e n g t h o f l a b o r a t o r y reared f i s h . See c a p t i o n , F i g u r e 8, f o r o t h e r details. ,  35  C l i n a l v a r i a t i o n i n numbers o f p l a t e s o f l e i u r u s . D a t a a r e b a s e d o n c o u n t s o f more t h a n 2000 f i s h . Samples o f a t l e a s t 200, a n d u s u a l l y 400-500 f i s h , were made a t e a c h S t a t i o n .  37  B r e e d i n g seasons f o r the races o f s t i c k l e b a c k i n t h e L i t t l e C a m p b e l l R i v e r , 1964, Data f o r 1965; f i r s t , b r e e d i n g l e i u r u s , March 2 and f o r t r a c h u r u s May 1 2 ; f i r s t f r y o f l e i u r u s , A p r i l 28 a n d f o r t r a c h u r u s J u l y 7; peak o f b r e e d i n g f o r l e i u r u s , A p r i l 12-26 and f o r t r a c h u r u s June 6 - A u g u s t 2; e n d o f s e a s o n f o r l e i u r u s , J u l y 10 a n d f o r t r a c h u r u s , September 1 8 .  40  13.  Recapture data f o r l e i u r u s Pond D t o S t a t i o n D .  59  14.  Recapture data f o r the t r a n s f e r o f l e i u r u s from S t a t i o n D to S t a t i o n N . Zero r e p r e s e n t s p o i n t of r e l e a s e . See t e x t f o r f u r t h e r d e t a i l s .  61  Recapture data f o r l e i u r u s t r a n s p l a n t e d from S t a t i o n D to S t a t i o n N . Zero r e p r e s e n t s p o i n t of r e l e a s e ,  63  Recapture data f o r trachurus t r a n s p l a n t e d from S t a t i o n N to S t a t i o n D . Zero r e p r e s e n t s p o i n t of r e l e a s e ,  65  Food items trachurus,  80  10.  11.  12.  15.  16.  17.  transferred  from  taken from stomachs o f l e i u r u s and See t e x t f o r e x p l a n a t i o n .  ix  ACKNOWLEDGEMENTS  I am g r a t e f u l I  can t e l l .  research  the  and f o r h i s  laboratory all  Peter  techniques  thank him f o r  facilities.  Johansen,  I also  a seine,  Curator  for his  of  sticklebacks.  J.  T . McFadden f o r For  their  must e s p e c i a l l y all  for  the  use  of F i s h e s ,  I thank D r s :  appreciation  the  profitable  discussions,  concerning  the  Liley,  t o my p a r e n t s  for  0  E.  Canada,  very  distribution  D„ S u z u k i ,  discussions  suggestions,  i n and  Dr. D  and  I have had w i t h  and enthusiasm,  thank D r s : Dennis C h i t t y and J .  I am g r a t e f u l  I am  assistance  N a t i o n a l Museum o f  N . R.  of  and e s p e c i a l l y M r .  i m p r o v i n g my mood.  k i n d l y s u p p l i e d me w i t h m u c h . d a t a  Most o f  this  Vancouver P u b l i c  e x t e n d my s i n c e r e  to M r . Peter E l l i c k s o n  McAllister,  To the  M r . A l e x Peden and M r . Ken P e t r i e .  a r o u n d t h e museum a n d f o r  them.  used i n  than  f r e e l y c h o o s e my  t o e x p r e s s my t h a n k s  t h o s e who h e l p e d me p u l l  grateful  and i d e a s  a l l o w i n g me t o  inspiration.  Aquarium I s h o u l d l i k e  to  D r . C . C . L i n d s e y i n more ways  d e v e l o p e d through d i s c u s s i o n s w i t h h i m , and I would  especially research  Many o f  to  I  D„ M c P h a i l .  their  encouragement.  INTRODUCTION  The determine marine  and freshwater  process  Finally,  objective  of this  which i s o l a t i n g mechanisms,  Gasterosteus the  foremost  research  i f any, s e p a r a t e the  forms o f the t h r e e - s p i n e d  aculeatus  L.  was t o  stickleback,  A s e c o n d p u r p o s e was t o  of speciation  that  gave r i s e  an a t t e m p t was made t o r e s o l v e  interpret  t o t h e two f o r m s . their  taxonomic  relationships. Isolating ever  since  mechanisms h a v e b e e n a c t i v e l y  D o b z h a n s k y ( 1 9 3 7 ) , Mayr  emphasized t h e i r  importance.  t h a t i s o l a t i n g mechanisms attributes  of species  related  are studies  sympatric  mechanisms  since  Today i t i s w e l l  these  species.  Indeed,  It  three-spined stickleback  form of t h i s  i t s abundance makes  laboratory able  research.  fish  Especially  offers  of  species  exceptional  Both the marine and  and t h i s  regions.  together  i t an i d e a l animal f o r f i e l d and  Furthermore,  pertinent  closely  these  i s widespread i n boreal  i n f o r m a t i o n a v a i l a b l e on the The  permitted  to u n d e r s t a n d i n g the o r i g i n  i s easy to m a i n t a i n i n the l a b o r a t o r y  with  have  a knowledge o f  o p p o r t u n i t y f o r such i n v e s t i g a t i o n s . freshwater  established  important  properties  t h a t now e x i s t .  (1942)  o f i s o l a t i n g mechanisms b e t w e e n  is essential The  and Huxley  a r e among t h e most  t h e v a s t ^ number o f s p e c i e s important  (1942),  investigated  life  f o u n d a t i o n and o r i e n t a t i o n  there  consider-  stickleback.  history,  of this  i s already  which p r o v i d e s the  research,  i s reviewed below  2  There the  are  sea  two d i s t i n c t  f o r most o f  Following marine  the  lives  spring  migrates  In e a r l y life  o f bony p l a t e s long dorsal  convenience for  is  easily  along i t s  and p e l v i c  One e x i s t s  remains  (1963)  in  are  the  return  called  races.  and i n  rivers  to  the  freshwater  year,  streams or  the  the  to  breed.  sea.  The  (van M u l l e m a n d v a n d e r  i d e n t i f i e d by a complete  sides,  spines,  large  size,  in  freshwater.  and o t h e r s  and the  they  most o f  freshwater  Vlugt,  series  a t e r e t e body  shape,  few v e r t e b r a e ,  and a  bright  leiurus,  lives  in  It  distinguished  colour. The f r e s h w a t e r  s t r e a m s a n d ponds from t r a c h u r u s size,  other  trachurus  p r o b a b l y one y e a r  Trachurus  silver  as  sea  into  the  autumn o f f s p r i n g a n d a d u l t s  span i s  1964).  i n the  stickleback.  of Munzing  to  and f o r  Trachurus  of  life;  referred  leiurus,  it  its  terminology  form i s  f o r m as  forms  throughout  by i t s  very  chunky body shape,  vertebrae, on t h e  and an o l i v e  back.  colours  The m a l e s  with red throats  season they  are  build  i n the  nests  exhibit  parental  form,  very  its  few l a t e r a l  short  and p e l v i c  small  spines,  mottled with i n d i s t i n c t  of both  forms  and b l u e  substrate,  assume b r i l l i a n t  irises.  During  fry.  hybridize  (Munzing,  1963)  breeding  t h a t one  is  season and l e d to  bars nuptial  breeding  eggs,  L e i u r u s and t r a c h u r u s  d u r i n g the so  the  more  territories,  g u a r d and f a n t h e i r  sympatric  There are  bony p l a t e s ,  a g g r e s s i v e and e s t a b l i s h  care of  their  dorsal  is  colour,  typically  two m a i n t a i n  life.  freshwater  and  are presumably  a s k how  the  identity. a h o s t o f mechanisms  t h a t may  isolate  3  closely  related  species  f r o m one a n o t h e r ,  but despite  i m p o r t a n c e v e r y few a t t e m p t s h a v e b e e n made t o determine which of for  a given p a i r of  however,  for  mechanisms  they  used i n t h i s 1,  potential  species.  demonstrate  and the  c l a s s i f i c a t i o n of is  the  prevent  following  d e v e l o p e d b y Mayr  interspecific  (1963)  crosses  (seasonal  isolation).  Copulation attempted place  Egg i s  b u t no t r a n s f e r  (mechanical  reduce  crosses  Sperm t r a n s f e r fertilized  full  (postmating  takes  (gametic  of  mechanisms),  p l a c e b u t egg i s  not  mortality), dies  mortality),  Zygote produces  a n F-^ h y b r i d o f  viability  (hybrid  F-j_ h y b r i d  zygote  partially  or  produces  of  isolation),  success  f e r t i l i z e d but zygote  (zygote  (d)  the  P o t e n t i a l mates meet b u t do n o t mate  interspecific  (c)  The  of  isolation),  Mechanisms t h a t  (b)  importance  P o t e n t i a l mates do n o t meet  sperm t a k e s  (a)  necessary,  mechanisms),  (ethological  2,  the  function  investigation:  and h a b i t a t  (c)  are  i s o l a t i n g mechanisms  (premating  (b)  Such s t u d i e s  i n t e r p l a y among t h e m .  Mechanisms t h a t  (a)  systematically  i s o l a t i n g mechanisms  relative  their  reduced  inviability)„ is  fully viable  completely s t e r i l e ,  d e f i c i e n t F^  (hybrid  but or  sterility),  4  S t i c k l e b a c k s are e x t e r n a l f e r t i l i z e r s so t h a t m e c h a n i c a l i s o l a t i o n can be ommitted from f u r t h e r c o n s i d e r a t i o n . The i n t e n t t h r o u g h o u t the r e s e a r c h has been t o examine the r e l a t i v e  importance o f the p o s s i b l e  isolating  mechanisms l i s t e d above combining i n t e n s i v e f i e l d and laboratory investigations.  5  THE STUDY AREA  MATERIALS AND METHODS An i n t e n s i v e Campbell  twice  spent  M',  a month.  but during  ecological  data,  and other  S a l i n i t i e s were registering  habitat  and S t a t i o n s  characteristics  o f 1.000-1.011,  gravity of NaCl.  T h e s e measurements  were  s a l i n i t y t h a t might e x i s t  d i s s o l v e d s o l i d s were bridge. the  Water  stream  samples  were  recorded.  1.010-1.021, and  Sometimes a B u c h l e r s a l i n i t y near the  compared w i t h  from the upper reaches o f the stream in  L , M, a n d  C o l l e c t i o n notes,  C o t l o v e c h l o r i d o m e t e r was u s e d t o d e t e r m i n e estuary.  Station  recorded with a set of three hydro-  densities  1,020-1.031 s p e c i f i c  leiurus habitat,  habitat,  which i n c l u d e d the h y b r i d zone.  averaged  chosen f o r d e t a i l e d  S t a t i o n D, a r e p r e s e n t a t i v e trachurus  Several  from March  the w i n t e r v i s i t s  T h r e e l o c a l i t i e s were  a representative  meters  the L i t t l e  e a c h week a t t h e s t u d y a r e a  September,  studies: N,  study began a t  R i v e r i n 1964 a n d c o n t i n u e d t h r o u g h 1 9 6 5 ,  d a y s were through  field  to d e t e c t  as a r e s u l t  recordings small  differences  of t i d a l f l u x .  Total  d e t e r m i n e d w i t h a n R. C , c o n d u c t i v i t y f o r pH measurements  were  collected  from  and i m m e d i a t e l y r e t u r n e d to the l a b o r a t o r y f o r  d e t e r m i n a t i o n w i t h a Beckman pH m e t e r . measured w i t h a G u r l y c u r r e n t f l o a t e d on the s u r f a c e  meter  Stream c u r r e n t s  or a table  o v e r a known d i s t a n c e .  tennis  stream.  ball  The l a t t e r  method was u s e d f o r m e a s u r i n g a v e r a g e c o n d i t i o n s o v e r s e c t i o n o f the  were  a  large  6  F i s h were c o l l e c t e d w i t h a common s e n s e o f woven n y l o n w i t h 3 mm s t r e t c h e d 3m x 2m,  a n d the  collecting a variety  transported  The d i m e n s i o n s were  l e a d l i n e was h e a v i l y w e i g h t e d t o  i n dense v e g e t a t i o n . of  mesh,  minnow s e i n e  Where s e i n i n g was i m p o s s i b l e ,  f i n e meshed d i p n e t s were  used.  i n 8 g a l l o n styrofoam coolers,  morphological  s t u d i e s were  Invertebrates  were  facilitate  Live  fish  were  and c o l l e c t i o n s  for  preserved i n 15% buffered formalin.  preserved i n 8 0 % e t h y l a l c o h o l .  THE HABITAT A small coastal was c h o s e n f o r is  It  and because  is  located  the  Campbell  River,  s t i c k l e b a c k because  l e i u r u s and t r a c h u r u s  into  extreme southwest Semiahmoo Bay a t  it  are corner  White  (Fig, 1 ) , The L i t t l e  long.  Cambell R i v e r i s  The w i d t h v a r i e s  reaches length  t o more it  is  of channels fluctuates  thin  On t h e  from l e s s  than 3 6 f e e t  are  as  deep as  considerably.  and c u r r e n t  t h a n 10 f e e t  the  estuary.  so  seven f e e t . During  16% miles  i n the  upper  F o r most o f  hand,  the  The w a t e r  the  spring  level  intermittent  t h a t meadows a l o n g t h e b a n k s a r e  sheets of water,  other  at  approximately  e a s i l y waded, b u t i s o l a t e d p o o l s and s t r e t c h e s  f l o o d i n g occurs with  Little  i n the  o f B r i t i s h Columbia and empties Rock  the  a d e t a i l e d study of  readily accessible  abundant t h e r e .  stream,  p a r t i c u l a r l y i n the  the w a t e r  may be s c a r c e l y  and s h a l l o w w a t e r s are  upper  covered reaches.  l e v e l d r o p s v e r y low i n summer noticeable.  suitable  The n a r r o w w i d t h  for efficient  s e i n i n g and  Figure  1.  Location  of  Collecting Stations  on t h e  Little  Campbell R i v e r .  8  f i e l d studies. Major h a b i t a t s and  t h e i r f e a t u r e s are found i n Table  I and F i g u r e 2 shows the d i s t r i b u t i o n of s t i c k l e b a c k s i n the stream. miles  The headwaters  long and  ( S t a t i o n s A-E)  are approximately  l a y a t an a l t i t u d e o f 220  i s marshy w i t h a mud  feet.  This h a b i t a t  bottom and dense a q u a t i c v e g e t a t i o n  chokes s e c t i o n s of the stream i n the summer, extremely abundant, and d e n s i t y estimates convey some i d e a of t h e i r numbers. congregate i n shallow  7%  that  Leiurus i s  shown i n Table  II  This race tends to  pockets along the margins of the stream  where v e g e t a t i o n i s most dense and  i s l e s s numerous where the  banks are steep and v e g e t a t i o n i s sparse.  Such a d i s t r i b u t i o n  g i v e s r i s e to marked l o c a l v a r i a t i o n s i n d e n s i t y In the s p r i n g males w i t h b r e e d i n g  (Table I I ) .  c o l o u r s can be found a l l  over the f l o o d e d meadows; ponds along the stream a l s o have dense p o p u l a t i o n s  of  leiurus.  The headwaters give way the r i f f l e h a b i t a t next 4 % m i l e s .  ( S t a t i o n s E-K)  a b r u p t l y a t S t a t i o n E where begins,  extending  f o r the  This s e c t i o n i s c h a r a c t e r i z e d by s w i f t water,  rocky bottom, l a c k o f v e g e t a t i o n ,  and no s t i c k l e b a c k s .  A t S t a t i o n K there i s again a r a p i d t r a n s i t i o n from the r i f f l e h a b i t a t to one more l i k e the headwaters, is  less vegetation,  silt,  s l i g h t l y f a s t e r c u r r e n t , and  But  there  some sand  and  L e i u r u s i s l e s s abundant here than i n the upper reaches  (Table I I ) .  A s h o r t d i s t a n c e downstream i s the h y b r i d zone,  which w i l l be d e s c r i b e d i n a separate The  lower reaches  subsection.  ( S t a t i o n N) occupy 3 / 4 m i l e  and  Table  I.  A s y n o p s i s of h a b i t a t c h a r a c t e r i s t i c s i n the L i t t l e Campbell R i v e r . Physical a n d c h e m i c a l d a t a r e p r e s e n t a v e r a g e c o n d i t i o n s f r o m May t h r o u g h A u g u s t .  HABITAT: STATION: stream g r a d i e n t water v e l o c i t y temperature salinity total dissolved PH water c o l o u r bottom  solids  Headwaters (leiurus habitat) (A-E)  vertebrates  Section  32 f t a m i l e 74 cm a second 14°C 0.189bo  65 ppm 6.8 medium t e a c o l o u r ; slightly turbid  7.1 clear  dense beds o f Nuphar. Myosotis, Oenanthe, G l y c e r i a . Carex. Typha, Po tamo ge ton; a l s o Lemna and green algae f i s h : Catostomus catostomus. Cottus asper. p a r r o f Oncorhynchus k i s u t c h . Salmo c l a r k i  amphibians: Bufo b o r e a s . Rana p r e t i o s a . Hyla r e q i l l a . Ambystoma g r a c i l e invertebrates m o l l u s c s : Lymnaea. Physa. A n c y l u s . Musculium, Marqaritifera. Gyraulus c r u s t a c e a n s & a n n e l i d s : amphipods and l e e c h e s abundent a q u a t i c i n s e c t s : n a i a d s o f dragon and damselflies, S i a l i s , Nepa, Chironomidae, Chrysomelidae, Ephemoptera  H y b r i d Zone  (E-K)  8 f t a mile 3 cm a second 16°C 0.189bo  s o f t and muddy  vegetation  Riffle  r o c k y and rubble v e r y sparse; some e n c r u s t i n g algae (Cladophora)  (L-M ) 1  12 f t a m i l e 23 cm a second 15°C 0.209OO  118ppm 7.8 l i g h t tea colour to c l e a r ; s l i g h t l y turbid intermingled with mud, medium g r a i n e d sand, and some s i l t some E l o d e a and Myriophyllum; some G l y c e r i a . Carex. Typha  C. asper. C. aleuticus. 0. k i s u t c h . S. c l a r k i  C. asper. C. a l e u t i c u s  B. boreas  R.  Ancylus  Trichoptera, Ephemeroptera, Plecoptera  , Lower Reaches (trachurus h a b i t a t ) (N) 11 f t a m i l e 10 cm a second 15°C 0.2l9bo 118ppm 7.8 colourless; slightly turbid  Estuary (0-P) 10 cm a second 16°C mixohaline (0.179bo t o 1 2 9 b o ) turbid  sand  mud, sand, s i l t , c l a y and some o y s t e r beds  dense beds o f Elodea  dense beds o f U l v a and some Enteromorpha  C. a s p e r . C. a l e u t i c u s . 0. k i s u t c h , Platichthys stellatus  C. a s p e r . C. a l e u t i c u s . P. s t e l l a t u s . Oligocottus maculosus. Leptocottus armatus  Musculium, Pisidium, Marqaritifera  Crassostrea  Gammaridae, Palaemonetes  Balanus. Gammaridae, Palaemonetes  pretiosa  Trichoptera, Chrysomelidae, Chironomidae, Dixidae, naiads of d a m s e l f l i e s  % 100 •  TRACHURUS  O  HYBRIDS  •  LEIURUS  50  0.2 P  OA 0  mm. ' 1.5 N DISTANCE  Figure 2  A 7.8 M'  2 M  2.8 L  3.5 K  FROM THE SEA IN MILES AND STATIONS  D i s t r i b u t i o n o f r a c e s and h y b r i d s o f s t i c k l e b a c k a t D a t a b a s e d on more t h a n 6000 f i s h . Campbell R i v e r , f o r S t a t i o n was 200,  S t a t i o n s on the L i t t l e M i n i m a l sample' s i z e  16 A  Table  II,  D e n s i t y e s t i m a t e s f o r l e i u r u s and t r a c h u r u s from s e v e r a l h a b i t a t s i n the L i t t l e Campbell R i v e r . E a c h f i g u r e r e p r e s e n t s t h e a v e r a g e c a t c h f r o m t h r e e a d j a c e n t 15 foot seine hauls. Consecutive f i g u r e s f o r a g i v e n date are s i m i l a r e s t i m a t e s made a t 200 f o o t i n t e r v a l s o f s t r e a m . D e n s i t i e s o f l e i u r u s i n 1965, n o t g i v e n , were l i k e t h o s e f o r 1 9 6 4 . Leiurus,  1964 Station K  Station D March 6 M a r c h 13 April  5  April  23  1.7 2.112 3.97 1.138 2.101 3.86 1.116 2.204 3.9 1.27 2.14 3.196  May 4  1.123 2.216  March  16  May  1.416 2.28 3.312 1.432 2.156 3.82 1.257 2.46 3.315  April  18  28  June 9 June  22  1. 2 3, 1. 2. 3, 1, 2, 3, 1. 2. 3.  May 3 June  12  Trachurus 1964 Station N June  16  June  23  July  6  July  21  August 8  1.212 2.193 3.247 1.184 2.202 1.164 2.221 3.202 1.175 2.124 3.256 1.167 2.206 3.198  Station M 8 22 10 17 5 6 19 3 9 6 31 14  March  16  April  17  May 3 June 12  1965 Station  Station N  0  1.9 2.3  June 8  1.2 2.8 3.7 1.5 2.3 3.5 1.26 2.7  June  1.3 2.11 3.2  18  June 22 J u n e 26  1.43 2.21 3.23 1.12 2.8 3.16 1.11 2.20  July  1  1.9 2.5  July  5  1.6 2.7  July  14  1.19 2.6  J u l y 20  1.12 2.3 3.8 1.7 2.5 3.12  1 7 2, 3 3, 15 1 0 2, 3 3, 1 1, 5 2, 16 3.8 1.11 2.1 3.5  12  are  1% m i l e s  from the  a mild current, aquatic  estuary.  The h a b i t a t  a n d dense b e d s o f  vegetation  present,  l o c a t e d here  spring  to b r e e d .  breeding not  after  season  c o v e r i n g most o f  I).  are  the  trachurus.  the  into  the  homes  with Elodea,  to  of  in density  a given stream returned  i n 1964.  and s i n c e  to  Trachurus  Elodea i s  the is  for  when t h a t r a c e  compared w i t h l e i u r u s  The e s t u a r y with  large  (Stations  and dense beds o f U l v a . the  upper  does reaches  trachurus  demonstrate  the  the  a striking  spawning grounds stream  and  this  The h e a v i l y w e i g h t e d and f r y  f o u n d i n 1965 to  suggests  spawn s i n c e  stream closely  i n 1965  many o f might  associated this  t h e more u n i f o r m numbers a l o n g the  fluctuations  found throughout  late  it  uniformly distributed  probably accounts is  in  p r o b a b l y d e s t r o y e d many n e s t s  f r y t h a t would have  have been k i l l e d  i n the  Trachurus  s e i n e d i n 1964.  The d e c r e a s e  trachurus  stream  numbers o f  also  i n water  0-P)  is  level,  (Table  ( T a b l e II)„  stream  II).  an u n s t a b l e  habitat  mixohaline water,  S m a l l numbers o f  summer  the  spawning grounds  made t h r o u g h o u t  large  section  only  f r e s h and t h a t  The d a t a  a short  seine  the  s a l i n i t y of water  habitat.  was e x t e n s i v e l y  lead l i n e of  that  is  i n numbers f o u n d i n 1965.  concentrated  habitat  the w a t e r  Table I I i n d i c a t e s  that breed i n this decrease  into  the  the bottom i n  Trachurus  S a l i n i t y measurements  d i f f e r much f r o m t h e  (Table  of  they migrate  show t h a t  a sandy bottom,  Elodea which i s  summer b u t d y i n g away i n t h e w i n t e r . are  has  trachurus  c a n be  13  THE HYBRID ZONE This Table  section  I  shows t h a t  current,  and water  headwaters (Stations  for  and the  by a % m i l e s t r e t c h  of  it  clay  is  is  intermediate reaches.  and s i l t  bottom t h a t  conversely,  very  II)  captured, of  and o n l y o c c a s i o n a l but at  Stations  l e i u r u s and trachurus  The numbers o f  developed. 10J8:1.  The r a t i o  At Station  is  like  that  of  trachurus  1).  shown f o r decrease  lower  its  leiurus,  hybrids,  migrate few  leiurus  trachurus  leiurus  are  headwaters trachurus  largest  are  numbers  h y b r i d swarms  leiurus increase  have is  and d e n s i t y  S t a t i o n K, b u t numbers o f h y b r i d s a n d (Figure to  the  2).  Hybrids  stream  the headwaters  (trachurus  at of  are  also  common i n  S t a t i o n M (Pond M i n t h e h y b r i d zone a r e and i t s  (leiurus habitat)  its  intermediate and  the  habitat).  The F]_ a n d b o t h b a c k c r o s s i d e n t i f i e d by t h e i r  devoid of  and t r a c h u r u s ,  sharply defined boundaries,  between  reaches  of  together  The o u t s t a n d i n g f e a t u r e s  narrowness, properties  caught  L numbers o f  a s m a l l pond a d j a c e n t Figure  of  individuals  M and M'^where the are  is  l e i u r u s and  much s m a l l e r when c o m p a r e d w i t h numbers i n t h e (Table  zone  2),  Mixed b r e e d i n g p o p u l a t i o n s of i n the h y b r i d z o n e .  the  spawning grounds  V e r y few t r a c h u r u s  (Figure  vegetation,  The h y b r i d  trachurus  r e g i o n and,  and  between  f r o m the  f o u n d downstream b e l o w i t  do n o t o c c u r  one m i l e l o n g ,  s u c h as b o t t o m t y p e ,  lower  and s t i c k l e b a c k s .  upstream beyond t h i s are  stream  features  seperated  1  vegetation  the  quality,  habitat  L-M ) is  of  classes  m o r p h o l o g y as w i l l  be  o f h y b r i d s may be  demonstrated  later.  14  I f the h y b r i d M  1  zone i s d i v i d e d i n h a l f between S t a t i o n s M and  the numbers of f i s h i n the upper and lower h a l f of the  hybrid and 38,  zone are, r e s p e c t i v e l y s  f o r l e i u r u s backcrosses  f o r t r a c h u r u s backcrosses 41 and 76,  132  When the data  se t e s t e d w i t h a contingency t a b l e a value of P <  ,005 i s  o b t a i n e d i n d i c a t i n g t h a t h y b r i d s are not randomly d i s t r i b u t e d , Backcrosses to t r a c h u r u s are mostly i n the lower h a l f of the hybrid  zone n e a r e s t S t a t i o n N, and backcrosses to l e i u r u s are  mostly i n the upper h a l f n e a r e s t S t a t i o n  K,  15  MORPHOLOGICAL ANALYSIS  MATERIALS AND METHODS Leiurus characters. plates,  sexual  gill  rakers,  sexual maturity,  maturity,  c h a r a c t e r s were  conspicuous  selected  because  i n those  f o r counts  the  two r a c e s  characters.  A l l lateral  were  not  a n d measurements  bony p l a t e s  were  differences  i n the  caudal k e e l .  than about  3 cm were  o f a l i z a r i n dye i n 2% KOH f o r render  the  were  Technological  Station,  the  Vancouver,  The  electrophoretic the  a i d of a  s t a i n e d w i t h a few  small anterior  Dr. H. Tsuyuki at  i n most  plates  and p o s t e r i o r Fisheries  d i d the  O n l y s e x u a l l y mature  l e n g t h and body depth i n t o  f i s h were  standard  were  plates  Research  Board  electrophoretic  u s e d t o measure  l e n g t h so t h a t  probe.  crystals  on m u s c l e p r o t e i n u s i n g p r e v i o u s l y p u b l i s h e d methods 1962)„  small  C o u n t s were made  and w i t h t h e  24 h o u r s b e f o r e  conspicuous.  dorsal  used.  counted i n c l u d i n g  u s i n g a b i n o c u l a r d i s s e c t i n g scope  counted to  Other  l e n g t h a n d shape o f  lateral  plates  ones a n d t h o s e  smaller  showed  and  of  at  These  counts  analysis.  Fish  rays.  (1958)„  were  anterior  length  s u g g e s t e d b y Hubbs and L a g l e r  exceptions  standard  t h e y c o u l d be most e a s i l y and  and p e l v i c s p i n e s , , and c o l o u r p a t t e r n s , Procedures  lateral  patterns,  body depth i n t o s t a n d a r d  number o f v e r t e b r a e ,  those  number o f  electrophoretic  and because  differences  example,  instances  i n a number o f m o r p h o l o g i c a l  a n d number o f d o r s a l a n d a n a l  o b j e c t i v e l y measured,  for  differ  Those c h o s e n f o r a n a l y s i s were  number o f  length at  and t r a c h u r u s  analysis  (Tsuyuki, standard  a more  uniform  16  c o m p a r i s o n among t h e callipers  races  were u s e d t o make a l l Sticklebacks  collected  f r o m the  summer o f  1964  the  a n d h y b r i d s c o u l d be made.  a n d 1965.  morphology of  each  allopatric  populations  assure,  much as  as  s e r v e as  In o r d e r  of  leiurus  possible,  sympatric the be  are  circumscribe  and t r a c h u r u s .  against  between  t h a t once  known t h e  the  the  their  samples  leiurus  effects  were  s p r i n g and and i d e n t i f y from  These  samples  of  races.  Furthermore,  morphological  c a n be c o m p a r e d w i t h  and t r a c h u r u s .  In t h i s  way  and e x t e n t o f h y b r i d i z a t i o n and i n t r o g r e s s i o n  can  determined. Samples  taken  from the  reasonably  b u t n o t one allopatric the  there are  from a l l o p a t r i c  headwaters at  assumed t h a t  Thousands o f  in  so  populations of  effects  to  r a c e s a m p l e s were c o l l e c t e d  controls  characteristics  morphological analysis  Campbell R i v e r d u r i n g the  h y b r i d i z a t i o n and i n t r o g r e s s i o n they  measurements.  used i n the  Little  Vernier  Stations  A,  B,  these populations  leiurus  and D.  are  trachurus  o r h y b r i d was  populations of  estuary, (Figure  and s i n c e 2)  it  trachurus neither  found.  may a g a i n be assumed  may be  headwaters,  Collections  were made a t  leiurus  It  were  allopatric.  i n d i v i d u a l s have b e e n e x a m i n e d i n t h e  from  Station  P  n o r h y b r i d s were f o u n d that  these  populations  allopatric. Individuals  Stations  L,  section  of  each  r a c e were f o u n d t o g e t h e r  M, a n d M ' d u r i n g t h e  numerous v a r i a b l e this  populations of  of  b r e e d i n g season  and i n t e r m e d i a t e  the  stream -  individuals.  c a l l e d the  hybrid  at  together Samples  with from  zone - were  used  17  to  determine  are  almost  their of  the  presence  always  of h y b r i d i z a t i o n .  intermediate  morphological features  such i n d i v i d u a l s  strong  (Hubbs,  together  circumstantial  between the  or  the  so  that  other  the  parent  b a c k c r o s s i n g has Thus,  individuals  more c l o s e l y  occurred  in their  (Hubbs,  with  samples  trachurus  from the  backcrossing, are  demonstrated distributed.  the  for  that  Knowing t h i s  from S t a t i o n L to  in  the  one  indicates  a comparison l e i u r u s and  o f h y b r i d i z a t i o n and  classes  c a r e was fish  indicated  counts  shown)  were n o n r a n d o m l y  taken to over  be  (which  the  sample the entire  hybrid  distance  Station M ' .  To c o n f i r m t h e samples o f each  the  characters  i d e n t i f y i n g h y b r i d s as w i l l  zone u n i f o r m l y be s e i n i n g f o r  hybrids  the  populations,  presence  the b a c k c r o s s  provides  Moreover,  morphology  A preliminary a n a l y s i s of plate  most r e l i a b l e  types  t h a t approach  zone o f o v e r l a p b e t w e e n  may i n d i c a t e  in  1955),  the morphology f o r  had been determined i n a l l o p a t r i c  fish  presence  i n t e r m e d i a c y between  of  in  types  1955), a n d t h e  evidence f o r h y b r i d i z a t i o n .  presence  once  parental  with both parental  F ^ h y b r i d s u s u a l l y show s t r i c t parents  Hybrids  race,  of h y b r i d s caught  in wild  a r t i f i c a l l y produced r e c i p r o c a l  and b a c k c r o s s e s laboratory.  presence  were  Parents  o f f s p r i n g were c o l l e c t e d  reared used to  from the  F^  under u n i f o r m c o n d i t i o n s o b t a i n l e i u r u s and  same a l l o p a t r i c  trachurus  populations  mentioned e a r l i e r .  R e c i p r o c a l F^ h y b r i d s were  produced by  crossing  each  from a l l o p a t r i c  parents  populations,  of  race,  again c o l l e c t e d  i n both combinations,  Backcrosses  were made b y  18  first  c o l l e c t i n g presumed  T h e s e i n d i v i d u a l s were only individuals r a c e s were are  (Seasonal  from the h y b r i d  i d e n t i f i e d by t h e i r  t h a t were  used.  F^ h y b r i d s ,  hybrids  strictly  that  these  but evidence presented i n another shows t h a t  the  hybrids with  parents  were  the  individuals  isolation.  are  largely  The  o f an i n t e r m e d i a t e h y b r i d b e i n g an F2 i s Backcrosses  and  section  sexes o f h y b r i d s  p r e v e n t e d from i n t e r b r e e d i n g by s e a s o n a l  unlikely.  counts,  i n t e r m e d i a t e between  One c a n n e v e r be c e r t a i n  Isolation)  possibility  plate  zone.  therefore  t h e n made b y c r o s s i n g p r e s u m e d F^  o f each  race  i n the  four  reciprocal  combinations. Fertilized  eggs  dechlorinated freshwater Methods o f rearing  a n d o f f s p r i n g were at  the  are  F r y were  1 7 ° C to  22°C,  when t h e y were  reared at  c h a r a c t e r s were  Aquarium, and  s e c t i o n on P o s t m a t i n g two  thermostats  temperatures,  suspended i n water  room t e m p e r a t u r e ,  The o f f s p r i n g were b e t w e e n 17  which v a r i e d from a n d 18 weeks o l d  preserved for morphological analysis,  t h e y were b e t w e e n 2 . 8  in  i n c u b a t i n g eggs,  Eggs were h e l d a t  1 4 ° C and 1 8 ° C , c o n t r o l l e d w i t h baths.  eggs,  found i n the  I s o l a t i n g Mechanisms.  t h a t age  Vancouver P u b l i c  t a k i n g and f e r t i l i z i n g  offspring  reared  and 3,7  cm l o n g .  a n a l y z e d o n r e a r e d a n d on w i l d  and  at  The same fish.  ALLOPATRIC POPULATIONS Lateral easily are  Plate  Counts.  L e i u r u s and t r a c h u r u s  i d e n t i f i e d b y number o f p l a t e s  (Figure  3)  w i d e l y d i v e r g e n t w i t h no o v e r l a p i n c o u n t s .  since  are they  Counts o f  3  %  TRACHURUS N=890  30  X=32.7  20 10 0  30 31 32 33 34 35  40 HYBRIDS N-493  30  X=17 20 10 0  8  40  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29  LEIURUS  30 20 10 0  4  5 6 7  L a t e r a l p l a t e counts f o r a l l o p a t r i c p o p u l a t i o n s o f l e i u r u s and t r a c h u r u s and of hybrids. N - sample s i z e ; X - mean.  20 to  7 are  diagnostic Gill  means o f that  for  leiurus,  Raker C o u n t s .  a n d 30  This  d i s t i n g u i s h i n g races  is  an o v e r l a p o f  estimating smallest  the  (Figure  4% i n c o u n t s ,  percent  numbers o f  of  4),  Electrophoresis. u s e d to unique  i d e n t i f y each  and i t  to  each  (Figure  this  size  and g r a v i d females)  less  this  character  Dorsal anal  ray counts  fewer  rays  rays  However,  there i s  (Figure  is  c a n be  that  are  length  (nuptially smaller  an o v e r l a p o f (Figure  leiurus  50% makes  6).  Standard Length of A d u l t s .  and A n a l  t h a n does  and i n a n a l  bands  leiurus having a  a more c h u n k y b o d y a n d t r a c h u r u s For  trachurus.  determined by  Populations of  u s e f u l than others  Body D e p t h i n t o  6).  seen  5)„  than t r a c h u r u s .  character  than  character  may be d i f f e r e n t i a t e d b y a d u l t  c o l o u r e d males average  this  there are  Standard Length of A d u l t s . and t r a c h u r u s  rakers  c a n be  overlapping zone.  plates  race because  trachurus.  i n that race with  i n the  Like  for  which i s  individuals  individuals  35  character provides a useful  l e i u r u s averages c o n s i d e r a b l y fewer  There  has  to  Rays. 6)  a more  and i n each  trachurus.  t e r e t e body  an o v e r l a p o f The r a c e s  Leiurus (Figure  14% b e t w e e n  races.  d i f f e r i n dorsal  character  Overlap i n dorsal  leiurus rays  and  averages  is  36%  31%. THE HYBRID ZONE  Data f o r presented  morphological analysis  along w i t h those f o r  comparisons present  the  allopatric  c a n be more e a s i l y made.  i n t h e h y b r i d zone  included i n this  analysis.  (Figure  2)  of hybrids  p o p u l a t i o n s so  are that  Although both races o n l y the h y b r i d s  H y b r i d s were  i d e n t i f i e d by  are  are their  GILL Figure 4 .  12 RAKER  13  14 15 COUNTS.  16  17  18  19  20  21  G i l l raker counts f o r a l l o p a t r i c populations o f l e i u r u s of h y b r i d s . N - sample s i z e ; X - mean.  22  23  24  25  and t r a c h u r u s and  Trachurus  Figure  5.  E l e c t r o p h o r e t i c banding patterns for a l l o p a t r i c p o p u l a t i o n s o f l e i u r u s and t r a c h u r u s a n d o f h y b r i d s . D i a g n o s t i c bands o f each r a c e are i n d i c a t e d b y the arrows. Sample s i z e s a r e 60 l e i u r u s ? 60 t r a c h u r u s ? 120 h y b r i d s ; w i t h i n t h e h y b r i d c l a s s i n d i v i d u a l s h a d e i t h e r b o t h bands d i a g n o s t i c f o r each r a c e p r e s e n t o r one b a n d d i a g n o s t i c f o r one o r t h e o t h e r p a r e n t . These may be i n t e r p r e t e d as e i t h e r F^ h y b r i d s o r either backcross c l a s s . P e r c e n t a g e s f o r the t h r e e classes were: 11.8% F±; 80.2% b a c k c r o s s e s t o leiurus? 8% b a c k c r o s s e s t o t r a c h u r u s .  23  DORSAL RAY COUNTS  F i g u r e 6.  ANAL RAY COUNTS  Morphological c h a r a c t e r i s t i c s f o r a l l o p a t r i c populations o f l e i u r u s and trachurus and of h y b r i d s . N - sample s i z e ; X - mean.  24  plate  counts,  intermediate each  race  which includes a l l to  counts  (i.e.  obtained for  with counts  when s a m p l e s were  over  collected  to  (see  The H y b r i d  Lateral of plate 3). in  counts  Plate  the  races  suggests  that  more o r  less  counts  between  data  to  However,  for  constructing  show a c o m p l e t e  the  parental  4,  5,  the  a n d 6)  are  further  that  morphological  race. features  t h a t o b t a i n e d from  and  always  measurements.  a l l characters but dorsal  l e i u r u s parent  that parent  (for  o b t a i n e d from e l e c t r o p h o r e t i c  divergent  The  s i n c e h y b r i d s are  i n counts  (Figure  so  place.  to  range  types  l e i u r u s parent  for other  s i m i l a r to  intermediate  skewed t o w a r d s  backcrossing  Hybrids  the m a j o r i t y o f b a c k c r o s s e s  (Figures  of  i n t h e h y b r i d z o n e were  a p p r o a c h i n g the  Frequency d i s t r i b u t i o n s f o r are  taken  that b a c k c r o s s i n g takes  o f h y b r i d s show a p a t t e r n counts  7 and u n d e r 3 0 ) .  Counts.  Inspection of  plate  populations  amount o f v a r i a t i o n f o r a c h a r a c t e r  suggests  predominance o f  counts  Index).  intermediate  Such a l a r g e  allopatric  provide material  a h y b r i d index a l l s t i c k l e b a c k s included  sticklebacks with  suggesting  percentages  analysis  see  rays  considerable  of h y b r i d  Figure  classes  5).  THE HYBRID INDEX Once t h e possibility  of  presence  of hybrids is  introgression exists.  p r o f o u n d and f a r  reaching effects  (Anderson,  1949;  Stebbins,  necessary.  If  one a t t e m p t  1959) to  demonstrated  Introgression  i n the  establish  can  e v o l u t i o n of  making a c r i t i c a l  the have  species  examination  introgression  25  or  its  direction,  t o one o r  character  the  reasons.  One needs  dominance, genotypes make the plates  results  the  other  to  use  o n l y to  consider  realize  this.  the  genetic  and o t h e r  o f one c h a r a c t e r  less  a number o f basis  reliable.  of  -  For  of  recombination  example,  i d e n t i f y a hybrid but  f o u n d t h a t a few h y b r i d s c a n n o t  single  interactions  More o f t e n e f f e c t s  c a n be r e l i a b l y u s e d t o  alone.  u s i n g any  may be v e r y m i s l e a d i n g f o r  oppositional effects, -  parent,  be r e c o g n i z e d u s i n g  it  was  plates  B a c k c r o s s i n g a n d r e c o m b i n a t i o n y i e l d s some o f f s p r i n g  with counts  like  those of  the  parent  i d e n t i f i e d with that character, hybrid origin.  A n d so  it  is  race  but raker  for  into  w e i g h i n g each  to  a more p o w e r f u l t o o l  to  relative detect  they cannot  counts  any s i n g l e  c o m b i n i n g a number of", c h a r a c t e r s character  so  character  a single  its  reveal  their  so  that  i n d e x and  usefulness  introgression  be  and  provides its  direction. Fresh collected  samples o f  from a l l o p a t r i c  40  taken throughout  returned  the  measurements preparation  laboratory  were made the for  the h y b r i d z o n e .  alive,  and a n a l  electrophoretic  because Reared  ray counts  t h e y were  a n d once  f i s h were  E a c h i n d i v i d u a l was a n a l y z e d f o r Dorsal  trachurus  populations along with  individuals to  l e i u r u s a n d 40  were  found u s e l e s s  144  These  counts  were  and  immediately frozen  analysis  of muscle  e x c l u d e d from the Morphology of  in  protein.  five morphological  (see  were  features.  index Laboratory  Fish). :  The method o f A n d e r s o n (1956)  for  constructing  a  hybrid  26  index proved s a t i s f a c t o r y used i n the success  i n d e x were  in this  analysis.  i n c l u d e d because  in identifying  the  races  to  review b r i e f l y  are  is  the  less  the u t i l i t y o f a h y b r i d  index.  following.  Any c h a r a c t e r  This  is  counts  o r measurements  or  other  the  realized. is  If  expressed  used. three  the  scores  counts  is  (backcrossing  races  are  o f each  of a character. diagnostic is  and  to  be  so much v a r i a t i o n scores  i n a character  c a n be  only  present  only  the  extreme  a score providing from h y b r i d s ,  for  for  and o f  a  races  another, Weights are  it  classes  f o r e a c h r a c e a n d one  grouped f o r of races  than  (F-|_ h y b r i d s )  d i v e r g e n t not  overlap is  race are  i n the h y b r i d  introgression)  i f much o v e r l a p e x i s t s  little  discrete  c l o s e l y a p p r o a c h i n g one  or  one  is  t h e r e c a n be  and c o n s e q u e n t l y f e w e r  more e f f i c i e n t s e p a r a t i o n f r o m one  less  c a n be a s s i g n e d ,  Where  less  that  so b e c a u s e  o r measurements  more o r  in hybrids,  Finally  hybrids. counts  parent  may be u s e f u l  g i v e n more s c o r e s  a wider spectrum of v a r i a t i o n expressed intermediate  and  b e h i n d s c o r i n g and w e i g h t i n g o f  divergent character.  allowing  above  Methods f o r s c o r i n g a n d but. i t  and w i d e l y d i v e r g e n t between r a c e s a  characters  demonstrated  (see  presented below,  The r a t i o n a l e characters  their  and h y b r i d s  Morphology of L a b o r a t o r y Reared F i s h ) , weighting characters  of  The f i v e  for  If  a s s i g n e d i n p r o p o r t i o n to  the  character  races and h y b r i d s ,  a s s i g n e d a maximum w e i g h t .  overlap increases  the  character  is  the  usefulness  o b j e c t i v e l y determined,  a n d w i t h no o r  little  As t h e s e c r i t e r i a i s weighted  less.  overlap,  diminish  or  27  After individual  a l l c h a r a c t e r s h a v e b e e n a n a l y z e d f o r an  and the weights  they a r e added t o g e t h e r  f o r the f i v e c h a r a c t e r s  t o g i v e an i n d e x v a l u e .  index values  are obtained f o r a l l i n d i v i d u a l s .  index values  are grouped and p r e s e n t e d  distribution. allopatric  This  and sympatric  compared and thus or  procedure  introgression  in a  is carried  determined  I n t h i s way Then  like  frequency  out independently f o r  p o p u l a t i o n s so t h a t  t h e two c a n be  the e x i s t e n c e and d i r e c t i o n o f b a c k c r o s s i n g evaluated.  Finally,  i t s h o u l d be r e a l i z e d t h a t one r a c e  always  receives  minimal weights  of zero,  and the o t h e r  always  receives  maximal w e i g h t s  f o r each  character.  (leiurus) race  S c o r i n g and Weighting Lateral for  the f o l l o w i n g  30-35 ing  Plates.  groups o f p l a t e  (trachurus).  leiurus),  T h i s was g i v e n s e v e n s c o r e s ,  17-20  (intermediate),  21-24 and 25-29  diagnostic  of races  3-5 8-12  (approaching  maximum w e i g h t o f 12 was a s s i g n e d o b j e c t i v e l y measured,  counts;  to plates  as a l l c h a r a c t e r s  one e a c h  (leiurus), and 13-16  trachurus). because  used a r e ,  it  (approachThe  it  is  is  and h y b r i d s and i s w i d e l y d i v e r g e n t  with  no o v e r l a p . Gill counts t 15-17 For  12-14  Rakers.  (leiurus),  (approaching this  phoretic  F i v e s c o r e s were 23-25  leiurus),  a s s i g n e d to  (trachurus).  20-22  18-19  (approaching  raker(intermediate),  trachurus).  c h a r a c t e r and the r e m a i n i n g ones,  excepting  patterns,  o f l e i u r u s and t r a c h u r u s  o n l y the extreme counts  electro-  28  were  grouped f o r reasons  criteria  Rakers  fulfill  the  f o r m a x i m a l w e i g h t i n g b u t f o r a s m a l l o v e r l a p o f 4%.  Accordingly of  d i s c u s s e d above.  t h e y were w e i g h t e d l e s s  than p l a t e s ,  with a value  10. Electrophoretic  be a s s i g n e d t h i s small. the  Patterns,  character  since  The one b a n d d i a g n o s t i c  other  diagnostic  patterns for  fulfill  to l e i u r u s r e c e i v e d a  is  score,  r e c e i v e d a second, and  received a third.  the r e m a i n i n g c r i t e r i a  although they are l e s s  could  d i v e r g e n c e between r a c e s  to trachurus  i n d i v i d u a l s with both present  Only three scores  However, b a n d i n g  a n d s o were w e i g h t e d 10,  divergent  than rakers  Standard Length o f A d u l t s .  S i z e s were  there  i s no  overlap.  only  three classes  Scores  were.  because  2.8-3.5  (intermediates). of considerable  o f t h e 50% o v e r l a p b e t w e e n  (leiurus).  5.4-6,3  A  3,1-3.5  Standard Length,  (leiurus).  4.5-4,9  Scores  (trachurus),  The c h a r a c t e r was w e i g h t e d 6,  The 14%  banding patterns  or  42. the  the h i g h e s t  Trachurus always  summed f o r  r e c e i v e d maximum w e i g h t s  i n d e x p o s s i b l e was 12 + 10 + 10 + 6 + 4 =  L e i u r u s r e c e i v e d minimum v a l u e s lowest  than  rakers.  W e i g h t s f o r t h e f i v e c h a r a c t e r s were individual,  as  3,6-4,4  a h e a v i e r weight than s i z e b u t l e s s  that  because  were  overlap warrants  so  races.  overlap.  (intermediates),  every  into  (trachurus), 2.9-5.3  A w e i g h t o f 4 was a s s i g n e d t o s i z e  Body D e p t h i n t o follows?  divided  f o r a l l c h a r a c t e r s so  i n d e x t h a t c o u l d be o b t a i n e d was z e r o .  More o r  29  less  intermediate  individuals  r e c e i v e d more o r  less  intermediate  indices, Indices populations  are  of  l e i u r u s and t r a c h u r u s  d i s t i n c t and w e l l  the  i n t e g r i t y of  are  no l o n g e r s e p a r a t e d b y a w e l l  features also  of each  the  races breaks  the  a result  of  gene  d e f i n e d gap,  individuals  that backcrosses  to  t h a t race are  presented next,  However,  are  It  is  trachurus, of  a  distinct  r e l a t i v e l y few,  approaching trachurus rare.  They  and the m o r p h o l o g i c a l  The a b s e n c e  t h a t F^ h y b r i d s  of  fish,  7)„  down i n t h e h y b r i d z o n e .  flow.  scarcity  reared  (Figure  l e i u r u s i s more a f f e c t e d t h a n  i n t e r m e d i a t e mode s u g g e s t s and  separated  r a c e become b l u r r e d w i t h one a n o t h e r .  obvious that  p r e s u m a b l y as  from a l l o p a t r i c  Results  indicates  from l a b o r a t o r y  f u r t h e r c o n f i r m these  interpretations,  LABORATORY REARED F I S H Lateral reared  fish  (Figure of  8)„  plates  and  Counts,  numbers o f  Although  t h e y were  in offspring  of  that  range part  had counts  F^ h y b r i d s  towards  of  f o r w i l d caught  trachurus The  for plates  are  trachurus  as  is are  clear  genetically controlled  r e a r e d u n i f o r m l y the  Furthermore,  laboratory reared fish,  except  low as  30  the fish  are  numbers distinct  range is  of  the  same  t h a t no l a b o r a t o r y  (nor  did their  largely intermediate. suggests  from l a b o r a t o r y  l e i u r u s and t r a c h u r u s  reared  parents),  But a s l i g h t  shift  some dominance b y t h a t p a r e n t .  of v a r i a t i o n expressed of  It  plates  s e p e r a t e d by a wide gap.  variation as  that  Plate  i n Fj_ h y b r i d s  accounts  the v a r i a b i l i t y encountered i n w i l d h y b r i d s ,  The  for only which  a  HYBRID INDEX VALUES 20 22  20  42  15 10  S  JUT  O  1 U J  10 15  20L Figure  LEIURUS  TRACHURUS  F r e q u e n c y d i s t r i b u t i o n o f the h y b r i d i n d e x . Inverted histograms represent a l l o p a t r i c p o p u l a t i o n s ? u p r i g h t h i s t o g r a m s r e p r e s e n t samples from the h y b r i d zone.  o  31  I  3  l I l I i  5  7  I I i i l l I I i i i i i I i  9  PLATE F i g u r e 8.  l i i i  i i i i i i i I  11 13 15 17 19 21 23 25 27 29 31 33 35  COUNTS  L a t e r a l p l a t e counts f o r l a b o r a t o r y r e a r e d (black histograms) and of w i l d caught f i s h (hatched h i s t o g r a m s ) . L e i u r u s used as parents to o b t a i n o f f s p r i n g had p l a t e counts from 3-5; trachurus counts 32-34, Data based on 6 i n t r a r a c i a l c r o s s e s w i t h l e i u r u s parents, 6 i n t r a r a c i a l c r o s s e s w i t h trachurus parents, 7 i n t e r r a c i a l c r o s s e s f o r F^ h y b r i d s , and 7 b a c k c r o s s e s . Approximately equal numbers o f r e c i p r o c a l c r o s s e s made.  32 further  c o n f i r m s the Backcross  l e i u r u s have p l a t e offspring  are  the  leiurus i f Of the  of  of o f f s p r i n g of populations  With backcrosses  range  are  Backcrossing,  then,  raker the  conclusions  (31%) h a v e  counts  accounts  drawn f r o m t h e h y b r i d  zone F  's  are  rare.  If  t h e y were  of  (Figure  those  i n t e r m e d i a t e between which,  index,  for  counts  are  the  the  races.  like  wild the  like  plate  i n the h y b r i d  zone.  a n d t h e modes f o r  raker  races. is  Although less  that  i n the to  the  than i n  found i n  progeny r e i n f o r c e  r e l a t i v e l y few a n d b a c k c r o s s e s present  for  d i f f e r e n c e i n numbers  of backcrosses  and p l a t e s  to  are  increased,  for  counts  repeated b a c k c r o s s i n g c o u l d produce t h a t rakers  alone.  32% have  counts  of v a r i a t i o n i n laboratory reared hybrids  Counts of  and  criterion  counts between the  than w i l d h y b r i d s ,  variation is  counts  The v a r i a t i o n and mode f o r  Fj_ h y b r i d s  presence  sole  such because  i n t e r m e d i a t e b e t w e e n F-^'s a n d t h e  wild hybrids wild.  the  remaining progeny  c o n t r o l of  races.  variable  c o n f i r m s the  using plate  o b t a i n e d from backcrosses  l e i u r u s and t r a c h u r u s  less  the  f r o m 4-7, a n d t h e y w o u l d be  Results of  genetic  o f each r a c e .  and are  counts,  counts  and the  Raker C o u n t s .  t h a t d i s t i n g u i s h e s the  races  offspring  F^ h y b r i d s ,  a g a i n demonstrate  66% o f  to  i n c o u n t s b e t w e e n F-^'s and l e i u r u s ,  spectrum of v a r i a t i o n i n p l a t e Gill  zone,  crosses  total,  p l a t e s were u s e d as  trachurus.  that overlap with  9)  Of t h e  o n l y 3 7% c a n be r e c o g n i z e d as  those  total  f r o m 4-17.  progeny have counts  identification.  like  counts  be r e c o g n i z e d as b a c k c r o s s e s  i d e n t i f i e d as  trachurus  o f b a c k c r o s s i n g i n the h y b r i d  progeny r e c o v e r e d from presumed  intermediate  can t h e r e f o r e But 2 9 % o f  presence  the  hybrid  trachurus  are  t h e y c o u l d be r e c o g n i z e d e i t h e r  by  the  Figure  9,  G i l l raker counts o f l a b o r a t o r y r e a r e d (black histograms) and o f w i l d (hatched histograms) f i s h presented f o r comparisons. See c a p t i o n f o r Figure 8 f o r other d e t a i l s .  caught  CO CO  34  plate  or raker  counts.  Equally,  t h a t most o f t h e b a c k c r o s s e s Body D e p t h i n t o  the data  10),  Standard Length.  but i n contrast  to w i l d  gap b e t w e e n t h e o f f s p r i n g o f e a c h  i s under g e n e t i c  fish  race.  The d i f f e r e n c e i n  there This  is a  i n this  feature;  control  noticeable  d i s c r e p a n c y might  be c a u s e d b y n o n - i s o m e t r i c g r o w t h so t h a t a d u l t s coverge  the c o n c l u s i o n  are to l e i u r u s .  b o d y shape b e t w e e n l e i u r u s a n d t r a c h u r u s (Figure  reinforce  o f the r a c e s  or the g r e a t e r v a r i a t i o n i n w i l d  adults  may b e i m p o s e d b y e n v i r o n m e n t a l v a r i a t i o n . Dorsal hybrids  and Anal  are v i r t u a l l y a l i k e .  have  d o r s a l r a y counts  11.2  (s = 0 . 2 8 3 ) ;  range rays  Rays.  F i n ray counts  L e i u r u s o f f s p r i n g (sample  trachurus Hence,  have  f o r trachurus  a range  a range  o f f s p r i n g (sample (s = 0 . 8 0 3 ) .  It  (s =  (phenotypic reason  that  and a n a l r a y s  were e x c l u d e d f r o m t h e h y b r i d i n d e x .  make i t p l a i n  t h a t d i f f e r e n c e s between r a c e s  together as  with intermediates  evidence  cannot  (or  be a c c e p t e d  0.048);  0.435). races  plasticity), dorsal The r e s u l t s  species)  without  question  for hybridization.  Other t h e y were  (s =  i n the w i l d between  is for this  the  For anal  o f 7-9 a n d a mean o f 7 . 6  must be due t o e n v i r o n m e n t a l v a r i a t i o n divergence.  o f 80)  o f 7-9 a n d a mean o f 7.35  the d i f f e r e n c e encountered  not genetic  o f 160)  r a n g i n g f r o m 10-12 w i t h a mean o f  i s 10-13 w i t h a mean o f 1 1 . 4 l e i u r u s have  f o r r a c e s and  differences  o b s e r v e d between r a c e s ,  n o t q u a n t i f i e d , i n c l u d e c o l o u r a n d shape  Laboratory reared  leiurus retain  their  m o t t l e d w i t h b a n d s o f brown o r b l a c k ,  though of spines.  greyish-olive colour and t h e i r  short  spines,  LEIURUS N=26  TRACHURUS N=26  3.1 24 3.7 4 4.3 4.6 4.9 5.2 5.5 5.%// 6.7 7 BODY DEPTH INTO BODY LENGTH LO  F i g u r e 10 „  Body depth i n t o standard length o f l a b o r a t o r y r e a r e d f i s h . F i g u r e 8, f o r other d e t a i l s .  See c a p t i o n ,  0 1  36  w h i l e trachurus remain a b r i g h t s i l v e r c o l o u r and have long d o r s a l and p e l v i c s p i n e s .  Genetic c o n t r o l of these  characters  i s thereby i n d i c a t e d .  VARIATION WITHIN LEIURUS P l a t e counts of l e i u r u s i n the L i t t l e Campbell R i v e r demonstrate the presence o f a c l i n e  (Figure 11).  The  of the c l i n e i s such t h a t mean counts i n c r e a s e w i t h  direction  distance  upstream so t h a t h i g h e s t counts are i n the headwaters  and  lowest counts are n e a r e s t the h y b r i d zone a t S t a t i o n F. d i r e c t i o n o f the c l i n e i s , then, if  the r e v e r s e to t h a t  i n t r o g r e s s i o n from the h y b r i d zone were e f f e c t i n g  The  expected the  character. In a d d i t i o n , l i n k a g e r e l a t i o n s h i p s between p l a t e s and r a k e r s i n w i l d h y b r i d s was table.  The  tested with a 2 x 2  contingency  number o f l e i u r u s with low raker counts and  high  p l a t e counts and w i t h h i g h r a k e r counts and low p l a t e counts are r e s p e c t i v e l y ;  13 to 1 and 2 6 to 6.  Only i n d i v i d u a l s with  extreme p l a t e and raker counts are i n c l u d e d . c h i - s q u a r e value of 18.98  and one  With an  adjusted  Degree of Freedom P <C  ,001  r e v e a l i n g t h a t i n d i v i d u a l s w i t h low raker counts tend to have high plate  counts.  3  STATION A  Figure  11.  4  5  6  7  3  STA. B  4  5  6  7  C l i n a l v a r i a t i o n i n numbers o f p l a t e s o f l e i u r u s . o f more t h a n 2000 f i s h . Samples o f a t l e a s t 200, were made a t e a c h S t a t i o n .  3  STA. C  4  5  6  7  Data a r e b a s e d on c o u n t s and u s u a l l y 400-500 f i s h ,  co  38  SEASONAL ISOLATION  MATERIALS AND METHODS B r e e d i n g d a t a were this  study  made t o to  the  to  over  the  two y e a r s  d e t e r m i n e what c o n t r i b u t i o n s e a s o n a l  reproductive Little  gathered  isolation.  The s t u d y was  Campbell R i v e r and a d j a c e n t  isolation  largely confined  ponds,  but  observa-  t i o n s were made i n s t r e a m s on V a n c o u v e r I s l a n d i n t h e of  1965.  Throughout the b r e e d i n g season  censused Station  at  least  twice weekly.  N and l e i u r u s a t  stickleback  s e i n i n g were  one m i l e s e c t i o n f i s h were several was  sites  of  for  stream  estimated  at  each  from s t a n d a r d  h a u l s were made a n d t h e  vegetation  15  foot  were at  the  heavily  and presumably  rotated  Station.  summer  sampled  S t a t i o n D a n d i n a s m u c h as aquatic  nests,  populations  T r a c h u r u s was  weighted lead l i n e destroyed  within  Numbers o f  seine  average c a t c h  of  breeding  hauls.  for  the  a  Usually series  calculated. R e c o g n i z i n g b r e e d i n g f i s h was s i m p l e  b r e e d i n g males  are  b r i g h t l y c o l o u r e d and g r a v i d females  e n o r m o u s l y s w o l l e n abdomens. captured  recently  hatched  c o u l d be r e c o r d e d . males  It  is  and g r a v i d females  The f i n e mesh o f  f r y so  that  became  i n the  to  seine  earliest  hatching  are  breeding f i s h  though t h i s  Since g r a v i d females  s p r i n g than n u p t i a l l y c o l o u r e d males  non-reproductive  reasonable  their  the  record  have  assumed t h a t n u p t i a l l y c o l o u r e d  n o t a l w a y s be a r e l i a b l e m e a s u r e . found l a t e r  because  earlier  i n the  summer i t  t h e b e g i n n i n g and e n d o f  the  seemed  may were and most  reproductive  39  season by t h e i r were,  then,  presence  used to  and absence.  indicate  The f o l l o w i n g  d i f f e r e n t phases  of  criteria  the  breeding  season: Beginning of  1.  first  g r a v i d females  Occurrence  2.  the  reproduction:  first  when t h e males 4.  collected  of reproductions fry collected  Peak o f b r e e d i n g :  3.  estimated  largest  i n the  from  season.  i n d i c a t e d by t h a t  period  nuptially coloured  and g r a v i d f e m a l e s were  captured.  End o f b r e e d i n g s e a s o n :  i n d i c a t e d by the  g r a v i d females  i n the  collected  the  season.  estimated  i n the  number o f  from  last  season.  RESULTS The Leiurus last,  for  about  earlier  i n the  (Figure  12).  one m o n t h . further  Little  and t r a c h u r u s  f o u r months, year,  However,  but  starting  The r a c e s  restricted  Campbell  are  the  b o t h have b r e e d i n g seasons l e i u r u s begins  in late  breeding at  opportunity for  other  the  for  leiurus  peak o f b r e e d i n g s e a s o n  trachurus  present  of b r e e d i n g season have  e a r l y March  same t i m e  for  interbreeding  f o r when maximum numbers o f one r a c e the  (<0.2 per  the  standard i n the of  sharply declined,  are  seine haul)  stream.  trachurus,  breeding.  there are of e a r l y  Conversely,  that  to b r e e d much  February or  b r e e d i n g minimum numbers o f  numbers  River  only  is are  During  very  low  breeding  d u r i n g the  numbers o f b r e e d i n g  peak  leiurus  40  •  MARCH  Figure  12.  I  APRIL  i  MAY  I  JUNE  I  JULY  I  AUG.  I  SEPT.  B r e e d i n g seasons f o r the races o f s t i c k l e b a c k i n t h e L i t t l e C a m p b e l l R i v e r , 1964. D a t a f o r 1965 § f i r s t b r e e d i n g l e i u r u s , March 2 and f o r t r a c h u r u s May 12; f i r s t f r y o f l e i u r u s , A p r i l 28 a n d f o r t r a c h u r u s J u l y 7; peak o f b r e e d i n g f o r l e i u r u s , A p r i l 1 2 - 2 6 a n d f o r t r a c h u r u s June 6 - A u g u s t 2; e n d o f s e a s o n f o r l e i u r u s , J u l y 10 a n d f o r t r a c h u r u s September 1 8 .  41  Data f o r a n d M) r e s e m b l e  the b r e e d i n g season  that  from the  stream but  I n b o t h ponds l e i u r u s c o n t i n u e s  it  June  July.  and c o n t i n u e d throughout  caudal  a l i v e had e i t h e r  peduncle.  and J u l y .  temperatures Lethocerus, July,  as h i g h as  sticklebacks  Many a d u l t s  were  t a i l were  were that  this  I have o b s e r v e d  on  the  mortality,  that  24°C a r e l e t h a l to a d u l t s ,  A  predator,  a b u n d a n t i n ponds d u r i n g June  occasions  I have  T h e y were  vegetation  repeatedly readily  were  eruptions to  late  20-27°C were n o t uncommon d u r i n g l a t e  was e x c e e d i n g l y  i n aquatic  summer  post-reproductive  corpses  fungused bodies or  laboratory  sticklebacks.  the w a t e r  by the  of  I n the  and on s e v e r a l  on a d u l t  Their  ponds.  the  which began i n  Two f a c t o r s may c o n t r i b u t e  I n ponds t e m p e r a t u r e s June  i n the  difference.  into  may be m e n t i o n e d t h a t in leiurus,  scarcely  f o r one  (D  J u l y 16).  m o r t a l i t y was v e r y n o t i c e a b l e  e s p e c i a l l y conspicuous  l e i u r u s i n ponds  to breed l a t e r  ( i n 1964 u n t i l A u g u s t 5; i n 1965 u n t i l In p a s s i n g ,  of  found t h i s  found near  the  insect  refused,  but  feeding  surface  hanging head-down. l i v e ones  and  of  Dead presented  taken.  Hybrids Numerous n u p t i a l l y c o l o u r e d male a n d g r a v i d h y b r i d s were unusual years  collected  i n t h e h y b r i d zone a n d i n Pond M„  r e l a t i o n s h i p was f o u n d  demonstrate  female  ( T a b l e HI),  an o b v i o u s s e a s o n a l  sexes o f h y b r i d s .  B r e e d i n g m a l e s were  and g r a v i d females  i n June  and J u l y .  The r e s u l t s  of  i s o l a t i o n between taken i n A p r i l In e f f e c t ,  then,  A most both  the  a n d May the  42  Table  III.  Numbers o f b r e e d i n g h y b r i d s  BREEDING HYBRIDS, Date  o*  April 6 A p r i l 10 A p r i l 12 A p r i l 18 A p r i l 25 May 3 May 5 May 13 May 2 7 June 3 June 7 June 9 June 15 June 18 June 2 6 J u n e 29 July 5  9 23 13 18 41 14 21 27 11 0 0 0 0 0 0 0 0  1964  0 0 0 0 0 0 0 0 0 0 14 26 28 18 15 22 10  t a k e n on d a t e s  BREEDING HYBRIDS, Date  cyr  April 2 April 8 A p r i l 14 A p r i l 20 A p r i l 24 A p r i l 29 May 8 May 16 May 30 June 3 June 12 June 14 June 24 June 28  17 21 20 9 8 23 15 16 6 5 0 0 0 0  indicated.  1965  0 0 0 0 0 0 0 0 7 12 24 27 16 19  43  b r e e d i n g season leiurus, of  o f h y b r i d males  overlaps  a n d the b r e e d i n g s e a s o n  of hybrid  b r e e d i n g season  females  overlaps  of that  trachurus. Pond M i s  extensive were  always  about  it  is  unlikely  stream  is  that results This  that  also  are  is  dominant i n h y b r i d f e m a l e s ,  Early  t h e y make t h e  were  efficient,  Campbell  collected  River, i n the  one month  (Figure  farther  seems t o be  inherited of  of  as  leiurus  trachurus  (1964) h a s  also  Spawning T r a c h u r u s is  devoted to  these  Early breeding individuals stream both y e a r s .  12).  into  They were  the  individuals  uninhabited clay belt,  i n the h y b r i d zone collected  at  trachurus  (Figure Stations  3). L,  the  trachurus the  s t r e a m by as much  than i n d i v i d u a l s  (Station  of  They p r e c e d e d  further  b y p a s s i n g the  i n d i v i d u a l s were  seems v  spawning m i g r a t i o n ,  themselves  spurious.  c o n t r i b u t i o n to h y b r i d i z a t i o n i n  upstream  N) and the  shore.  in hybrid birds.  p r i n c i p a l breeding congregations  migrating  with  sticklebacks  and a g a i n i t  Selander  subsection  chief  are  and b r e e d i n g season  sex-limited characters  A separate  results  such t h a t b r e e d i n g season  dominant i n h y b r i d males  Little  and  spurious,  is  for  an a c r e ,  vegetation  the  physiological trait  a sex-limited character  reported  three-fourths  c o l l e c t e d by s e i n i n g c o m p l e t e l y around the  S e i n i n g i n the unlikely  small,  s h a l l o w s a n d dense e m e r g e n t  Consequently,  as  the  characterized  by  i n the p r i n c i p a l breeding thus So i t  grounds  establishing is  M, a n d M',  that in  these early  44  May.  A t t h e e n d o f May when t h e  arrived  i n the  stream  was no n o t i c e a b l e that  time or  principal  they s e t t l e d  increase  of  at  spawning m i g r a t i o n s  S t a t i o n N, a n d t h e r e  trachurus  i n t h e h y b r i d zone  later.  I n May o f  1965  I attempted  i n a more c r i t i c a l w a y .  During  the  to  c o n f i r m these  clay belt,  between the and l e f t .  for  o f n i n e t r a c h u r u s were  days. the  and a t o t a l  E a c h day t h e  The p r o c e d u r e was r e p e a t e d  the  p r i n c i p a l breeding migration of  Station N.  None was c a p t u r e d .  data  Again  s p a w n a i n g m i g r a t i o n s were (early  migrants)  o f May w i t h i n Station N.  were  for  During  captured during  the  first  S t a t i o n N (see  Table  were c a p t u r e d d u r i n g h y b r i d zone?  the  i n 1965  t h r e e were  t h r e e weeks o f June  14  I n 1965,  drastic  zone.  I n 1964, the  26  first  13  trachurus t h r e e weeks  June,  taken  19  D  present,  D u r i n g the  were c a p t u r e d i n t h e  at  Station N (recall  trachurus  counts  trachurus the first  hybrid  that  occurred i n  A c o m p a r i s o n was made b e t w e e n p l a t e  at  trachurus  e a r l y migrant  Station N  trachurus  reduction i n density of  at  e a r l y spawning  t h r e e weeks o f May w i t h i n  taken at  zone a n d h u n d r e d s were p r e s e n t  after  had a r r i v e d  f o u r i n d i v i d u a l s were  first  six  s e c o n d week i n June  t h r e e weeks o f  II)„  searched  captured over  were c a p t u r e d i n t h e h y b r i d zone a n d t h o u s a n d s were at  stream  c a t c h e s o f e a r l y and p r i n c i p a l  gathered.  t h e h y b r i d zone?  n e t was  trachurus  m i g r a n t s were c o l l e c t e d i n t h e h y b r i d The f o l l o w i n g  the  a  t r a c h u r u s b r e e d i n g grounds  and the h y b r i d zone, fish  observations  f i r s t week i n May 1965,  h a l f - i n c h m o n o f i l a m e n t g i l l n e t was s u s p e n d e d a c r o s s w i t h i n the  at  a  1965), of  45  individuals 404) of  s  2  and i n d i v i d u a l s from the e a r l y  34).  for  from the p r i n c i p a l b r e e d i n g m i g r a t i o n  The r a n g e s ,  means,  X = 34.55, that  f o r the e a r l y s  2  = 1.76.  and v a r i a n c e s  of P =  plate  counts of e a r l y  from the p r i n c i p a l  breeders  migration,  point  one month b e f o r e  move f a r t h e r  33-36,  (Bailey,  e q u a l so  1959).  A  v e r i f y i n g the o b s e r v a t i o n average h i g h e r  counts  that  than  to the c o n c l u s i o n  upstream  that  breeding migrants  enter  the p r i n c i p a l trachurus and s e t t l e  have  s i g n i f i c a n t l y higher  different  R =  32.7,  c o u l d n o t be assumed  early  s i t e s where h y b r i d swarms  and have  X =  those  migration.  To r e c a p i t u l a t e , stream about  R = 30-35,  o f t - t e s t was u s e d  .001 was o b t a i n e d ,  (sample  are the f o l l o w i n g ;  breeding migration,  Variances  the approximation  value  at  breeding migration  the p r i n c i p a l b r e e d i n g m i g r a t i o n , = 0.993;  (sample o f  developed  plate these  the  spawning  i n t h e h y b r i d zone ( S t a t i o n s M and M ' ) ,  counts.  A l l these  i n d i v i d u a l s are  facts  genetically  from i n d i v i d u a l s w i t h i n the p r i n c i p a l b r e e d i n g  migration.  Vancouver B e g i n n i n g J u n e 15, made  to v a r i o u s  seasonal that was in  Island 1965,  s t r e a m s on V a n c o u v e r  had completed  Island  the b r e e d i n g  a t t h e peak o f b r e e d i n g s e a s o n .  intervals  an e i g h t  i s o l a t i o n was more w i d e s p r e a d .  leiurus  Table  Populations  I V . The f o u r  streams  day f i e l d  t r i p was  to f i n d out i f  If so,  i t was p r o b a b l e  season and trachurus  The r e s u l t s  are  l i s t e d were s a m p l e d a t  from the e s t u a r y to the headwaters,  summarized frequent  b u t no l e i u r u s  Table  IV.  Stream  P r o p o r t i o n o f each race b r e e d i n g i n Vancouver  and Date  B i g Qualicum River June 15  Island  -  hundreds  Sample s i z e  -  30  Breeding f i s h - 0 ( f r y t o 2 . 1 cm a b u n d a n t i n a d j a c e n t marshes)  Breeding f i s h  -  F r e n c h Creek June 17  None p r e s e n t  Sample  163  B o n s a l l Creek June 18  Sample s i z e  Sooke R i v e r June 20  -  Breeding f i s h None  present  96 -  1965.  Trachurus  Leiurus  Sample s i z e  streams,  3(3%)  size  -  Breeding f i s h  -  Sample s i z e  72  Breeding  fish  Sample s i z e Breeding  -  -  fish  -  26(87%)  126(773  72  86 -  86  47  c o u l d be  f o u n d i n two o f  trachurus  trachurus  the  of  leiurus  and i t s  provides  the  is  under  were t r i g g e r e d freshwater  leiurus  confined.  were t a k e n  strong  and i t  like early  35,2,  to  i n b r e e d i n g between  genetic  is  to  control.  factors  insure  the  i n the  were  from  leiurus if  the  trachurus  and difference  trapped as  not.  t r a c h u r u s were  noteworthy  that  condition.  i n breeding condition,  ponds were  breeders  For  of  French  with evidence  that nine  i n l a n d j u s t below Home  had u n u s u a l l y h i g h p l a t e a mean o f  investigated  (along  B i g Qualicum River  7 miles  individuals,  evidence  freshwater  estuary,  adjacent  a l m o s t a l l were i n b r e e d i n g  difference  breeding  A landlocked population  s h o u l d not have been  in local  the  e x p e c t e d no  of  A b o u t one h u n d r e d t r a c h u r u s  by environmental  In the near  were f o u n d ,  additional  that  trachurus  in  a n d as  b a n k s were c a r e f u l l y  s e i n e d and i n s p e c t e d ?  hybrids)  A l l populations  e x i s t e d i n a s m a l l b e a v e r marsh  p o p u l a t i o n was  This  streams.  s a m p l e d were b r e e d i n g ,  populations  Creek,  the  Lake,  Little  counts w i t h a range  breeding,  individuals These  Campbell  from 33-36  River, and  48  ETHOLOGICAL  Specific choice  of  courtship displays that provide for  c o n s p e c i f i c mates,  constitute  ISOLATION  and t h u s  an i m p o r t a n t c a t e g o r y  of  prevent  When i s o l a t i n g mechanisms a r e  essential  t h a t mate p r e f e r e n c e  behaviour of nothing  is  isolating  the  at  issue  investigated.  three-spined stickleback  known r e g a r d i n g i t s  hybridization,  i s o l a t i o n mechanisms  animals.  be  the  it  in  is  Reproductive  is well  known,  c o u r t s h i p b e h a v i o u r as  but  an  mechanism.  MATERIALS AND METHODS Methods d e s c r i b e d b y v a n I r s e l for  holding  Large  sticklebacks  p r i o r t o mate p r e f e r e n c e  numbers o f m a l e s o f b o t h r a c e s were h e l d  g a l l o n cement partition  tank,  each  that divided  dechlorinated  16 h o u r s o f  of  race b e i n g separated  the  tap-water  uniform temperature gave  tank  excitation ready  their  light.  The low w a t e r  Little  at  w i t h a wooden  fresh  flow  of  nearly  and  l e v e l of  G r a v i d females i n the  c o l l e c t e d at  laboratory  cm x 29  drop  the  p r e c e d i n g mate p r e f e r e n c e (47  sexual  that, were  they would spontaneously  f e m a l e s were  Campbell River j u s t Sixteen aquaria  temperature  i m p o s s i b l e to h o l d  t h a n a day b e c a u s e hence,  200  A constant  a decreased  and m i n i m i z e d f i g h t i n g .  eggs,  in a  8°C while a c o n t r o l l e d photoperiod  t o mate were a l m o s t  more  in half.  efficient  tests.  p r o v i d e d c i r c u l a t i o n and a  crowded c o n d i t i o n s k e p t f i s h  for  (1953) were  experiments.  cm), each h o l d i n g  8 gallons,  49  were  u n i f o r m l y p r e p a r e d f o r mate p r e f e r e n c e  was c o v e r e d w i t h a of  R i c c i a algae  the a q u a r i a  layer  s a n d 5 cm d e e p ,  f l o a t e d on the  f l u c t u a t e d between  Sixteen of selected,  of  tests.  surface.  a n d a dense  Water  l e i u r u s and t r a c h u r u s ,  f r o m the h o l d i n g  tank to  the  shift  aquaria.  is  nests of  algae  are  of  a n d an e x i t  constructed  may be u s e d as building  were  1961).  females  into  the  Only a f t e r  about  this  i n the  transferred  called the  in  conditions  activities.  sand. algae  Once a  nest  l e a v i n g an  "creeping through"  t r a n s i t i o n from (van I r s e l ,  nest  1953;  tests.  I n most  instances  t h e y were  males  introduced  aquaria.  t h e y were  as  Difference  f e m a l e s was c a r e f u l l y s e l e c t e d  nearly equal i n s i z e  a n d g r a v i d i t y as  i n s i z e between females  Once a p a i r o f  f e m a l e s was c h o s e n  t h e y were  known t h a t  females  a male b y p e r f o r m i n g " h e a d u p "  her body a t  a 45° angle).  placed i n glass  tubes  Females  r e s p o n d to  (facing  It the  f i l l e d w i t h water  to  a  is  well  courtship  t h e male and  t o be u s e d i n  that  t h a n 3 mm.  presented  that purpose.  r e a d y t o mate w i l l  so  possible.  n e v e r v a r i e d more  " t e s t e r " male k e p t s p e c i f i c a l l y f o r  were  of  then  a c t i v i t y had been observed  t h r e e days a f t e r  Each p a i r of  of  solitary  to c o u r t s h i p a c t i v i t i e s  introduced for  completed nests  action,  an i n d i c a t o r s i g n i f y i n g  activities  Sevenster,  and t h i s  and  reproductive  made t h e male b u r r o w s t h r o u g h t h e b a l l  entrance  in  The s u d d e n i n c r e a s e  from crowded to  u s u a l l y produced a r a p i d onset First,  temperature  t h e most b r i g h t l y c o l o u r e d m a l e s were  each of  and t h e  coverin  1 7 ° and 1 9 ° C .  eight  temperature  The b o t t o m  tilting  experiments  and suspended i n  50  the  aquarium of  to t h e  the  " t e s t e r " male.  c o u r t i n g male w i t h  indication  t h a t each  experiment  continued.  two r e s p o n s i v e  If  ones were  of equal s i z e ,  then c o n c u r r e n t l y mental male.  the m a l e ,  there.  or for  Female a preference t h r e e 40  b o t h ends  nest,  two s t r i p s  depth o f  of  aquarium,  After  results  the  f i s h were  of  also  the  c e n t r e of  was e q u i d i s t a n t algae covered  from the  the  surface  the  of  35  cm)  recorded  discarded  to A  were u s e d  detect series  as  i n t e r v e n i n g space  cm b e t w e e n a q u a r i u m so  the  were  s a n d 5 cm w i d e was p l a c e d  and the  strips  test  significance  same r a c e .  s a n d a n d g r a v e l was u n i f o r m l y 5 cm.  i n the  eggs  conducted  s a n d was c o v e r e d w i t h c o a r s e  11  experi-  closed sequential  continuing u n t i l  cm x  A s t r i p of  were  respond  and d e p o s i t h e r  t e s t s were  (70  the  that  aquarium.  f o r males  p a r t i t i o n w i t h a space o f exactly  until  t o mate,  females would  i n the  i n another  gallon aquaria  of each  the  tested  aquarium of  the  females,  preference  tanks.  reliable  would b e g i n and e v e n t u a l l y ,  experiments  by females  experimental  the  the  one o f  p a i r of  began  a  l e i u r u s and t r a c h u r u s ,  n o n - s i g n i f i c a n c e was a c h i e v e d . a male a n d the  were  and p r e p a r e d n e s s  was r e c o r d e d  (1962),  and experiments  of  gravidity,  15 m i n u t e s ,  Cole  females  one e a c h o f  introduced into  result  t a k e n as  found.  f o l l o w h i m to h i s  This  design of  not other  Courtship a c t i v i t i e s  usually within to  " h e a d u p " t h i s was  responded  f e m a l e was p r e p a r e d t o mate a n d  Two f e m a l e s , were  If both females  of  sand.  water.  at  between  gravel.  The  A double  glass  partitions t h a t each  was  placed  glass  A uniform layer  plate of  51  Males o f approximately  equal s i z e  (- 3 mm) were  i n t r o d u c e d i n t o the aquarium, a l e i u r u s on one s i d e o f the c e n t r a l p a r t i t i o n s and a t r a c h u r u s on the other s i d e .  The  arrangement o f s u b s t r a t e f o r c e d males to b u i l d nests i n the sand - they never attempted t o b u i l d i n g r a v e l - and males were l e f t u n t i l both had b u i l t through" had been observed.  nests and u n t i l  "creeping  A g r a v i d female was s e l e c t e d  and i n t r o d u c e d between the g l a s s p a r t i t i o n s .  Sometimes the  female swam r a p i d l y to the bottom and was not n o t i c e d by e i t h e r male b u t e v e n t u a l l y she would b e g i n to swim about, as females u s u a l l y d i d when f i r s t her.  i n t r o d u c e d , and males began t o c o u r t  The s i t u a t i o n d i d not a r i s e i n which one male c o u r t e d  w h i l e the o t h e r male d i d not c o u r t .  Once the experiments  were under way o b s e r v a t i o n s were made by s i t t i n g 3 m from the aquarium.  I f a f t e r 15 minutes a female had not  responded t o a male's c o u r t s h i p w i t h r e p l a c e d w i t h another.  quietly  "head up" she was  When a female responded t o e i t h e r  l e i u r u s o r a trachurus male w i t h  "head up" the r e s u l t was  recorded i n Cole's c l o s e d s e q u e n t i a l t e s t d e s i g n . r o t a t e d between the three a q u a r i a and c o n t i n u e d  Tests were  until  s i g n i f i c a n c e o r n o n - s i g n i f i c a n c e was o b t a i n e d .  RESULTS  Mate Preference  Tests  R e s u l t s o f mate p r e f e r e n c e t e s t s are g i v e n i n Table V.  The outcome o f the f i r s t c l o s e d s e q u e n t i a l t e s t s e r i e s f o r  52  Table V.  R e s u l t s o f mate p r e f e r e n c e t e s t s (above) and female p r e f e r e n c e t e s t s (below) taken from Cole's c l o s e d sequential test design. Mate Preference  Tests  Number o f times females o f each race mated w i t h male. Each s e t r e p r e s e n t s r e s u l t s o b t a i n e d i n one t e s t .  With l e i u r u s  l e i u r u s i£ = 7 trachurus £ = 6  leiurus ^ = 6 trachurus °_ = 7  l e i u r u s °_ = 6 trachurus $ = 6  l e i u r u s f£ = 7 trachurus = 9  's  T o t a l l e i u r u s °_ = 26 T o t a l trachurus = 28  With t r a c h u r u s  t r a c h u r u s °_ = 6 l e i u r u s °_ = 6  trachurus = 9 l e i u r u s °_ = 7  t r a c h u r u s °_ = 8 l e i u r u s °_ = 11  trachurus ^ = 9 l e i U r u s °_ = 7  's  T o t a l t r a c h u r u s .£ = 32 T o t a l l e i u r u s .°. = 31 Female Preference  Tests  Number o f times females f i r s t "head up" to a male. With l e i u r u s °J s  l e i u r u s 0*^= 7 trachurus 8  performed  leiurus d ™ 6 trachurus c f s 7 7  T o t a l l e i u r u s 0*= 13 T o t a l trachurus o"= 15  n, With trachurus °.'s  trachurus & = 8 l e i u r u s 0 * = 10  trachurus o *^ 11 l e i u r u s 0^= 8  T o t a l t r a c h u r u s (f- 19 T o t a l l e i u r u s 0^= 18  3  53  leiurus  and t r a c h u r u s  m a l e s was n o t  p r e f e r e n c e b y the m a t e s . it  is  p o s s i b l e to  accepting  the  To m i n i m i z e  as  this  from each s e t  the It  error  the  of  test,  necessity  II  a series  four replicates  shown,  powerful  commit a Type  Cole  (1962)  error  with  and the  results  for s t a t i s t i c a l  on t h e all)  of  these  randomly.  For  then males  average,  for present  if  race  there  were  and females o f  l e i u r u s and f o r  Pooled t o t a l s  for  the  for s t a t i s t i c a l  necessity  the  the  t e s t s were  females  trachurus  treatment.  or  level  t h e y p l a y an i n s i g n i f i c a n t r o l e  isolation.  they e x i s t  often  race.  replicated  at  the  first  (Table V ) .  females  It  not e x i s t , that  they do,  opposite  less  same  obviate  may be c o n c l u d e d  t h a t e t h o l o g i c a l i s o l a t i n g mechanisms on b e h a l f o f if  with  Tests  trachurus  l e i u r u s and f o r  purposes.  differences  be s t i m u l a t e d t o mate  experiments  preclude  reproduce  n u l l h y p o t h e s i s was a l w a y s a c c e p t e d - i n  for  of  t h a t f u n c t i o n e d as  than males and females o f  Again i n these  sequence  were  The outcome  pooled totals  treatment  Female C h o i c e  the  exists.  In each  n u l l h y p o t h e s i s was a c c e p t e d .  i s o l a t i n g mechanisms,  because  test  had been p e r f o r m e d .  i n c o u r t s h i p b e h a v i o u r between r a c e s  not at  this  that  t e s t s was t h e n p o o l e d t o p r o v i d e f o r a more  males o f b o t h r a c e s  (or  p o i n t s out  of closed sequential tests  may be c o n c l u d e d t h a t f e m a l e s o f e i t h e r  race would,  i n d i c a t i n g no  n u l l h y p o t h e s i s when a s l i g h t p r e f e r e n c e  repeated u n t i l these,  However,  significant,  females  do  s u c h an e x c e e d i n g l y low i n reproductive  54  Hybrid Reproductive A c t i v i t i e s No mate of e i t h e r  race.  preference It,  would demonstrate  c o u l d be d e t e c t e d b y m a l e s o r  therefore,  seemed u n l i k e l y  a preference,  females  that  hybrids  and such experiments  d i d not  worth w h i l e .  However, h y b r i d i n v i a b i l i t y  many a n i m a l s ,  and i t was p o s s i b l e t h a t h y b r i d s m i g h t n o t be  able  to  perform c o u r t s h i p a c t i v i t i e s  properly, a series  or perform parental care o f t e s t s were  is well  known i n  properly, build  properly.  seem  With  nests  this  i n mind  d e s i g n e d u s i n g p r e s u m e d Fj_ a n d b a c k c r o s s  hybrids. E i g h t males t h a t were  t h a t were p r e s u m e d F]_'s  presumed b a c k c r o s s e s  to  leiurus  and f i v e  (see  males  s e c t i o n on  M o r p h o l o g i c a l A n a l y s i s ) were c o l l e c t e d a n d p l a c e d i n a q u a r i a like  those  males female  d e s c r i b e d f o r mate  successfully built leiurus.  preference  nests,  For a l l these  courted, activities  when c o m p a r e d w i t h male o f e i t h e r had been d e p o s i t e d i n a m a l e ' s each day. and o t h e r days,  H y b r i d s were activities.  the  aggregate of  the  When a c l u t c h o f  In a l l these  nest, the  activities  I n one c a s e a h y b r i d h a d two s e p a r a t e  nest  clutches  of  on t h e  After  the  first  glueing six  Males a probe  into a  loose  t h e y seemed n o r m a l .  one d e p o s i t e d o n one d a y ,  day.  eggs  observed  nest,  attacked  deposited i n h i s nest, following  were  two more d a y s .  a q u a r i u m , and p u l l e d  algae.  t h e y seemed n o r m a l  eggs h a d h a t c h e d a t  continued for  instance  and spawned w i t h  n u d g i n g the  r e t u r n e d f r y t h a t h a d s t r a y e d f r o m the introduced into  In e v e r y  nest h i s a c t i v i t i e s  seen f a n n i n g , Most o f  and o b s e r v a t i o n s  race.  tests.  eggs the  other  c l u t c h h a d h a t c h e d he  55  was  v e r y busy f a n n i n g the remaining  wandering f r y to the n e s t .  There was  c l u t c h of eggs and r e t u r n i n g no s u g g e s t i o n of h y b r i d  i n v i a b i l i t y o r breakdown i n c o u r t s h i p or r e p r o d u c t i v e I had  the impression  activities.  t h a t h y b r i d males executed  care more v i g o r o u s l y than d i d males of e i t h e r r a c e .  parental  This  was  e s p e c i a l l y n o t i c e a b l e d u r i n g f a n n i n g and r e t u r n i n g f r y t h a t had wandered from the Finally,  nest.  s i x h y b r i d females,  to trachurus, were c o l l e c t e d and were examined.  presumably  t h e i r reproductive  activities  These females were p l a c e d i n a q u a r i a c o n t a i n i n g  trachurus males t h a t had b u i l t nests, one aquarium.  backcrosses  female i n each  F i v e o f the h y b r i d females responded to the male's  courtship with  "head up",  and when the male l e d , the female  f o l l o w e d to the nest, entered,  and d e p o s i t e d her eggs.  One  female d i d not spawn, but no s i g n i f i c a n c e i s a t t a c h e d to t h i s r e s u l t because there were f r e q u e n t l y cases i n which females of both races f a i l e d to spawn.  There were no suggestions  h y b r i d i n v i a b i l i t y or breakdown i n h y b r i d  females.  of  56  ECOLOGICAL ISOLATION  The  two races o f s t i c k l e b a c k occur i n v e r y  d i s s i m i l a r . e n v i r o n m e n t s o u t s i d e the b r e e d i n g season, i n freshwater and trachurus i n the s e a .  leiurus  Observations a t the  L i t t l e Campbell R i v e r d u r i n g the b r e e d i n g season make i t e v i d e n t t h a t even when the two races are sympatric i n the same stream  segregation e x i s t s  (Figure 2 ) .  This  suggests  t h a t each race i s a s s o c i a t e d w i t h d i f f e r e n t aspects o f the environment producing what seems to be a v e r y i s o l a t i n g mechanism.  A s e r i e s o f experiments  powerful was t h e r e f o r e  designed to t e s t f o r e c o l o g i c a l f a c t o r s t h a t might be important i n m a i n t a i n i n g the observed e x c l u s i o n .  DISPERSAL AND TRANSFER  EXPERIMENTS  MATERIALS AND METHODS D i s p e r s a l experiments  were c a r r i e d out w i t h l a r g e  numbers o f l e i u r u s by marking, r e l e a s i n g , and r e c a p t u r i n g , i n d i v i d u a l s moved from one p a r t o f t h e i r h a b i t a t to another. T r a n s f e r experiments,  on the other hand, were conducted  with  l e i u r u s and trachurus by t r a n s f e r r i n g i n d i v i d u a l s o f each race from t h e i r own h a b i t a t to the h a b i t a t o f the o t h e r . experiments  f i s h were marked by c l i p p i n g the f i r s t d o r s a l  spine a t i t s base. to i d e n t i f y .  F o r both  This mark proved to be r e l i a b l e and easy  S i x weeks b e f o r e experiments  began 2 0 f i s h had  t h e i r s p i n e s c l i p p e d w i t h heavy n a i l c l i p p e r s and were h e l d  57  in  the  laboratory,  screen  live-boxes  f i s h h e l d i n the stream d i e d . their  a n d 15  spines  i n the  first  A f t e r f o u r weeks  a n d none o f  No s t i c k l e b a c k  erect  showed e v i d e n c e  their  spines,  s p i n e was c o n s p i c u o u s .  that spines  are  two o f  those h e l d i n  e v e n t h o u g h t h o s e h e l d i n the  dorsal  no e v i d e n c e  stream.  laboratory  Captured s t i c k l e b a c k s the  o t h e r s were c l i p p e d a n d h e l d i n  of  the  the  regenerating  laboratory  had grown.  and the  absence  Moreover,  there  u s e d i n l o c o m o t i o n and  of is  their  absence s h o u l d not h i n d e r m o b i l i t y . Dispersal a typical  experiments  leiurus habitat.  were p e r f o r m e d a t  There are  several  a c c e s s w i t h narrow w i d t h and s h a l l o w w a t e r , for  recapture experiments.  N were u s e d f o r  For  transplant  the  S t a t i o n D,  miles of  ideal  same r e a s o n s  easy  conditions S t a t i o n D and  experiments.  RESULTS  Dispersal Several  observations  suggesting  that  located  Station  with  at  a parasite,  leiurus  the  either  is  p o i n t i n the  a sedentary  about  50% o f  d u c k s may a c t  i d e a l and l a r g e  downstream  the  as  there are  infected  s p o t s on t h e for  leiurus  are  body. the  definitive  y e t w i t h i n 300 dense  a population  individuals  the  number o f  direction  First,  probably responsible  densely vegetated shallows, up o r  same  fish.  producing black  stream i s  i n f e c t i o n since  The h a b i t a t in  E has  Neascus,  A d u c k f a r m b y the heavy  is  Experiments  host. present  meters  populations of  leiurus  58  and p r a c t i c a l l y none are p a r a s i t i z e d .  Second, the c l i n e i n  p l a t e numbers occupies the v e r y s h o r t d i s t a n c e of 7% m i l e s . T h i s would be expected o n l y i n a v e r y sedentary f i s h . h y b r i d s are c o n f i n e d to an amazingly Other  narrow zone i n the  f a c t o r s are no doubt i n v o l v e d , but t h e i r  to p o i n t s of h y b r i d i z a t i o n suggests  low d i s p e r s a l  stream.  localization  (coexistence of l e i u r u s and  trachurus)  rates.  On 19 March 1965,  a sample o f 2000 l e i u r u s were  c o l l e c t e d a t Pond L> a d j a c e n t to the stream and immediately  Last,  to S t a t i o n D f o r r e l e a s e .  The s p i n e s were c l i p p e d  and f i s h were r e l e a s e d i n t o the stream. of marked f i s h began two  carried  Seining f o r recovery  days a f t e r t h e i r i n t r o d u c t i o n and  continuous up and downstream from the p o i n t of r e l e a s e . procedure was  Data  (Figure 13)  show t h a t v i r t u a l l y no l e i u r u s were r e c a p t u r e d beyond f e e t from the p o i n t o f r e l e a s e .  was  600  Most were r e c o v e r e d w i t h i n  f e e t up or downstream, and t h i s was  when the experiment at  The  terminated a f t e r a s e r i e s of s i x or more  c o n s e c u t i v e h a u l s y i e l d e d no marked f i s h .  100  was  terminated.  so even 30 days l a t e r  Six months l a t e r  collections  S t a t i o n D turned up l a r g e numbers of marked l e i u r u s .  l e i u r u s always do, marked f i s h demonstrated a s t r o n g  As  tendency  to gather i n densely v e g e t a t e d shallows a l o n g the stream,  and  peaks of d e n s i t y shown i n the frequency -tek®. d i s t r i b u t i o n corresponds w i t h such  locations.  Transfer These experiments  Experiments  were undertaken  together w i t h o t h e r s  300-  0  +300  600  300-  0  DISTANCE  gure  13.  Recapture data f o r  leiurus  +300 IN  transferred  600  300-  0  FEET  f r o m Pond D t o  Station  D.  +300  600  60  to seek an e x p l a n a t i o n to the observed Experiments began 6 March 1965 experiments) when 3081  e x c l u s i o n between r a c e s .  ( e a r l i e r than the p r e v i o u s  l e i u r u s were c o l l e c t e d a t S t a t i o n D,  marked, and r e l e a s t e d a t S t a t i o n N j u s t below the p o i n t where Highway 99 c r o s s e s the stream. e n t e r e d the stream.  A t t h a t time trachurus had  not  Recapture of marked f i s h began s i x days  a f t e r t h e i r r e l e a s e , but not one  l e i u r u s was  recovered  although a d i s t a n c e of one m i l e up and downstream was  seined.  Spot s e i n i n g began a t the e s t u a r y and c o n t i n u e d upstream to S t a t i o n N but none was disappearance  was  found. T h i s unexpected and  sudden  probably a r e s u l t of d i s p e r s a l from the  area because f i s h ,  l i k e salmon, t h a t c o u l d be p r e d a t o r s were  present as f r y , and no other predators were known i n the a r e a . Another t r a n s p l a n t was  attempted on 14 March u s i n g 6000 l e i u r u s ,  and r e c a p t u r e began the f o l l o w i n g day.  Very few  individuals  were recovered even on the f i r s t day a f t e r r e l e a s e (Figure 14), and a f t e r three days o n l y three l e i u r u s were taken a one m i l e s e c t i o n of stream.  The  data suggest  over  t h a t most of  the f i s h moved upstream d e s p i t e a c u r r e n t of 92 cm a second they had  to forge w h i l e p a s s i n g through  beneath Highway 99.  a 230  foot conduit  The r e s u l t s o f these experiments c o n t r a s t  v i v i d l y w i t h d i s p e r s a l experiments i n which l e i u r u s was from one  p a r t o f i t s h a b i t a t to  moved  another.  S e v e r a l prominent changes occur a t S t a t i o n N between e a r l y s p r i n g and e a r l y summer.  A t the time the f o r e g o i n g  experiments were performed the area i s d e v o i d o f v e g e t a t i o n , and c u r r e n t i s s w i f t there b e i n g no p l a c i d backwaters along  Figure  14.  DISTANCE  R e c a p t u r e d a t a f o r the t r a n s f e r o f Zero r e p r e s e n t s p o i n t of r e l e a s e .  IN F E E T  l e i u r u s from S t a t i o n D to S t a t i o n See t e x t f o r f u t h e r details.  N,  <r>  62  the margins o f the stream,  i n e a r l y June dense beds o f Elodea  cover the bottom and the c u r r e n t i s m i l d difference at Station N called for s t i l l experiment.  0 /  This  seasonal  another  transplant  On 31 May, 3000 l e i u r u s were marked and r e l e a s e d .  A p o p u l a t i o n was s u c c e s s f u l l y founded and 24 days l a t e r when recapture  stopped l a r g e numbers were s t i l l  Recovery f o r marked f i s h continued they remained abundant u n t i l  present  (Figure 15).  a t monthly i n t e r v a l s and  7 December when none was caught.  By t h a t time the Elodea had d i e d down and the c u r r e n t was again fast. The  r e s u l t s demonstrate a very n o t i c e a b l e  shift  upstream i n the d i s t r i b u t i o n p a t t e r n from the p o i n t o f r e l e a s e , which p e r s i s t e d throughout the summer.  As r e s u l t s o f the  d i s p e r s a l experiment had shown, l e i u r u s proved t o have a low d i s p e r s a l , s e t t l i n g w i t h i n 600 f e e t from the p o i n t o f r e l e a s e and  g r a v i t a t i n g toward l o c a l i t i e s where v e g e t a t i o n was most  dense and c u r r e n t l e a s t . By 2 June trachurus  were present  a t S t a t i o n N, and  the two races e x i s t e d s y m p a t r i c a l l y f o r the r e s t o f the summer (but r e c a l l t h a t d e n s i t y o f trachurus Extensive  was v e r y reduced i n 1965).  c o l l e c t i o n s were not p o s s i b l e but those t h a t were made  suggest a marked i n c r e a s e  i n the i n c i d e n c e  a r e s u l t o f the a r t i f i c a l  i n t r o d u c t i o n as can be seen i n the  f o l l o w i n g c o l l e c t i o n data: 21 h y b r i d s ;  of h y b r i d i z a t i o n as  September 18, 17 h y b r i d s ;  December 7, 6 h y b r i d s .  October 12,  Over a two year p e r i o d p r i o r  to the i n t r o d u c t i o n o n l y 24 h y b r i d s were c o l l e c t e d a t S t a t i o n N. R e c i p r o c a l t r a n s p l a n t s , moving trachurus  from S t a t i o n  Figure  15.  DISTANCE IN FEET Recapture data f o r l e i u r u s t r a n s p l a n t e d from S t a t i o n Zero r e p r e s e n t s p o i n t o f r e l e a s e .  D to  Station  N.  64  N to S t a t i o n D, began on 7 June but marking the f i s h was  not  necessary inasmuch as trachurus were unknown i n the upper  reaches.  Because trachurus were f a r l e s s abundant i n 1965  o n l y 350 were  c o l l e c t e d and t r a n s p l a n t e d .  days l a t e r .  Recapture began two  The c h a r a c t e r i s t i c upstream s h i f t i n d i s t r i b u t i o n can be seen  (Figure 16) but d i d not p e r s i s t .  Trachurus  again  dispersed  over a g r e a t e r d i s t a n c e than l e i u r u s so t h a t a f t e r nine days they had moved a t l e a s t 1000 d i s t a n c e upstream.  f e e t downstream but j u s t a s h o r t  There was  a c o n t i n u a l r e d u c t i o n i n the  numbers caught and by 2 7 June few s t r e t c h was captured,  again s e i n e d on 19,  nor was  remained.  20,  A one  mile  21 J u l y and none  was  any found w h i l e spot s e i n i n g many s i t e s i n  the upper reaches of the stream.  The  continual reduction i n  numbers o f trachurus a t S t a t i o n D together  with  little  d i s p e r s a l upstream and i n c r e a s i n g numbers downstream i n d i c a t e s a downstream m i g r a t i o n from the a r e a . During  the 20 days t h a t trachurus remained a t  S t a t i o n D they e x h i b i t e d a s t r o n g p r e f e r e n c e f o r the l o c a l i t i e s where g r a v e l was mild  few  present and where the c u r r e n t  was  (most o f the c u r r e n t i s s l u g g i s h d u r i n g the summer i n the  headwaters).  I n v a r i a b l y the l a r g e s t numbers were s e i n e d over  such p l a c e s g i v i n g r i s e to temporary c l u s t e r s of t r a c h u r u s t h a t lodged i n a v e r y d i f f e r e n t p o r t i o n o f the h a b i t a t than d i d leiurus.  I t i s u n l i k e l y that competition played a role i n t h i s  h a b i t a t e x c l u s i o n such t h a t r e s i d e n t l e i u r u s f o r c e d trachurus i n t o those areas, f o r 14,000 l e i u r u s had been c u l l e d from the area e a r l i e r i n the s p r i n g .  A c t i v e s e l e c t i o n o f these  sites  100 r  JUNE 9  JUNE 13  50  600  400  200-  0  +200 400  ^50  800  600 400 200-  +200 400  JUNE 16  JUNE 20  & 0 800  600  400  200-  0  800  +200 400  50  61.  400  200-  0  +200 400  JUNE 27  800 Figure  600  600 400  200-  0  +200 400  DISTANCE IN FEET  Recapture data f o r trachurus t r a n s p l a n t e d Zero r e p r e s e n t s p o i n t of r e l e a s e .  from S t a t i o n  N to  Station  D.  Ul  66  seems more l i k e l y . Many of the trachurus unhealthy.  t h a t were caught seemed v e r y  Eleven dead f i s h were found and males l o s t  their  b r i l l i a n t n u p t i a l c o l o u r s - which a l l i n t r o d u c e d males had  -  w h i l e females assumed a d u l l s i l v e r c o l o u r r a t h e r than the b r i g h t s i l v e r that i s t y p i c a l .  They were a l s o emaciated.  M o r t a l i t y as w e l l as d i s p e r s a l from the area probably  accounted  f o r the c o n t i n u a l r e d u c t i o n i n numbers. In summary, d i s p e r s a l experiments w i t h l e i u r u s confirm observations  t h a t t h i s i s a v e r y sedentary  fish  which congregates where v e g e t a t i o n i s dense and c u r r e n t n i l . When i t i s i n t r o d u c e d i n t o an environment d e v o i d  of  v e g e t a t i o n w i t h no r e t r e a t from c u r r e n t i t r a p i d l y d i s p e r s e s from such an i n h o s p i t a b l e h a b i t a t .  Trachurus i n t r o d u c e d i n t o  a l e i u r u s h a b i t a t moves i n t o l o c a l i t i e s of g r a v e l and  current  e v e n t u a l l y d i s a p p e a r i n g from the area p a r t l y as a r e s u l t of downstream m i g r a t i o n and p a r t l y as a r e s u l t of m o r t a l i t y . The  inappropriateness  of the h a b i t a t i s demonstrated by  t h e i r g e n e r a l l y poor c o n d i t i o n .  PREFERENCE TESTS A s e r i e s of preference  t e s t s was  conduced i n the  l a b o r a t o r y i n an attempt to d e f i n e some of the components o f the environment t h a t were important isolation. First,  i n securing e c o l o g i c a l  Four components seemed p a r t i c u l a r l y  important.  l e i u r u s were always observed n e s t i n g on a mud  bottom  67  and t r a c h u r u s always  on a s a n d b o t t o m .  a n d n e s t s were  and n e s t s tea  l e i u r u s nests  f o u n d among b r o a d - l e a v e d v e g e t a t i o n  among E l o d e a o r M y r i o p h y l l u m . water  Second,  Third,  found there,  c o l o u r e d water  and t r a c h u r u s  the  races  thus p r o v i d i n g  Fourth,  clear  frequents  leiurus  water.  inhabits  A l l four  t h e y may s e r v e  into different habitats  for reproductive  nests  leiurus dwells i n standing  components may be i m p o r t a n t i n a s m u c h as separate  and t r a c h u r u s  while trachurus  i n gently flowing water.  were  isolation  within (and  the  to stream  competitive  exclusion).  MATERIALS AND METHODS  Substrate  Preference  In February stocks wharves the  i n the  laboratory  sea water, of  to  i n 40  the  L e i u r u s were  i n freshwater.  A dense  layer  also  On 1 A p r i l  of  trachurus  was  stocks  the  stream  from  and h e l d  in  supplied with c i r c u l a t i n g  t a k e n and h e l d the the  were  s u p p l i e d to  all  to  1957).  freshwater,  glass-bottomed either  to  aquaria.  (Baggerman,  had a t t a i n e d  c o u l d n o t be any r e c e n t h a b i t u a t i o n t o  increased  was g r a d u a l l y e l e v a t e d  transferred  were h e l d o v e r  same  p h o t o p e r i o d was  sexual maturity  28 May many l e i u r u s a n d t r a c h u r u s Because  of  temperature  algae  T h e s e c o n d i t i o n s promote On 15 A p r i l  estuary  were c o l l e c t e d  o f 8°C, a n d a p h o t o p e r i o d o f 8 h o u r s  16 h o u r s a day a n d t h e  18°C.  trachurus  gallon aquaria  a temperature  l i g h t a day.  except  sea .near  of  Tests  and b y  nuptial colours.  aquaria  there  a s a n d o r a mud  68  substrate. 8  Six preference providing plate of  tests, the  the  other  The  aquaria  on  the  a  mud  and  on  were  a  were  same b a s i c  were  used  i n these  half  with  Elodea  in  which  on  parallel  design  half  with  the rows  An  8  one  a  a  Station  in half  was  and  and  with  placed,  depth after  5  of  N a  on cm.  sediment  layer of  algae  had  placed  to  cm  apart  side  of  the  the  opposite  of  the  f r o s t e d p l a t e was  top  of  the  water  the so  the  side  of  aquarium  partition  that  the  partition  that  the  space  end  and  aquarium  firmly  cm. cm  the  effects  of  did  d i s t u r b water  not  a  cm  plant was  was  planted  current  with  firmly  in 8  above  planted  in half  other 12  by  10  fish  Tests  was  the  wedged  projected  of  of  observation.  divided  one  were  vegetation  Preference was  stocks  a headwater  facilitate  g a l l o n aquarium such  A  and  and  aquaria  Penanthe,  4  Tests  aquaria  The  side  of  The  of  observer's  p l a t e of f r o s t e d g l a s s  of  at  water  frequently nests.  Current  side  mud  substrate  from  divided  Preference  experiments.  and  leiurus  open  against  sand  layer of  removed  for  surface.  The  left  were  with  Vegetation  in  and  a l l uniformly  filled  partitions  prepared  Station D  side  sand  then  were  Aquaria  one  layer of  the  from  materials.  glass  settled  gallon aquaria  fitted  end  fell  The of  sand. of  generated  on  the  short  bottom  surface  on  a  other  one side.  69  The  open end p e r m i t t e d  f r e e passage of a f i s h from one  the other o f the aquarium, d r i v e n motor w i t h a 2,5  s i d e to  A v a r i a b l e speed e l e c t r i c a l l y  cm p r o p e l l e r was  suspended j u s t beneath  the s u r f a c e i n a corner o f the aquarium a t the open end, the device was  h e l d f i r m w i t h a c l a p and a r i n g stand.  b l i n d ends of the aquarium mesh baskets suspended and f i l l e d w i t h a l g a e .  and At  the  w i t h open bottoms were  On the bottom a t the open  end of the aquarium pebbles were p l a n t e d . T h i s produced a tank i n which f i s h c o u l d  freely  choose to nest i n e i t h e r c u r r e n t or s t a n d i n g water. open end where a s l i g h t c u r r e n t was s t i c k l e b a c k s from b u i l d i n g n e s t s .  d e t e c t e d pebbles Fine threads  b r i n e shrimp sank d i r e c t l y to the bottom on one the other s i d e such items flowed  In  the  prevented  or p i e c e s of s i d e , but  on  g e n t l y through the water.  Males p l a c e d i n the c u r r e n t tank q u i c k l y l e a r n e d to pass f r e e l y from s i d e to s i d e and there was  no i n d i c a t i o n t h a t  avoided  the p r o p e l l e r , which a t any r a t e was  corner.  Before and d u r i n g n e s t i n g trachurus  a s i d e and  they  in a  spent a great  d e a l o f time r i d i n g the c u r r e n t j u s t i n f r o n t o f the r o t o r .  Water Q u a l i t y Preference  Tests  For these experiments t e a - c o l o u r e d water from S t a t i o n D and c l e a r water from S t a t i o n N was laboratory.  The  t r a n s p o r t e d to the  apparatus and methods used were l i k e  d e s c r i b e d by Mclnerney  (1964) f o r s a l i n i t y p r e f e r e n c e  In h i s experiments one  chamber was  c o n c e n t r a t i o n of s a l t water, and  those tests.  f i l l e d w i t h a given  freshwater  from the  adjacent  70  chamber was allowed to r i s e above a c e n t r a l p a r t i t i o n a b r i d g e o f water.  forming  The d i f f e r e n t i a l d e n s i t i e s o f s a l t and  freshwater maintained  the two separate while  the b r i d g e o f  water p r o v i d e d f r e e passage so t h a t the f i s h c o u l d choose between s a l i n i t i e s . In my experiments i t was necessary to determine how much mixing o c c u r r e d when water o f equal d e n s i t y was p l a c e d i n both chambers. i n the plugged  A s e r i e s o f t e s t s were made by dyeing water  chamber w i t h Methyl  to form a b r i d g e .  Blue and then f l o o d i n g  A v e r y s m a l l p o r t i o n o f the dye moved i n t o  the f l o o d e d chamber as a r e s u l t o f turbulence, b u t the two kinds o f water c l e a r l y remained s e p a r a t e .  RESULTS  Substrate Preference  Tests  Three n u p t i a l l y c o l o u r e d male l e i u r u s and trachurus were removed from the stock tanks and i n t r o d u c e d , one each, i n t o the s i x prepared a q u a r i a .  They were l e f t u n t i l  b u i l t nests and were then r e p l a c e d by o t h e r males.  they had Each  time  a s u b s t r a t e was s e l e c t e d by males o f e i t h e r race the c h o i c e was e n t e r e d i n Cole's c l o s e d s e q u e n t i a l t e s t design and t e s t s c o n t i n u e d u n t i l s i g n i f i c a n c e o r n o n - s i g n i f i c a n c e was a c h i e v e d . When s t i c k l e b a c k s b u i l d nests they d i g a p i t i n the sand and l a t e r c a r r y algae to the p i t g l u e i n g t h i s w i t h a s e c r e t i o n , presumably from the kidney.  together  I t may be t h a t  71  s e c r e t i o n s around the nest i n f l u e n c e s i t e s chosen by subsequent males so the s u b s t r a t e around and beneath the nest was always removed a f t e r each experiment. R e s u l t s are h i g h l y s i g n i f i c a n t , a s t r o n g p r e f e r e n c e f o r n e s t i n g on mud s t r o n g p r e f e r e n c e f o r n e s t i n g on sand  leiurus  demonstrating  (7:0) and trachurus a (9sl).  V e g e t a t i o n Preference  Tests  L e i u r u s males show a p r e f e r e n c e f o r n e s t i n g among Qenanthe (17-4) and trachurus males a p r e f e r e n c e f o r n e s t i n g among Elodea  (9-rl) .  Males twice b u i l t nests between rows of  Elodea and Qenanthe and the r e s u l t s were d i s c a r d e d . The  s t r u c t u r e o f the v e g e t a t i o n and i t s arrangement  i n the aquarium complicates drawing c o n c l u s i o n s from the experiment.  Qenanthe has long s t a l k s f r e e o f leaves w i t h a  dense canopy of leaves above, b u t Elodea has s m a l l leaves t h a t c l u s t e r densely along the e n t i r e l e n g t h o f the s t a l k .  Thus,  among Qenanthe the bottom i s r e l a t i v e l y open and there i s a dense canopy 10 to 15 cm above the bottom. cover above i s l e s s b u t the bottom o f the tank i s much more s e c l u d e d . demonstrating  (where the nests are found) Hence, r a t h e r than  a p r e f e r e n c e f o r type o f v e g e t a t i o n i t i s p o s s i b l e  t h a t the two forms are demonstrating  a preference f o r e i t h e r a  more open o r a more secluded n e s t i n g s i t e . field,  Among Elodea the  to be d e s c r i b e d , i n s i n u a t e the l a t t e r  Observations possibility.  i n the  72  Current Preference  Tests  Each race demonstrates a s t r o n g preference f o r n e s t i n g i n s t a n d i n g water preclude any  (7:0 f o r each), but t h i s c o n c l u s i o n does not  importance o f c u r r e n t i n s e g r e g a t i n g the races i n  t h e i r c h o i c e of a n e s t i n g l o c a t i o n .  L e i u r u s v e r y r a r e l y moved  i n t o the h a l f o f the aquarium w i t h c u r r e n t and a b r i e f time.  Before  then o n l y f o r  n e s t i n g began trachurus was  most o f t e n i n  the h a l f w i t h c u r r e n t , and even a f t e r i t had b u i l t i t s nests i n the h a l f w i t h s t a n d i n g water i t s a c t i v i t i e s a s i d e from nest b u i l d i n g were l a r g e l y spent i n moving water. w i l l be c o n s i d e r e d f u r t h e r i n a l a t e r  These o b s e r v a t i o n s  sub-section.  Water Q u a l i t y Preference  Tests  Equal numbers o f f i s h were p l a c e d i n each chamber, one  c o n t a i n i n g t e a - c o l o u r e d water and  c l e a r water from S t a t i o n N.  the other c o n t a i n i n g  A f t e r f l o o d i n g , the numbers i n  each chamber were recorded a t 15 minute i n t e r v a l s . backs do not s c h o o l d u r i n g the b r e e d i n g aggregation  probably d i d not b i a s the  Stickle-  season so t h a t s o c i a l  results.  P r e l i m i n a r y experiments w i t h stocks of f i s h t h a t had been kept i n the l a b o r a t o r y i n c l e a r water r e v e a l e d a s t r o n g p r e f e r e n c e by both races f o r c l e a r water. leiurus  ( S t a t i o n D) and  trachurus  and used i n l a t e r experiments  Fresh stocks of  ( S t a t i o n N) were c o l l e c t e d  (Table V I ) .  Again each race, i n  both r e p l i c a t e s , e x h i b i t s a s t r o n g p r e f e r e n c e f o r c l e a r water ( S t a t i o n N) which became e v i d e n t a t 45 minutes and p e r s i s t e d f o r the d u r a t i o n of the experiment.  73  Table V I .  Preference t e s t s w i t h stream water. Number o f f i s h i n each compartment (tea coloured, c l e a r ) a t i n d i c a t e d times f o l l o w i n g i n t r o d u c t i o n to apparatus.  15  30  Time - minutes 45 60 75 90 105  tea c o l o u r e d water Station D  11  12  10  3  2  3  c l e a r water Station N  19  18  20  27  28  27  7  4  1  13  16  19  19  17  27  13  9  7  3  17  21  tea c o l o u r e d water  16  17  c l e a r water  14  13  Leiurus #1.  #2.  30  20  120  fish  29-  28  4  3  13  16  ,17  3  4  7  2  23  27  26  23  28  12  6  3  1  0  0  18  24  27  29  30  30  fish  tea c o l o u r e d water c l e a r water  1  3  7  Trachurus #1.  30  fish  tea c o l o u r e d water c l e a r water #2.  30  fish  74  ECOLOGICAL OBSERVATIONS ON VANCOUVER ISLAND Observations  on the e f f e c t i v e n e s s of e c o l o g i c a l  were extended to four s m a l l streams on Vancouver I s l a n d IV). to  The  streams were thoroughly  the e s t u a r y w i t h a s e i n e .  isolation  (Table  sampled from the headwaters  R e s u l t s may  b e s t be t r e a t e d i n  summary. E s t u a r i e s of a l l four streams have b i o t i c and p h y s i c a l c h a r a c t e r i s t i c s v e r y much l i k e the e s t u a r y of  gross the  Little  Campbell River, i n c l u d i n g a sandy bottom,  waters  (recorded w i t h hydrometer), dense beds o f Ulva. and  same s p e c i e s of f i s h .  Breeding  mixohaline the  p o p u l a t i o n s o f trachurus were  present i n the e s t u a r i e s of a l l f o u r streams. French  Creek, B i g Qualicum, and  Sooke R i v e r are r a t h e r  s w i f t l y f l o w i n g streams w i t h rocky bottoms and  little  vegetation.  S t i c k l e b a c k s were never found where these f e a t u r e s p r e v a i l . As mentioned b e f o r e , nine trachurus were s e i n e d i n the headwaters o f B i g Qualicum, a l l b r e e d i n g males, and were found among beds of Myriophyllum  nests  (see next s u b s e c t i o n ) .  The h a b i t a t had a sandy s u b s t r a t e , a m i l d c u r r e n t , and some Myriophyllum.  A l s o mentioned e a r l i e r was  the headwaters of French of  trachurus.  Creek t h a t h e l d a l a n d l o c k e d p o p u l a t i o n  The h a b i t a t was  impounded water was  a t y p i c a l f o r t h a t race, f o r the  choked w i t h broad-leaved  Nuphar, Oenanthe. and  dark tea c o l o u r , and there was  bed  l e d i n t o French Creek, testimony A t one  v e g e t a t i o n such as  Potamogeton, the bottom was  was  the c r e e k .  the beaver pond near  no c u r r e n t .  time trachurus may  mud,  the water  A d r i e d stream  t h a t the waters once f e d have migrated  each year  75  to spawn i n the s i t e now  o c c u p i e d by the impounded waters,  were trapped there upon completion  of the dam.  o f 46 i n d i v i d u a l s three were found  t h a t had p l a t e counts  those o f h y b r i d s - 15, questionable. unfavourable its  16,  and 2 7.  From a sample like  That they were h y b r i d s i s  The s w i f t waters of French Creek are h a b i t a t f o r l e i u r u s and none was  an  found  throughout  l e n g t h , nor were any found i n the e n t i r e a r e a .  where l e i u r u s i s present i t i s an abundant f i s h , i n ponds.  but  Typically,  e s p e c i a l l y so  The r e g i o n i s remote and i t i s not l i k e l y t h a t  had r e c e n t l y i n t r o d u c e d l e i u r u s , and i f l e i u r u s had from the pond a f t e r h y b r i d i z i n g ,  i t s disappearance  man  vanished must have  been r e c e n t and complete because s t i c k l e b a c k s have a s h o r t  life  span. L e i u r u s were p l e n t i f u l o n l y i n h a b i t a t s w i t h  mud  bottom, dense a q u a t i c v e g e t a t i o n , s t a n d i n g or v e r y p l a c i d tea c o l o u r e d water.  Although  they were absent  and  from most  streams they were abundant i n ponds and marshes.  Bonsall  Creek i s a s m a l l stream about h a l f the l e n g t h of the  Little  Campbell R i v e r and the d i s t r i b u t i o n o f races there i s worthy of s p e c i a l a t t e n t i o n . was  A l a r g e b r e e d i n g c o n g r e g a t i o n o f trachurus  found 1% m i l e s from the e s t u a r y where there was  bottom, a m i l e c u r r e n t , and beds of Elodea and W i t h i n 200  Mvriophyllum.  f e e t upstream the h a b i t a t changed to mud  dense stands of Potamoqeton p u l c h e r . Qenanthe, and In t h a t h a b i t a t l e i u r u s alone was  collected.  a sandy  The  bottom w i t h Campanula.  shift in  h a b i t a t s along w i t h a complete s w i t c h i n races over so b r i e f a d i s t a n c e was  c o m p e l l i n g evidence  supporting e c o l o g i c a l  isolation.  76  More f o r c e f u l l evidence y e t came from the i n t e r v e n i n g s e c t i o n where h y b r i d s were common.  In t h i s zone the edges o f the stream  were muddy, h e a v i l y vegetated, was  most common, while  sand, Elodea captured  and without c u r r e n t , and l e i u r u s  i n the c e n t r e o f the stream there was  and Myriophyllum. and c u r r e n t .  there i n l a r g e numbers.  Trachurus were  Hybrids were common throughout  t h i s s e c t i o n composed o f i n t e r m i n g l e d h a b i t a t s b u t not one was found j u s t upstream o r downstream although l a r g e samples were taken.  From preserved  samples o f 82 f i s h i n the h y b r i d zone  21% were h y b r i d s ; and from a sample o f 224 l e i u r u s taken 400 f e e t upstream and 183 trachurus  downstream none were h y b r i d s .  i  NESTING.SITES AND MATERIALS Data f o r the l o c a t i o n o f n e s t i n g s i t e s and the m a t e r i a l s used to c o n s t r u c t nests were gathered a t S t a t i o n D f o r l e i u r u s and a t S t a t i o n N f o r t r a c h u r u s . aggressive  C l a r i t y o f the water and  behaviour o f males w i t h  nests made them s u r p r i s i n g l y  easy t o f i n d by s i t t i n g on the banks and watching f o r males t h a t chased o t h e r s t i c k l e b a c k s o r f r y o f coho salmon. such males would r e t u r n to t h e i r nests  Eventually  to f a n o r to c a r r y on  w i t h i t s f a b r i c a t i o n . When a nest was found i t was removed and p l a c e d i n a v i a l o f i s o p r o p y l a l c o h o l . substrate, current,  Depth o f water,  temperature, nearby v e g e t a t i o n ,  i n the stream were recorded.  and l o c a t i o n  Nests were l a t e r p l a c e d under a  b i n o c u l a r d i s s e c t i n g microscope and the m a t e r i a l s used i n t h e i r c o n s t r u c t i o n were The  recorded.  f o l l o w i n g r e s u l t s were o b t a i n e d  from 26 l e i u r u s  77  nests:  nests were always p l a c e d on a, mud  bottom and u s u a l l y  s i t u a t e d so t h a t no c u r r e n t c o u l d be d e t e c t e d i n the average temperature was  vicinity;  21°C w i t h a range of 16° to 23°C;  nests  were u s u a l l y c l o s e to the banks and o c c a s i o n a l l y w i t h i n 6 cm  of  the w a t e r l i n e w i t h an average depth o f 24 cm and sometimes as shallow as 4 cm;  they were most commonly i n pockets or  i n d e n t a t i o n s along the banks near logs or s t i c k s , and  although  they were i n v a r i a b l y found i n open they were always c l o s e by stands  of Qenanthe, Potamogeton, Nuphar, or Carex. R e s u l t s from a sample of 25 t r a c h u r u s nests were the  following:  always on a sand bottom and  so p l a c e d t h a t c u r r e n t s  ranging from 3 to 6 cm a second passed over the s i t e ; averaged 21°C w i t h a range from 14° to 16°C;  temperature  nests were u s u a l l y  near the c e n t r e o f the stream i n water w i t h an average depth o f 50 cm;  they were always e i t h e r w i t h i n or on the downstream s i d e  of dense patches of  Elodea.  Nests of trachurus were s l i g h t l y r a i s e d from the s u b s t r a t e and had more n o t i c e a b l e entrances  than those  l e i u r u s because o f the c o n t r a s t i n g background of c o l o u r e d sand.  category  only.  lightly  M a t e r i a l s used i n the f a b r i c a t i o n o f  were o f t e n decomposed and c o u l d be  of  nests  i d e n t i f i e d to a v e r y  general  I c o u l d f i n d no d i f f e r e n c e s whatsoever i n the  kinds o f m a t e r i a l used by each race, which c o n s i s t e d o f p l a n t fibers,  grass r o o t s  stems, and  ( u s u a l l y f r e s h ) , s m a l l b i t s of leaves,  twigs. L e i u r u s nests were always found i n the open where  c u r r e n t was  absent but trachurus  nests were s h e l t e r e d w i t h i n or  78  on the downstream s i d e of t h i c k e t s of Elodea.  It i s possible  then t h a t the d i f f e r e n c e found between l e i u r u s and i n the v e g e t a t i o n  preference  more open or more secluded  t e s t s represents  trachurus  a preference  for  nesting locations.  TROPHIC ADAPTATIONS Stomachs of a d u l t s of both races were examined f o r p o s s i b l e d i f f e r e n c e s i n f e e d i n g t h a t might e x p l a i n  the  e c o l o g i c a l e x c l u s i o n between r a c e s .  A sample of 30 i n d i v i d u a l s  of each race was  Stomachs t h a t were empty  or t h a t had  collected i n July.  food which c o u l d not be  i d e n t i f i e d were  and o t h e r s were s e l e c t e d whose contents c o u l d be  discarded,  identified.  Samples were taken on the same days a t S t a t i o n D f o r l e i u r u s and  Station N for trachurus.  Contents of the  stomach  were p l a c e d i n a s i n g l e l a y e r on a g r i d d i v i d e d i n t o 5 mm and  food items were s o r t e d i n t o common l o t s .  each item was obtained  e s t i m a t e d f o r every stomach and  f o r a l l stomachs were then summed and  number of stomachs i n v e s t i g a t e d .  squares,  The  percentage o f  the  figures  d i v i d e d by  total  T h i s method adequately  expresses the food h a b i t s o f a p o p u l a t i o n  f o r a given  time  and  place. Stomach contents were d i v i d e d i n t o three  categories:  bottom d w e l l i n g organisms, f l o a t i n g or f r e e swimming organisms, and  those t h a t may  categories,  and  fall  i n t o e i t h e r of the two  therefore, c a l l e d uncertain.  previous Results  suggest  t h a t l e i u r u s feeds to a l a r g e e x t e n t on b o t t o n d w e l l i n g (78%)  and  t h a t trachurus  organisms  feeds on f l o a t i n g o r swimming organisms  79  (40%).  However, trachurus  eats a l a r g e amount o f Chironomidae,  which must be c l a s s i f i e d as u n c e r t a i n because they may be f r e e swimming o r attached  (Figure 1 7 ) .  The v a l i d i t y o f the d i f f e r e n c e i n f e e d i n g h a b i t s may be  questioned s i n c e the two races were sampled from d i f f e r e n t  h a b i t a t s - h a b i t a t s t h a t may be expected to accommodate i n v e r t e b r a t e faunas(Table I ) . common a t S t a t i o n N d u r i n g  different  Palaemonetes, f o r example, i s  June and J u l y b u t i s unknown from  the upper reaches w h i l e Musculium, zygoptera, and c o l e o p t e r a l a r v a e are common a t both S t a t i o n s y e t they a r e important food items f o r l e i u r u s o n l y .  Even when food items are commonly  found i n the two h a b i t a t s one cannot be c e r t a i n t h a t they a r e equally a v a i l a b l e i n both.  However, the d i f f e r e n c e s found  imply a d i f f e r e n c e i n f e e d i n g behaviour, one t a k i n g l a r g e l y from the bottom, and the other water.  food  food suspended i n the  2  Uj  ft: 2  Lu i—  0.1*1  CO ^. N  % 0  5  UJ  Q  UJ  a. UJ  i 1 1 C:  Uj  o  a!  i  Uj  UJ  §  o  Q O CL  tr.  S  Uj  o Uj  o  8  1  Q  5  i to o  Uj  ci Q  O  10 20 .  TRACHURUS  30 LEIURUS 20 10 0  i BOTTOM  DWELLERS  UNCERTAIN  SUSPENDED oo  Figure  17.  F o o d items t a k e n explanation.  f r o m stomachs o f  leiurus  and  trachurus.  See  text  for  o  81  POSTMATING ISOLATING MECHANISMS Postmating isolating mechanisms include a host of factors that come into play after gametes have been shed and, therefore, do not prevent wastage of gametes as premating mechanisms do.  The isolation springs from either genetic or  chromosomal incompatibility between populations and may produce any  of the following:  gamete mortality, zygote mortality,  hybrid inferiority, hybrid s t e r i l i t y , or hybrid breakdown. Any of these may serve to either block gene flow between populations or produce hybrid offspring that are at a selective disadvantage as compared to either parental type.  MATERIALS AND METHODS Parents for crosses were taken from allopatric populations or from the hybrid zone, and eggs were usually f e r t i l i z e d on the same day that fish were collected. A r t i f i c a l f e r t i l i z a t i o n was produced by stripping eggs into a small moist petri rfish. A male's testes were immediately removed and placed in a watch glass with a few drops of tap water, chopped into fine pieces with forceps and probe, and poured over the eggs as the container was gently rotate d.  Eggs were l e f t to harden  for 20 minutes and were then observed through a binocular dissecting microscope to assure that the perivitelline membrane had separated from the egg, as an indication of successful fertilization.  Mucous around the egg mass was removed by placing  the t i p of a dropper into the centre of the mass and pressing  82  i t a g a i n s t the bottom o f the d i s h .  S u c t i o n from the r e l e a s e of  the b u l b s u c c e s s f u l l y removed the mucous and adhering b i t s of t i s s u e and sperm. to a t t a c k by  T r e a t e d i n t h i s way  eggs were much l e s s . p r o n e  fungus.  F e r t i l i z e d eggs were t r a n s f e r r e d to cup-shaped r a f t s of c e l l u l o i d mesh t h a t f l o a t e d on the s u r f a c e i n j a r s h o l d i n g 500 c c of d e c h l o r i n a t e d tap water.  Each j a r was  v i g o r o u s l y a e r a t e d c a u s i n g the r a f t of eggs to s l o w l y r o t a t e j u s t beneath the s u r f a c e . temperature  Eggs were i n c u b a t e d a t a c o n s t a n t  of e i t h e r 14° or 18°C  o f 16 hours.  and under a d a i l y  photoperiod  I f fungus began to develop on eggs they were  t r e a t e d w i t h a 1:200,000 d i l u t i o n o f M a l a c h i t e Green s o l u t i o n f o r 6 hours.  Once the eggs hatched  the contents of the j a r  were t r a n s f e r r e d to an enamel t r a y so t h a t the numbers of f r y and unhatched eggs c o u l d be  counted.  When h y b r i d c r o s s e s were made the egg mass of each female was  s p l i t i n t o approximately equal h a l v e s .  the eggs was  One h a l f of  f e r t i l i z e d w i t h homoganetic sperm to serve as a  c o n t r o l , and the o t h e r h a l f was  f e r t i l i z e d with  heterogametic  sperm and each h y b r i d c r o s s was  compared w i t h i t s c o n t r o l .  Thus, o t h e r causes o f egg m o r t a l i t y were not a t t r i b u t e d to gametic  incompatibility. H y b r i d f r y and c o n t r o l s were kept under uniform  c o n d i t i o n s , but because of l i m i t e d f a c i l i t i e s combinations  of c r o s s e s extended  the  over two summers.  different The c l a s s e s  of o f f s p r i n g were h e l d i n separate gauze baskets 22 cm x 22 and 16 cm deep w i t h about 200  cm,  f i s h i n each basket; a l l baskets  83  were suspended i n a 200 g a l l o n cement tank.  Dechlorinated  tap water c i r c u l a t e d through the tank c o n s t a n t l y , h o l d i n g the temperature  between 17° and 19°C.  Newly hatched f r y were f e d  i n f u s o r i a as soon as the y o l k sac was absorbed and a t f o u r days they began t o f e e d on n a u p l i i o f l i v e b r i n e  shrimp.  A f t e r e i g h t weeks they were f e d f r o z e n b r i n e shrimp u n t i l the experiments  were abandoned a t 3% months.  RESULTS  S u r v i v a l to Hatching Comparing h a t c h i n g success o f r e c i p r o c a l a t both temperatures  w i t h t h e i r c o n t r o l s shows t h a t the s u r v i v a l  of h y b r i d s t o h a t c h i n g approximates There  hybrids  the s u r v i v a l o f c o n t r o l s .  i s no i n d i c a t i o n o f g e n e t i c i n c o m p a t i b i l i t y  (Table V I I ) .  Backcrosses were made w i t h h y b r i d males c o l l e c t e d from the h y b r i d zone.  During mid-May b a c k c r o s s e s to l e i u r u s  were a c h i e v e d and a t the same time adequate h y b r i d males were taken and h e l d i n the l a b o r a t o r y t o provide f o r backcrosses to t r a c h u r u s i n e a r l y June.  When these backcrosses are compared  w i t h t h e i r c o n t r o l s i t may be seen t h a t h a t c h i n g success o f backcrosses i s l i k e t h a t f o r c o n t r o l s , and there i s no s u g g e s t i o n of g e n e t i c i n c o m p a t i b i l i t y o r h y b r i d breakdown (Table V I I ) . A r e s t r i c t e d number o f b a c k c r o s s e s to l e i u r u s w i t h h y b r i d females was a l s o made as w e l l as a l i m i t e d number o f F2 crosses  (seasonal i s o l a t i o n  between sexes o f h y b r i d s made i t  d i f f i c u l t to c o l l e c t both sexes a t once).  C o n t r o l s were not  Table V I I .  Hatching success o f h y b r i d c r o s s e s . Numbers i n p a r e n t h e s i s r e p r e s e n t p l a t e counts o f p a r e n t s . C o n t r o l s presented i n l e f t hand column and c o r r e s p o n d i n g h y b r i d c r o s s e s i n r i g h t hand column. L e i u r u s °- x Trachurus err  L e i u r u s 2 x Leiurus <f  %(5) x (7(31) . T o t a l no. eggs = 33  £(5) x #(4). T o t a l no. eggs = 38 Hatch = 3 1 M o r t a l i t y = 18% Die = 7  1.  °-(3) 5(5). T o t a l no. eggs = 37 Hatch = 35 M o r t a l i t y = 5% Die = 2  2.  $(5) x 5(5). T o t a l no. eggs = 45 Hatch = 38 M o r t a l i t y = 15% Die = 7  3.  ?(4) x 3(5). T o t a l no. eggs = 55 Hatch = 51 M o r t a l i t y = 7% Die = 4  4.  5.  ?(6) x 5(5). T o t a l no. eggs = 63 Hatch = 59 M o r t a l i t y = 6% Die = 4  5.  ?(6) x 3(32). Hatch = 4 6 Die = 2  T o t a l no. eggs = 49 M o r t a l i t y = 4%  6.  $(5) x 5(6). T o t a l no. eggs = 49 Hatch = 47 M o r t a l i t y = 4% Die = 2  6.  $(5) x 5(34). Hatch = 5 8 Die = 3  T o t a l no. eggs = 61  £(4) x 5(5). T o t a l no. eggs = 38 Hatch = 3 5 M o r t a l i t y = 8% Die = 3  7.  1.  2.  3.  4.  7.  x  TOTAL MORTALITY = 8.9%  Hatch = 2 9 Die = 4  •°-(3) x 5(33). Hatch * 42 Die = 0 £(5) x 5(34). Hatch = 62 Die = 6  ?(4) x  3(33).  Hatch = 45 Die = 2  M o r t a l i t y = 12% T o t a l no. eggs = 42 M o r t a l i t y = 0% T o t a l no. eggs = 68 M o r t a l i t y = 9% T o t a l no. eggs = 47 M o r t a l i t y = 4%  M o r t a l i t y = 5%  £(4) x 0^(32) . T o t a l no. eggs = 49 Hatch = 46 M o r t a l i t y = 4% Die = 2 TOTAL MORTALITY = 5.8% 01  Leiurus 1.  x Leiurus  L e i u r u s °1 x Trachurus o*^  $(4) x d*(4). T o t a l no. eggs = 58 Hatch = 57 „ . , / Die = 1 M o r t a l i t y = 2%  1.  £(4) x 0^(35). Hatch = 33 Die = 1  T o t a l no. eggs = 34 „ . ,. . M o r t a l i t y = 3%  £ ( 4 ) x <?(5). T o t a l no. eggs = 52 Hatch = 50 „ . . . . , Die = 2 M o r t a l i t y = 4%  2.  £(4) x 6 ^ 3 3 ) . Hatch = 39 Die = 3  T o t a l no. eggs = 42 „ . . . . M o r t a l i t y = 7%  ? ( 5 ) x 3{5). T o t a l no. eggs = 38 Hatch = 31 -, £ _ M o r t a l i t y = 18%  3.  $(5) x <?(32). Hatch = 33 Die = 4  T o t a l no. eggs = 37 ,. , M o r t a l i t y = 11%  $ ( 5 ) x <?(4). T o t a l no. eggs = 42 Hatch = 4 1 Die = 1 M o r t a l i t y = 2%  4.  %(5) x 3(34). Hatch = 3 5  T o t a l no. eggs = 39  °.(6) x cT(4). T o t a l no. eggs = 47 Hatch = 43 .„ . , ., Die = 4 M o r t a l i t y = 8%  5.  #(4) x 3»(5). T o t a l no. eggs = 31 Hatch = 28 . ,. . , , ^ _ M o r t a l i t y = 10%  6.  $ ( 5 ) x <?(4). T o t a l no. eggs = 56 Hatch = 54 *i • A. Die = 2 M o r t a l i t y = 3%  7.  0o  2.  A o  3.  00/  D  4.  5.  6.  e  7  D  7.  e  3  TOTAL MORTALITY = 6.1%  e  =  3  M o r t a l i t y = 8%  £ ( 6 ) <?(33). T o t a l no. eggs = 32 Hatch = 29 . .. . Die = 3 M o r t a l i t y = 9% x  M  rto  D  i  1 0 /  n o /  $(4) x <?(33). Hatch = 46 Die = 10  T o t a l no. eggs = 56 ,„ . , . . , . M o r t a l i t y = 18%  °-(5) x <?(35). Hatch = 5 3 Die = 1  T o t a l no. eggs = 54  a a  M  M o r t a l i t y = 2%  TOTAL MORTALITY = 8 . 5 %  oo  Trachurus £ x Trachurus <f 1.  2.  $(33) x <?(32). T o t a l no. eggs = 88 Hatch = 8 6 Die • 2 M o r t a l i t y = 2%  1.  $(33) x 3(33). T o t a l no. eggs = 52 Hatch = 47 , . _ M o r t a l i t y = 10%  2.  ?(35) x 3(32). Hatch = 6 3 Die = 1 4  3.  D  3.  4.  5.  6.  i  e  5  T o t a l no. eggs = 82 M  o  r  t  a  l  i  t  y  J  =  1  7  %  £(32) x 3(33). Hatch = 84 Die = 12  T o t a l no. eggs = 96 . .. . , M o r t a l i t y = 13%  4.  ?(34) x 3(35). Hatch = 8 8 Die = 3  T o t a l no. eggs = 91  5.  M o r t a l i t y = 3%  £(33) x 3(31).  T o t a l no. eggs = 77  Die = 2 I C  7.  Trachurus Q x L e i u r u s c r * *  5 6  £(32) x 3(33). Hatch = 79 Die = 7  0 o /  M o r t a l i t y = 8%  TOTAL MORTALITY = 11.1%  7.  M o r t a l i t y = 1%  M o r t a l i t y = 6%  £(35) x 6*(6) . Hatch = 6 8 Die = 17  T o t a l no. eggs = 85 • . , M o r t a l i t y = 20%  T o t a l no. eggs = 63  0ft0  £(32) x 3 ( 5 ) . T o t a l no. eggs = 83 Hatch = 75 . i• . i na/ £ _ Q M o r t a l i t y = 10% E  ?(34) x 3 ( 4 ) . Hatch = 98 _ 4 D  6.  T o t a l no. eggs = 75  $(33) x 5 ( 5 ) . Hatch = 5 9 Die = 4  D  M o r t a l i t y = 27% T o t a l no. eggs = 86  £(33) x <?(5). Hatch = 74 Die = 1  i  e  ?(33) x 3 ( 5 ) . Hatch = 45 Die = ZJ £(32) x 3(6). Hatch = 88 . , Die = 6 n  T o t a l no. eggs = 102 M o r t a l i t y = 4% T o t a l no. eggs = 68 M  o  r  t  a  l  i  t  =  3 4  o^  T o t a l no. eggs = 94 „ . ,. . , M o r t a l i t y = 6% 0 /  TOTAL MORTALITY = 1 1 %  00  O  Trachurus ¥ x L e i u r u s <f  Trachurus °_ x Trachurus cr* 1.  2.  3.  4.  5.  6.  7.  ¥(31) x 5(35). Hatch = 59 Die = 6 ¥(34) x  S(33).  Hatch = 45 Die = 1  $(32) x 3(35). Hatch • 90 Die = 2 ¥(33) x 5(33). Hatch = 65 Die = 8 ?(32) x 3(33). Hatch = 80 Die = 4  T o t a l no. eggs = 65  1.  M o r t a l i t y = 9% T o t a l no. eggs = 46  2.  M o r t a l i t y = 2% T o t a l no. eggs = 92 Mortality =  3.  4.  ¥(32) x 3 ( 5 ) . T o t a l no. eggs = 98  «°rtalit - 8X  9 0  y  $(33) x 3 ( 4 ) . T o t a l no. eggs = 61 Hatch =55 c Die = 6 M  o  r  t  a  l  i  t  m1 0  / c  J  5.  ¥(32) x 3 ( 5 ) . T o t a l no. eggs = 76 Hatch = 74 Die = 2 M o r t a l i t y = 3% ¥(32) x 3 ( 5 ) .  M o r t a l i t y = 5%  ?(32) x 3(31). Hatch = 72 Die = 12  T o t a l no. eggs = 87 M o r t a l i t y = 14%  6.  2(33) x 3(32). Hatch = 75 Die = 3  T o t a l no. eggs = 78  7.  M o r t a l i t y = 4%  M o r t a l i t y = 4%  ¥(34) x 3 ( 6 ) . T o t a l no. eggs = 67 Hatch = 59 .„ . , . . . Die = 8 M o r t a l i t y = 12%  Se* 8  M o r t a l i t y = 11% T o t a l no. eggs = 84  Hatch = 78 Die = 3  O Q /  2%  T o t a l no. eggs = 73  2(31) x 5 ( 5 ) . T o t a l no. eggs = 81  H  *  t  c  h  o „  Die = 2 4  9  1  T o t a l no. eggs = 112 Mortality =21% *  ¥(33) x 3 ( 6 ) . T o t a l no. eggs = 52 Hatch = 5 2 Die = 0  M o r t a l i t y = 0%  TOTAL MORTALITY = 6.8% TOTAL MORTALITY = 9 . 3 %  oo  Leiurus 1.  2.  3.  4.  5.  1.  ? ( 5 ) x <?(5). Hatch = 3 1 Die = 0  Total  n o . eggs = 31 , i• , M o r t a l i t y = 0%  2.  £ ( 4 ) x <?(6). T o t a l n o . eggs = 27 Hatch = 2 5 Die = 2 M o r t a l i t y = 7%  3.  ? ( 4 ) x 3(5). T o t a l n o . eggs = 33 Hatch = 3 2 Die = 1 M o r t a l i t y = 3%  4.  £ 5 ) x 3(5). T o t a l n o . eggs = 36 H a t c h = 34 ^ _ 2 M o r t a l i t y = 5%  5.  ? ( 4 ) x 3(5). T o t a l n o . eggs = 19 H a t c h = 19 ^ _ o M o r t a l i t y = 0%  6.  S?(4) x 3(4) . T o t a l n o . eggs • 44 Hatch = 3 9 _ M o r t a l i t y = 11%  7.  $ ( 4 ) x 3(4). T o t a l n o . eggs = 78 H a t c h = 72 M o r t a l i t y = 8% Die = 6  8.  nn o /  v  D  8.  7  T o t a l n o . eggs = 22 . , .. M o r t a l i t y = 18%  e  D  7.  Le i u r u s  cf  $ ( 6 ) x 3(5). Hatch = 1 8 Die = 4  D  6.  °: x L e i u r u s  e  i  e  5  TOTAL MORTALITY = 6.8%  °. x H y b r i d o"^  $ ( 6 ) x <?(20). H a t c h = 56 Die = 2  T o t a l n o . eggs = 58 .„ , , . , M o r t a l i t y = 3%  ^ ( 5 ) x 3(20). H a t c h = 39 Die = 3  T o t a l n o . e g g s = 42 , , .. _ M o r t a l i t y = -7%  £ ( 4 ) x o*(16). H a t c h = 66 Die = 1  Total  °-(4) x d ' ( 1 2 ) . Hatch = 48 Die = 0 £ ( 5 ) x 3(19). Hatch = 3 3 Die = 1 $ ( 4 ) x e?(10). H a t c h = 49 Die = 3 $.(4) x 0^(12). H a t c h = 43 Die = 4 ? ( 1 8 ) x 3(4). H a t c h = 62 Die = 2  O D /  0 /  n o . e g g s = 67  M o r t a l i t y = 1% Total  n o . e g g s = 48  M o r t a l i t y = 0%. Total  n o . e g g s = 34  M o r t a l i t y = 3% Total  n o . e g g s = 52  M o r t a l i t y = 6% Total  n o . e g g s = 47  M o r t a l i t y = 8% Total M  o  r  t  a  n o . e g g s = 64 l  i  t  y  =  3  %  TOTAL MORTALITY = 3.8% oo  Trachurus ° r -  Trachurus J°_ x Trachurus o*"^ 1.  2.  3.  ¥(32) x c?(33). Hatch = 104 Die = 3  6.  7.  8.  T o t a l no. eggs = 9 2  2  M o r t a l i t y = 4%  ¥(33) x 5(32).  T o t a l no. eggs = 197  t  Die  5.  1  ¥(32) x 5(34). Hatch = 88 Die = 4  ?  4.  u  T o t a l no. eggs = 107 ^ ,.^ M o r t a l i t y = 3%  G  h  T o  =18  1  7  A o /  3  M o r t a l i t y = 9%  9  ¥(34) x 5(33). Hatch = 211 Die = 7  T o t a l no. eggs = 218  T o t a l no. eggs = 112  Hatch 109 Die = 3  3(32) x $ ( 1 6 ) . Hatch = 72 Die = 0  M o r t a l i t y = 3% T o t a l no. eggs = 72 M o r t a l i t y = 0%  T o t a l no. eggs = 153  5(33) x ¥ ( 2 0 ) . Hatch = 1 3 7 Die m 7  T o t a l no. eggs = 144  6  $(35) x 3 ( 3 3 ) . Hatch = 230 Die = 11  T o t a l no. eggs = 241 . ,. . M o r t a l i t y = 4%  7  T o t a l no. eggs = 212 . ,. . , M o r t a l i t y = 3%  8  A o /  TOTAL MORTALITY = 4.8%  -P(32) x 5(11).  5(32) x ¥ ( 1 4 ) . Hatch = 1 4 9 Die = 4  T o t a l no. eggs = 184 , , M o r t a l i t y = 1%  Oo  M o r t a l i t y = 7%  T o t a l no. eggs = 115  5(33) x $-(16) . T o t a l no. eggs = 137 Hatch = 135 M o r t a l i t y = 1% Die = 2  ¥(34) x 5(33). Hatch = 182 Die = 2  M  ¥(32) x 5(21). Hatch = 1 0 7 Die = 8  5  M o r t a l i t y = 9%  ¥(32) x 3(34). Hatch = 206 Die = 6  s9  3(33) x £(15) . T o t a l no. eggs = 56 Hatch = 52 M o r t a l i t y = 7% Die = 4  ¥(33) x 3 ( 3 3 ) . Hatch = 202 Die = 21  M  Hybrid 0  4  M o r t a l i t y = 3% T o t a l no. eggs = 223  x  3(34) x ¥ ( 1 8 ) . Hatch = 86 Die = 1  M o r t a l i t y = 3%  M o r t a l i t y = 5% T o t a l no. eggs = 87 M o r t a l i t y = 1%  TOTAL MORTALITY = 3.3%  CD  H y b r i d ¥ x L e i u r u s <f 1.  2.  $ ( 2 0 ) x 3(4)'., Hatch = 4 4 Die = 0  T o t a l n o . eggs = 44 „ . , • . M o r t a l i t y = 0%  ?(I9) x 5(5).  Total  H a t c h = 53 D i e = 3; 3.  H y b r i d ¥ x H y b r i d cf  ¥ ( 1 5 ) x 3(5). H a t c h = 36 Die = 1  n  M o r  o  /  n o . eggs = 56  tality  J  = 5%  T o t a l n o . eggs = 37 „„ . , . . M o r t a l i t y = 3%  TOTAL, MORTALITY = 2 . 9 %  1.  ¥ ( 2 0 ) x 6^(15). H a t c h = 62 Die = 5 £ ( 1 9 ) x 3(16) . H a t c h = 67 Die = 7 2(15) x c?(21) . H a t c h = 81 Die = 2  Total  7  n o . e g g s = 67  Mortality Total  n o . e g g s = 74  Mortality Total  = 7%  = 9%  n o . e g g s = 83  Mortality  = 2%  TOTAL MORTALITY = 6.2%  co  85  p r o v i d e d b u t h a t c h i n g success w i t h i n these groups compares favourably with VII).  Again  s u r v i v a l o f other homogametic c r o s s e s  (Table  the r e s u l t s do not i n d i c a t e g e n e t i c i n c o m p a t i b i l i t y  or h y b r i d breakdown.  Survival of Fry M o r t a l i t y i n F^ h y b r i d s was r e l a t i v e l y h i g h d u r i n g the first  two weeks a f t e r h a t c h i n g , as i t was i n a l l c l a s s e s o f  laboratory reared f i s h  (Table V I I I ) .  d i e d , b u t a t one month a protozoan  A f t e r two weeks f i s h  rarely  parasite, Ichthyophthirius,  was i n t r o d u c e d i n t o the nursery from w i l d caught f i s h .  Fry  were t r e a t e d w i t h standard s a l t water remedy (Davis, 1961) which checked spread o f the d i s e a s e and cured many o f the i n f e c t e d individuals.  M o r t a l i t y a t the end o f three months was comparable  between F^ h y b r i d c l a s s e s and c o n t r o l s . There was complete m o r t a l i t y i n the one  backcross  c l a s s , which o c c u r r e d a t 33 days and p e r s i s t e d f o r a f o u r day p e r i o d .  I t seemed improbable t h a t d i s e a s e o r l a b o r a t o r y  c o n d i t i o n s were r e s p o n s i b l e because a l l f i s h were h e l d i n a common supply o f f r e e l y c i r c u l a t i n g water, and there was no comparable m o r t a l i t y i n other groups o f f r y . h y b r i d breakdown was suspected  A t the time,  and both backcross  c l a s s e s were  made the f o l l o w i n g year. C o n d i t i o n s f o r r e a r i n g f r y were the same i n 1965 except  t h a t a t two months a l l f r y were t r a n s f e r r e d from  baskets  t o 40 g a l l o n a q u a r i a p r o v i d e d w i t h f i l t e r s and  aeration.  In a n t i c i p a t i o n o f the developmental m o r t a l i t y  Table  VIII.  Survival of laboratory reared hybrids with columns.  controls  presented i n l e f t  hand  SURVIVAL TO 3 MONTHS Conspecific Leiurus  x  Leiurus  T o t a l h a t c h l i n g s = 600 M o r t a l i t y a t 2 weeks = 413(31%) M o r t a l i t y a t 1 month = 397(34%) F i s h r e m a i n i n g a f t e r 3 months = 184 - i . e . 69% m o r t a l i t y Trachurus  x  Trachurus  T o t a l h a t c h l i n g s = 994 M o r t a l i t y a t 2 weeks = 674(32%) M o r t a l i t y a t 1 month = 624(36% F i s h r e m a i n i n g a f t e r 3 months = 2  Heterospecific Leiurus £ x Trachurus  cf  T o t a l h a t c h l i n g s = 596 M o r t a l i t y a t 2 weeks = 438(27%) M o r t a l i t y a t 1 month = 415(30%) F i s h r e m a i n i n g a f t e r 3 months = 75(85%) Trachurus £  x L e i u r u s cf*  T o t a l h a t c h l i n g s - 1006 M o r t a l i t y a t 2 weeks = 632(37%) M o r t a l i t y a t 1 month = 591(41%) F i s h r e m a i n i n g a f t e r 3 months = 109(89%) Hybrid %  x  Leiurus  (f  T o t a l h a t c h l i n g s = 122 M o r t a l i t y a t 2 weeks = 92 M o r t a l i t y a t 1 month = 100% No s u r v i v o r s  OD  0)  SURVIVAL TO 4 MONTHS Conspecific Leiurus  Heterospecific  x L e i u r u s cf  Leiurus  Total hatchlings =218 M o r t a l i t y a t 2 weeks = 24%(165*) M o r t a l i t y a t 1 month = 27%(157*) M o r t a l i t y a t 4 months = 39%(132*)  5- x H y b r i d cf  7  T o t a l h a t c h l i n g s = 243 M o r t a l i t y a t 2 weeks = 18%(199*) M o r t a l i t y a t 1 month = 25%(182*) M o r t a l i t y a t 4 months = 33%(162*) Leiurus  cf x H y b r i d °-  T o t a l h a t c h l i n g s = 61 M o r t a l i t y a t 2 weeks = 23%(47*) M o r t a l i t y a t 1 month = 29%(42*) M o r t a l i t y a t 4 months = 36%(39*) T r a c h u r u s cf x H y b r i d •?• T o t a l h a t c h l i n g s - 136 M o r t a l i t y a t 2 weeks = 16%(114*) M o r t a l i t y a t 1 month = 21%(107*) M o r t a l i t y a t 4 months = 32%(93*) T r a c h u r u s 2- x H y b r i d cf T o t a l h a t c h l i n g s = 74 M o r t a l i t y a t 2 weeks = 22%(58*) M o r t a l i t y a t 1 month = 30%(52*) M o r t a l i t y a t 4 months = 34%(49*)  *  =  survivors  87  a t 33 days, i n 1965 r e p l i c a t e s o f backcrosses were made a t two week i n t e r v a l s so t h a t i f death were a r e s u l t o f h y b r i d breakdown there s h o u l d be a two week i n t e r v a l among r e p l i c a t e s when death occurred. Indeed,  The expected m o r t a l i t y d i d not eventuate  (Table V I I I ) .  a t the end o f f o u r months s u r v i v a l o f backcrosses was,  i f anything, s l i g h t l y b e t t e r than s u r v i v a l i n the r a c i a l c r o s s e s . No e x p l a n a t i o n can be o f f e r e d f o r the d i s c r e p a n c y i n r e s u l t s , but i t i s important to r e c a l l t h a t o n l y one backcross combination was made i n 1964 w h i l e a l l f o u r were made i n 1965 producing l a r g e r and more complete  samples the second y e a r .  D e s p i t e good s u r v i v a l and seemingly normal and v i g o r o u s h y b r i d o f f s p r i n g , f i v e F-^' s and three backcrosses were abnormal.  The d e f o r m i t i e s i n a l l the abnormal h y b r i d s were  a l i k e and c o n s i s t e d o f the f o l l o w i n g : a.  no swim b l a d d e r  b.  i n t e s t i n e without the normal p o s t e r i o r loop, and c o n s i s t i n g o f a s t r a i g h t tube from mouth to anus  c.  abnormally l a r g e eyes  d.  dwarfed s i z e , about h a l f t h a t o f normal h y b r i d s  e.  v e r y t h i n and bow-shaped b o d i e s .  These i n d i v i d u a l s swam by f l u t t e r i n g the p e c t o r a l f i n s , and t h i s along w i t h an upset i n t h e i r d e n s i t y r e s u l t i n g from the absence o f a swim b l a d d e r produced v e r y e r r a t i c and j i t t e r y motion when swimming.  They spent most o f the time s h e l t e r e d i n algae on  the s u r f a c e and i f they were f o r c e d out they sank to the bottom and c o u l d r e t u r n to the s u r f a c e o n l y w i t h much e f f o r t .  Hubbs  88  (192 7) since  reported similar some e g g s h a d  the  t o an  At  but  suggesting  Gravid  f e m a l e s were n o t  to induce  captured  i n t h e w i l d and  a t an  gravidity  extremely  of  t o 19°C  kept  o f mature eggs i n h y b r i d s races.  Once i t h a s  are  present  as  found  but  i n the  the The  surprising  s i n c e egg  l a r g e l y a f u n c t i o n of size, between the  races  in  size.  less  females  than  two  counts  information  g i v e no  information  produce.  races  are  producing  trachurus.  This  i n each race  since hybrids  two  barriers  show t h a t f e m a l e h y b r i d s  production and  i t  c o u n t s from the  number o f c l u t c h e s h y b r i d s may  l e i u r u s but  a  Baggerman  d e r i v e d from  provide  they  i n f e c u n d i t y between the  more e g g s t h a n not  compared w i t h  IX)  maturity  Fecundity  although  (Table  had  laboratory.  such c o u n t s o f eggs can  results  If  level.  and  n e i t h e r was  been e s t a b l i s h e d t h a t s t e r i l i t y  about f e c u n d i t y of h y b r i d s ,  is  of  16 h o u r s and  ( b u t see  A measure o f h y b r i d f e c u n d i t y was  intermediate  stage  low  sexual  i n p a r t i a l l y mature  Hybrid  concerning  and  abnormalities  become a g g r e s s i v e  photoperiod  t e m p e r a t u r e o f 17°  possible  not  the  t h e y were a p p r o a c h i n g  ensued a t a constant  r e l a t i v e l y high 1957).  former i t e x i s t s  f o u r months m a l e s h a d  iris,  this  p o s s i b l e to a t t r i b u t e  late  fish,  i n c i p i e n t h y b r i d breakdown o r t o p a r a s i t i s m .  c a u s e were t h e  a blue  in parasitized  been exposed to fungus a t a  development i t i s not either  abnormalities  are  result  is  intermediate  Table  IX.  Numbers o f m a t u r e eggs f o r h y b r i d s a n d f o r e a c h r a c e . each race and f o r h y b r i d s . Leiurus  Length 4.8 4.9 5.2 5.3 4 3.6 4.4 4 4.2 4 4.4 4.3 4.1 4.3 3.3 4.6 5.2 5.1 4.5 4.7  No. eggs  cm 112 115 148 120 72 75 101 93 76 88 123 60 56 69 71 84 116 124 92 97  Hybrids  Length  No. eggs  4.7 3.9 5 5.2 5.2 5 4.4 4.2 5.4 5.8 7.5 3.4 3.4 4.5 4.2 4.1 3.8 4.5 4 4.3  96 83 123 205 170 172 124 71 229 234 229 51 49 76 79 57 52 70 67 76  X = 105  cm  Length  No. eggs  5 . 4 cm 4.7 4.7 5.1 6  143 106 142 207 236  6.3 5.9 4.8 5.1 5.3 4.6 4 5.3 4.9 4.8 6 5.7 5.4 5.2  296 226 112 132 126 93 83 123 126 123 224 201 212 169  Sample s i z e  o f 40  for  Trachurus  Length  No. eggs  Length  No. eggs  Length  No. eggs  5.8 6 5.9 5.8 5.9 5.5 6 5.7 5.4 5.6 6.1 5.6 6 5.9 5.4 4.8 4.7 5.2 5.9 6.1  217 229 284 236 249 186 204 217 227 231 239 217 227 237 152 118 150 216 224 283  5.4 5.2 5.8 6 5.9 5.9 5.8 5.8 6 5.7 5.9 5.7 6.3 6.4 5.7 6 6.1 5.9 5.7 5.9  210 157 227 239 286 247 253 171 205 228 247 243 259 271 213 228 257 283 219 266  5.6 5.8 6.1 6.2 5.7 6 5.8 5.7 5.9 5.9 6 5.8 5.2 5.4 5.9 5.8 6.2 6.5 5.6 6  225 257 231 284 226 223 191 267 256 284 257 239 162 221 253 248 271 292 209 232  X = 190  cm  cm  X = 241  cm  90  Hybrid  Inferiority  I t i s e x c e e d i n g l y d i f f i c u l t to g a i n i n f o r m a t i o n p e r t a i n i n g to the s e l e c t i v e disadvantage  of h y b r i d s r e l a t i v e  to  p a r e n t a l types, f i r s t because the s e l e c t i v e agents o p e r a t i n g a g a i n s t h y b r i d s may  be v e r y low i n i n t e n s i t y or v e r y s u b t l e  i n a c t i o n , and second because any negative evidence a t t a i n e d i n the l a b o r a t o r y of the absence o f s e l e c t i o n a g a i n s t h y b r i d s i s meaningless s i n c e the s e l e c t i v e agents themselves may  have  been removed.  mile  In e a r l y September of 1964  h y b r i d zone was estimated. of 1965  sampled and percentages  The procedure  was  the e n t i r e one  of h y b r i d s and  races  repeated a g a i n i n e a r l y June  thus g i v i n g estimates of r e l a t i v e numbers of h y b r i d s  i n autumn a f t e r r e p r o d u c t i o n ceased,  and again i n l a t e s p r i n g  w h i l e young o f the year c o u l d be i d e n t i f i e d and excluded counts.  I f h y b r i d s were a t a s e l e c t i v e disadvantage  to e i t h e r race, one  from  relative  s h o u l d be a b l e to d e t e c t a decrease  in  p r o p o r t i o n of h y b r i d s over t h i s nine month l a p s e . For September census f i g u r e s were: 82 h y b r i d s , and 14 t r a c h u r u s . 246  leiurus,  217  leiurus.  Estimates f o r June were:  73 h y b r i d s , and 12 t r a c h u r u s .  T h i s measure,  rough though i t i s , g i v e s no i n d i c a t i o n t h a t h y b r i d s any m a n i f e s t disadvantage  suffer  as compared to p a r e n t a l types,  t h i s together w i t h an abundance of backcrosses  and  i n the h y b r i d  zone i n d i c a t e s t h a t e i t h e r h y b r i d s are not a t a s e l e c t i v e disadvantage  or t h a t they are a t a v e r y low s e l e c t i v e  i n the h y b r i d zone.  disadvantage  91  DISCUSSION  ISOLATING MECHANISMS AND  HYBRIDIZATION  I s o l a t i n g Mechanisms A m o r p h o l o g i c a l a n a l y s i s and o b s e r v a t i o n s i n the p r o v i d e f i r m c i r c u m s t a n t i a l evidence  t h a t l e i u r u s and  trachurus  f r e e l y i n t e r b r e e d i f they come together, producing  vigorous  and f e r t i l e h y b r i d s .  Campbell  I n v e s t i g a t i o n s i n the L i t t l e  R i v e r and B o n s a l l Creek r e v e a l t h a t the two  races are  segregated,  but where they c o h a b i t presumed h y b r i d s  backcrosses  are p l e n t i f u l .  And  field  largely  and  a g a i n when l e i u r u s i s t r a n s -  p l a n t e d i n t o a b r e e d i n g p o p u l a t i o n of trachurus numerous h y b r i d s arise.  T h i s evidence would seem to b e l i t t l e  the importance of  b e h a v i o u r a l or g e n e t i c o b s t a c l e s to h y b r i d i z a t i o n . The v i r t u a l absence of e t h o l o g i c a l and  postmating  i s o l a t i n g mechanisms ( e x c l u d i n g h y b r i d i n f e r i o r i t y ) i s confirmed by experimental  a n a l y s i s i n the l a b o r a t o r y .  Behavioural patterns  t h a t f u n c t i o n as i s o l a t i n g mechanisms c o u l d not be  detected,  and mating between races i s random i n p r e f e r e n c e t e s t s .  Hybrids  were found to s u c c e s s f u l l y complete c o u r t s h i p and p a r e n t a l activities.  S t e r i l i t y b a r r i e r s are a p p a r e n t l y not present  i s genetic incompatibility,  nor  f o r i f the few abnormal h y b r i d s  t h a t were found i n the l a b o r a t o r y are a r e s u l t of  incompatibility  (and not p a r a s i t i s m ) , i t f u r n i s h e s a v e r y i n e f f e c t i v e b l o c k to gene f l o w .  Most o f the h y b r i d s are v i g o r o u s and no decrease  h a t c h i n g success or s u r v i v a l of o f f s p r i n g c o u l d be  found.  in  92  Field demonstrate ecological Seasonal  that reproductive isolation  isolation  same t i m e However,  for  trachurus  is  a lesser  at  the  is  since  the  hybridization  i n the  Little  g r e a t e s t numbers  farther  than  settle  upstream  these  of  leiurus  sites.  This  occur.  of  the  two  situation  habitats  hybridization.  and,  of  and c o n v e r s e l y ,  i n the  when  only  a major  contribution  They a r r i v e are  in  breeding,  sparse but  from e a r l y  to  the move  and  permanent developed  contrasts with that not  very  stream.  leiurus  where  the  d e c r e a s e d when  season there are  therefore,  isolation  The r a c e s  different habitats,  Leiurus  breeding.  d i m i n i s h e d i n a s m u c h as  makes  the  in Bonsall  migrants  contiguous  but  populations  races. Ecological  exclusion  t h a t each i s  H y b r i d swarms h a v e  C r e e k where h y b r i d i z a t i o n r e s u l t s from contiguous  our  p r i n c i p a l spawning m i g r a t i o n ,  i n an i n t e r m i n g l e d h a b i t a t  populations at  the  isolation.  two r a c e s b r e e d a t  Campbell R i v e r .  s t r e a m w h i l e the  p r i m a r i l y from  the  present  spawning t r a c h u r u s  research  degree from s e a s o n a l  stream to b r e e d ,  peak o f b r e e d i n g  in this  springs  b r e e d i n g have  spawning t r a c h u r u s Early  of  further  t h a t are  move i n t o  few e a r l y  isolation  partial  isolation  leiurus  is  and t o  data gathered  o n l y one month o u t  this  numbers o f  leiurus  and l a b o r a t o r y  which are  of h a b i t a t s  inhabits  a muddy b o t t o m , tea-coloured  dense  intimately  p o w e r f u l l y to  separate,  biotic  sympatry  and  Trachurus,  aquatic on the  this  (Grant,  and s t a n d i n g backwaters o r stands of  prevent  associated with  largely  prevents  quiet  water.  are  serves very  vegetation, other hand,  1963).  ponds w i t h and u s u a l l y , breeds  in a  93  h a b i t a t c h a r a c t e r i z e d by a sandy bottom and a m i l d c u r r e n t , beds o f recumbent v e g e t a t i o n , u s u a l l y Elodea or and c l e a r water.  Myriophyllum,  These c o n d i t i o n s h e l d i n a l l streams examined,  but B o n s a l l Creek was  especially instructive.  In t h a t stream  a switch i n h a b i t a t s occurs w i t h i n 200 f e e t , and with i t there is  a complete s h i f t i n the two  races.  L e i u r u s i s an abundant  f i s h upstream i n i t s t y p i c a l h a b i t a t , and t r a c h u r u s i s numberous downstream i n i t s t y p i c a l h a b i t a t .  In the b r i e f  intermediate  zone trachurus i s present i n mid-stream where there i s a sand bottom, m i l d c u r r e n t , and stands of Elodea, w h i l e along margins of the stream l e i u r u s occurs over a mud  the  bottom w i t h  dense emergent and submerged a q u a t i c v e g e t a t i o n , and  still  water. More evidence from d i s p e r s a l and from one  f a v o u r i n g e c o l o g i c a l i s o l a t i o n comes  t r a n s f e r experiments.  p o r t i o n of i t s h a b i t a t to another  When l e i u r u s i s moved i t tends to s o r t  out  i n t o q u i e t backwaters w i t h dense v e g e t a t i o n along the margins of  the stream.  Large numbers of l e i u r u s t r a n s f e r r e d to the  lower p a r t o f the stream i n w i n t e r - the summer b r e e d i n g l o c a t i o n o f trachurus - r a p i d l y move out of the a r e a .  At that  time of the year there i s no r e t r e a t from s w i f t c u r r e n t , v e g e t a t i o n i s not p r e s e n t .  and  Such a h o s t i l e environment would  p r o h i b i t l e i u r u s from e s t a b l i s h i n g ; permanent p o p u l a t i o n s i n the lower reaches between r a c e s .  of the stream, and thus prevent i n t e r b r e e d i n g  However, i f l e i u r u s i s t r a n s f e r r e d to the same  l o c a l i t y i n the summer i t e s t a b l i s h e s r e s i d e n c e w h i l e is  l e s s and s t i l l water can be  the c u r r e n t  found along the margins of the  94  stream,  and Elodea covers the bottom. Once a g a i n l e i u r u s vanishes from the area i n w i n t e r as  c u r r e n t i n c r e a s e s and v e g e t a t i o n d i s a p p e a r s .  Conversely, when  t r a c h u r u s i s p l a n t e d i n a l e i u r u s h a b i t a t t h e i r numbers d i m i n i s h over one month u n t i l none remains.  There i s evidence  s u g g e s t i n g a'downstream m i g r a t i o n from the area, but the l o s s o f b r i g h t b r e e d i n g c o l o u r s , t h e i r t h i n and emaciated and the many dead i n d i v i d u a l s found, i s i l l - a d a p t e d to the h a b i t a t . i n the upper reaches  appearance,  a l l i n d i c a t e t h a t trachurus  For the month they were present  they congregated  over r e l a t i v e l y  swift  r i f f l e s w i t h g r a v e l bottoms. R e s u l t s of preference t e s t s i n the l a b o r a t o r y s u b s t a n t i a t e evidence from the f i e l d f o r e c o l o g i c a l  isolation.  L e i u r u s has a s t r o n g p r e f e r e n c e f o r n e s t i n g on a mud and t r a c h u r u s s e l e c t s sand as a n e s t i n g l o c a t i o n .  substrate,  Leiurus  demonstrates a p r e f e r e n c e f o r n e s t i n g among Oenanthe  (an exposed  l o c a t i o n from the v i e w p o i n t of the f i s h but o f t e n w i t h a dense canopy o f leaves above), among Elodea  and t r a c h u r u s a p r e f e r e n c e f o r n e s t i n g  (secluded a t the nest s i t e ) .  Even though both  races  choose to nest i n s t a n d i n g water trachurus spends much more time i n the h a l f of the tank w i t h c u r r e n t than does l e i u r u s . R e s u l t s of p r e f e r e n c e t e s t s were not always e x c l u s i v e between races, f o r even though l e i u r u s i s a s s o c i a t e d w i t h t e a - c o l o u r e d water i n the w i l d i t shows a p r e f e r e n c e f o r c l e a r water Station  from  N. In a d d i t i o n , evidence from morphology and  i n d i c a t e s t h a t the two  races are thoroughly adapted  behaviour to d i f f e r e n t  95  habitats.  The t e r e t e shape found i n trachurus i s a w e l l known  a d a p t a t i o n to l i f e  i n c u r r e n t , and the deeper shape o f l e i u r u s  an a d a p t a t i o n to l i f e  i n s t a n d i n g water  (Hubbs, 1940).  I t has  been demonstrated t h a t d i s t i n c t t r o p h i c a d a p t a t i o n s e x i s t between the two races, t r a c h u r u s p o s s e s s i n g many g i l l  rakers with a long  and f i n e a r c h i t e c t u r e , and l e i u r u s w i t h few g i l l s h o r t and t h i c k .  r a k e r s t h a t are  An a n a l y s i s o f stomach contents" suggest  l e i u r u s feeds predominately  that  on bottom d w e l l i n g organisms, and  t h a t trachurus feeds on p l a n k t o n i c o r suspended organisms. Observations  on f e e d i n g behaviour  i n 40 g a l l o n a q u a r i a  i n d i c a t e t h a t l e i u r u s feeds mostly on l a r g e r p a r t i c l e s o f food t h a t have s e t t l e d to the bottom, and trachurus feeds on suspended p a r t i c l e s .  largely  T h i s was e s p e c i a l l y n o t i c e a b l e when  f i s h were f e d b r i n e shrimp.  L e i u r u s was o f t e n observed i n  the stream c a r r y i n g on what I assume were f e e d i n g a c t i v i t i e s , and these were v e r y c h a r a c t e r i s t i c .  The f i s h would swim s l o w l y  a l o n g j u s t above the bottom and e v e n t u a l l y come to an abrupt w i t h the body t i l t e d a t about a 45° angle,  stop  l i n g e r f o r a moment  and d a r t forward i n t o the mud c a u s i n g a s m a l l c l o u d to r i s e . A f t e r backing away i t c o u l d be seen making a m u l l i n g motion w i t h the mouth.  This a c t i v i t y ,  or others t h a t seemed a s s o c i a t e d  w i t h f e e d i n g , was never d i s c e r n e d i n t r a c h u r u s .  In both the  m o r p h o l o g i c a l and b e h a v i o u r a l aspect o f t r o p h i c s p e c i a l i z a t i o n then, the two races are w e l l adapted to t h e i r h a b i t a t s one f o r g a t h e r i n g food from the bottom i n q u i e t , muddy, and vegetated h a b i t a t s , and the other f o r t a k i n g suspended organisms i n e i t h e r a stream o r i n the sea where trachurus l e a d a p e l a g i c  96  existence. Other d i f f e r e n c e s c o n c e r n i n g may  w e l l be a d a p t a t i o n s to l i f e  l o c a t i o n of n e s t i n g s i t e s  i n s t a n d i n g or f l o w i n g water,  which would f u r t h e r f u n c t i o n to i s o l a t e the races  ecologically.  L e i u r u s i n the w i l d and i n a q u a r i a d u r i n g p r e f e r e n c e t e s t s choose open n e s t i n g l o c a t i o n s .  In s t i l l water, where they dwell, there  would be no s e l e c t i v e pressure f a v o u r i n g a p r o t e c t i o n o f nests and f r y from c u r r e n t t h a t may But there may  sweep them away from the  parent.  be s e l e c t i o n f a v o u r i n g an exposed n e s t i n g s i t e  where a male i n a dense p o p u l a t i o n : (and p o p u l a t i o n s of are u s u a l l y dense) would have a b e t t e r o p p o r t u n i t y to a female,  leiurus encounter  or the female to l o c a t e a n e s t i n g male. Evidence  from the c o l l e c t i o n o f nests i n the w i l d  and from p r e f e r e n c e t e s t s demonstrates t h a t trachurus p l a c e s i t s n e s t w i t h i n the s h e l t e r of dense v e g e t a t i o n . where c u r r e n t p r e v a i l s . o t h e r way  I t a l s o nests  In t h a t h a b i t a t s e l e c t i o n may  around, as i t were.  There may  be  the  then be a s e l e c t i v e  premium f o r p l a c i n g nests where they are p r o t e c t e d from c u r r e n t t h a t c o u l d sweep f r y away from the parent, and s i n c e the v e g e t a t i o n present i n t h a t h a b i t a t i s recumbent the males are more exposed to females I t was  for courtship.  observed  t h a t t r a c h u r u s i s a more shy  r e t i r i n g f i s h than l e i u r u s .  I t i s more d i f f i c u l t to  t r a c h u r u s to breed i n a q u a r i a and i t spends more time  and  induce hidden  from view, w h i l e i n the w i l d i t i s e q u a l l y shy and t h e i r more d i f f i c u l t to l o c a t e .  A s l i g h t movement from an  nests  observer  on the bank of the stream would send trachurus d a r t i n g i n t o  97  a bed of  Elodea.  This c o n t r a s t s w i t h l e i u r u s which i s  o b s e r v e d and l e s s  shy.  be a d a p t a t i o n s  the h a b i t a t s  in  a habitat  dense  to  Again these d i f f e r e n c e s  canopy o f v e g e t a t i o n  w i t h no c a n o p y o f  leaves  Most o f is  of  habitat  somewhat other  that and i t  selection  preference,  basis.  behaviour, genetic  ethological  a coadapted  habitat  vegetation  ensued between  that other  and i n t e g r a t e d  an e x t e n s i v e  isolation,  genetic  which  of  differences, isolation,  differentiation.  Mayr  The  under c o n t r o l  and s e a s o n a l  F o r as  expressed  the m o r p h o l o g i c a l  are  and p o s t m a t i n g ,  the  contrasting  and b e h a v i o u r .  Most o f races  i n two  that are  i s o l a t i n g mechanisms  an i n c i d e n t a l p r o d u c t .  mechanism h a s  t h a t has  may b e e x p e c t e d  surprising that  is  leiurus  i n an e x p o s e d  and a d a p t a t i o n  d i s t i n g u i s h the  considerable  animals,  genotype  race;  s a n d and o n l y r e c u m b e n t  divergence  genetic  preference,  represent  trachurus  s e l e c t i o n has been e c o l o g i c a l  a considerable  polygenes,  as  of  above,  producing manifold effects  this  differences  f r e q u e n t e d by each  above.  the morphology, h a b i t a t  outcome has  the  a result  environments, in  i n b e h a v i o u r may  w i t h an o b s c u r i n g mud b a c k g r o u n d u s u a l l y w i t h a  with a l i g h t background of  races  easily-  It  so  have  is  also then  important not  in  developed  (1963) makes c l e a r unit,  as  and each  the  isolating  basis.  The H y b r i d Zone Hybrids the h y b r i d zone, localities  constitute but  (Stations  about  t h e y may be as M and M')  21% o f  the  sticklebacks  a b u n d a n t as  and a t  46% a t  these s i t e s  the  some  in  98  p r o p o r t i o n o f trachurus, h y b r i d s , and  l e i u r u s i s 1:8:10.  Early  spawning trachurus are s c a r c e and w i l l perpetuate h y b r i d i z a t i o n by o u t c r o s s i n g w i t h the more abundant l e i u r u s .  Bagkcrossing  is  most f r e q u e n t to l e i u r u s , f o r which there are probably s e v e r a l reasons.  L e i u r u s i s much more abundant than trachurus, so t h a t  once F i s are present t h e i r chances of encountering  leiurus  1  greatest.  Furthermore, l e i u r u s i s a permanent r e s i d e n t i n the  h y b r i d zone and  t h i s provides considerable opportunity f o r  b a c k c r o s s i n g to t h a t race b e f o r e trachurus a r r i v e s Finally,  are  (Figure 12).  the seasonal i s o l a t i o n between sexes o f h y b r i d s  diminishes  the chances f o r an F2 g e n e r a t i o n .  But,  as the h y b r i d  index shows, there i s some b a c k c r o s s i n g to trachurus,  and  probably o c c a s i o n a l i n t e r b r e e d i n g among h y b r i d s . An aspect o f h y b r i d zones i n s t i c k l e b a c k s t h a t i s difficult  to understand  not occurred?  Seasonal  i s t h e i r narrowness; why i s o l a t i o n reduces the  have f a i l e d why  has  swamping not ensured  b e h a v i o u r a l or postmating  barriers?  swamping  opportunity  f o r h y b r i d i z a t i o n and e c o l o g i c a l i s o l a t i o n v e r y minimizes i n t e r b r e e d i n g but, nonetheless,  has  effectively  where these mechanisms i n the absence of  I t i s c l e a r t h a t the  d u r a t i o n o f h y b r i d zones cannot be e s t a b l i s h e d , but they repeated many times  i n d i f f e r e n t streams.  Despite  t h a t w i t h random mating panmixia c o u l d occur w i t h i n  the  are  fact  one  g e n e r a t i o n , h y b r i d zones remain narrow and the two  races remain  d i s t i n c t and i s o l a t e d .  involved:  1.  Evidence  S e v e r a l f a c t o r s seem to be  t h a t l e i u r u s i s a v e r y sedentary  has come from s e v e r a l l i n e s ;  fish  from p a r a s i t i z e d  99  populations  t h a t are  localized,  from the  cline  i n p l a t e numbers i n l e i u r u s , the r e s t r i c t i o n of hybrids  to p o i n t s o f h y b r i d i z a t i o n (where the  races c o e x i s t ) , and  from d i s p e r s a l experiments.  A sedentary h a b i t reduces interchange hybrids Fryer  two  between  and e i t h e r r a c e .  (1959) has  demonstrated t h a t p a r e n t a l  i n c i c h l i d s serves  care  to r e s t r i c t d i s p e r s a l o f  o f f s p r i n g , which become h a b i t u a t e d surroundings as a r e s u l t of t h e i r emancipation from p a r e n t s . care i n s t i c k l e b a c k s may  The  to the  local  late  intense  parental  w e l l f u n c t i o n to reduce  d i s p e r s a l as i t does i n c i c h l i d s . The  two  races have s t r o n g h a b i t a t  and  s e l e c t h a b i t a t s w i t h d i f f e r e n t environmental  characteristics.  preferences  The h y b r i d zone, however,  c h a r a c t e r i s t i c s t h a t are i n t e r m e d i a t e the h a b i t a t s of e i t h e r race, and  has  between  the d r a s t i c  r e d u c t i o n i n d e n s i t y of both races  i n t h a t zone  t e s t i f i e s that i t i s less desirable.  Hybrids  have a t t r i b u t e s t h a t are i n t e r m e d i a t e  between  the races and preference  i t i s probable they have a  f o r and  s e l e c t the intermediate  but a v o i d l e i u r u s and j u s t up and  trachurus  downstream.  The  h a b i t a t s that  this explanation  are  nonrandom d i s t r i b u t i o n  found i n the h y b r i d c l a s s e s provides supporting  zone,  evidence  (there was  no  evidence  100  t h a t trachurus was  nonrandomly d i s t r i b u t e d ) .  As d i s c u s s e d e a r l i e r the two  races are w e l l  adapted to the d i v e r g e n t h a b i t a t s t h a t each l i v e s i n and i l l other.  adapted to the h a b i t a t of the  T h i s i s apparent  i n the morphology of the  food g a t h e r i n g apparatus, behaviour,  body shape, f e e d i n g  and n e s t i n g behaviour.  But h y b r i d s  are i n t e r m e d i a t e i n number and s t r u c t u r e of gill  their  r a k e r s , and i t i s not u n l i k e l y t h a t they are  a l s o i n t e r m e d i a t e i n f e e d i n g behaviour.  Presumably  they are not as w e l l adapted i n e i t h e r h a b i t a t as the two  races are i n t h e i r own  habitat.  For  example, h y b r i d s are probably p o o r l y adapted to c o l l e c t bottom d w e l l i n g organisms or p l a n k t o n i c and d r i f t i n g organisms, both because f e e d i n g apparatus t h e i r behaviour  their  i s i n t e r m e d i a t e and because  i s a d j u s t e d to c o l l e c t n e i t h e r .  With i n t e r m e d i a t e body shapes they are  less  adapted to s t a n d i n g and to f l o w i n g water. they are i n t e r m e d i a t e i n n e s t i n g behaviour,  If such  an upset c o u l d cause males to p l a c e nests i n c u r r e n t t h a t would sweep the f r y away, or i n dense v e g e t a t i o n where there i s a s e l e c t i v e premium f o r males t h a t have exposed and conspicuous  sites.  There are  undoubtedly many o t h e r more s u b t l e , but e q u a l l y important,  adaptive t r a i t s  the races possess,  many of these must impose a s e l e c t i v e  and  disadvantage  101  against hybrids.  Furthermore, there a r e dense  p o p u l a t i o n s o f l e i u r u s upstream and dense p o p u l a t i o n s o f trachurus below the h y b r i d zone. reasonable  Thus, i t i s  to suppose t h a t h y b r i d s a r e a t a  c o n s i d e r a b l e s e l e c t i v e disadvantage  on both  sides  beyond the h y b r i d zone, f o r there they are l e s s w e l l adapted and must compete w i t h w e l l adapted populations i n a saturated h a b i t a t . S e l e c t i o n a g a i n s t h y b r i d s c o u l d not be found, however, i n s i d e the h y b r i d zone, nor would i t be expected foregoing discussion.  i n view o f the  I n s i d e the h y b r i d zone where the h a b i t a t  i s i n t e r m e d i a t e both races are a t a s e l e c t i v e disadvantage  since  they a r e l e s s w e l l adapted and, hence, t h e i r numbers reduced. But h y b r i d s w i t h t h e i r i n t e r m e d i a t e s e l e c t i v e disadvantage  t r a i t s may not be a t a  r e l a t i v e to e i t h e r race because the  h a b i t a t i s i n t e r m e d i a t e and because c o m p e t i t i o n would be g r e a t l y reduced. The circumstances  e x p l a i n e d above c o u l d e a s i l y g i v e  r i s e to a s t a b l e h y b r i d zone i n which n e i t h e r swamping nor reinforcement  c o u l d take p l a c e f o r c o n s i d e r a b l e  B e h a v i o u r a l and postmating  time.  i s o l a t i n g mechanisms ( e x c l u d i n g h y b r i d  i n f e r i o r i t y ) have not accompanied divergence  between the two races  so t h a t where they come i n t o c o n t a c t there i s no means to prevent interbreeding.  On the other hand e c o l o g i c a l i s o l a t i n g mechanisms  are w e l l developed  and c o n t a c t between races a r i s e s o n l y where  h a b i t a t s a r e contiguous.  Once c o n t a c t i s e s t a b l i s h e d the extent  o f h y b r i d i z a t i o n w i l l depend upon the s i z e o f the i n t e r m e d i a t e  102  h a b i t a t where h y b r i d s are not a t a s e l e c t i v e disadvantage. the i n t e r m e d i a t e Campbell R i v e r  zone i s r e l a t i v e l y broad as i t i s i n the  (1 mile) h y b r i d i z a t i o n may  i s narrow as i t i s i n B o n s a l l Creek be s e v e r e l y r e s t r i c t e d .  (200  If Little  be e x t e n s i v e , but i f i t feet) h y b r i d i z a t i o n w i l l  In e i t h e r i n s t a n c e the h y b r i d zone cannot  broaden, nor swamping take p l a c e , becuase of the disadvantage o u t s i d e the zone.  selective  S t i l l h y b r i d i z a t i o n continues  i n s i d e the h y b r i d zone because there i s no i s o l a t i n g mechanism to prevent there.  i t and because h y b r i d s are not a t a disadvantage  Even i f reinforcement  c o u l d occur  h y b r i d s are not a t a disadvantage,  i n a h y b r i d zone where  the r e i n f o r c e d genotypes  t h a t developed i n the zone would v e r y l i k e l y be swamped by v a s t l y g r e a t e r number of genotypes of e i t h e r race t h a t o u t s i d e the zone.  This complex o f f a c t o r s together  the  occur  with  s e l e c t i v e disadvantage a g a i n s t h y b r i d s absent i n s i d e the zone but present o u t s i d e the zone c o u l d produce s t a b i l i t y  over l o n g  periods. According i n d i v i d u a l s o f two those  to the concept of reinforcement, species  (or d i v e r g e n t p o p u l a t i o n s )  where overlap,  i n d i v i d u a l s t h a t have p o o r l y developed i s o l a t i n g mechanisms  w i l l be most prone to h y b r i d i z e , and w i l l produce h y b r i d s are a t a s e l e c t i v e disadvantage.  I n d i v i d u a l s t h a t have b e t t e r  developed i s o l a t i n g mechanisms w i l l be and w i l l  that  l e a s t prone to h y b r i d i z e  t h e r e f o r e leave more o f f s p r i n g .  Thus, n a t u r a l s e l e c t i o n  w i l l p e r f e c t i s o l a t i n g mechanisms because genotypes t h a t h y b r i d i z e w i l l be  l o s t from e i t h e r s p e c i e s  hybridization w i l l  (or p o p u l a t i o n ) , and  no longer o c c u r .  Moore (1957) has  eventually criticized  103  t h i s concept, p o i n t i n g out t h a t reinforcement mechanisms i s an ad hoc  of  isolating  mechanism; t h a t i s , reinforcement  i s o l a t i n g mechanisms i s o n l y of s e l e c t i v e value  of  i n s i d e the  hybrid  zone because o n l y w i t h i n the zone are genotypes t h a t h y b r i d i z e a t a s e l e c t i v e disadvantage.  Outside the h y b r i d zone such  r e i n f o r c e d genotypes would be  s e l e c t i v e l y n e u t r a l or even  disadvantageous.  The  l a t t e r argument may  apply  to  isolating  mechanisms such as e t h o l o g i c a l i s o l a t i o n , p r e v e n t i n g  their  spread beyond the h y b r i d zone; t h a t i s , p r o v i d i n g they c o u l d arise despite  swamping from the dense p o p u l a t i o n s  genotypes t h a t occur o u t s i d e  the zone.  unreinforced  However, Moore's  c r i t i c i s m i s concerned o n l y w i t h the spread o f i s o l a t i n g mechanisms o u t s i d e  of  perfected  the h y b r i d zone, and  i t seems  u n l i k e l y t h a t t h i s c r i t i c i s m a p p l i e s to e c o l o g i c a l i s o l a t i n g mechanisms.  T h i s i s so f o r the f o l l o w i n g reasons:  E c o l o g i c a l i s o l a t i o n develops o u t s i d e zone i n response to s e l e c t i o n and a d a p t a t i o n environments, and  to two  hybrid diverse  s e l e c t i o n would be expected to operate  c o n s t a n t l y to p e r f e c t a d a p t a t i o n isolation.  the  and  the consequent e c o l o g i c a l  Those i n d i v i d u a l s w i t h i n e i t h e r race t h a t have  p o o r l y p e r f e c t e d e c o l o g i c a l i s o l a t i n g mechanisms w i l l be more prone to move i n t o the h y b r i d zone than i n d i v i d u a l s w i t h w e l l developed i s o l a t i n g mechanisms.  Once i n the h y b r i d zone the  former are a t a severe s e l e c t i v e disadvantage. are p o o r l y adapted to the i n t e r m e d i a t e second they w i l l p r o b a b l y h y b r i d i z e and  First,  they  h a b i t a t i n the zone,  and  t h e i r genotypes w i l l  l o s t from t h a t race because t h e i r h y b r i d o f f s p r i n g cannot  get  be  out o f the h y b r i d zone, o r i f they d i d they would be selected against.  Thus, there  severely  i s no concern to have r e i n f o r c e d  i s o l a t i n g mechanisms spread beyond the h y b r i d zone. contrary,  On  the  those i n d i v i d u a l s w i t h p o o r l y developed e c o l o g i c a l  i s o l a t i o n move i n t o the h y b r i d zone arid are s e l e c t e d  against.  I n d i v i d u a l s w i t h b e t t e r developed i s o l a t i n g mechanisms w i l l increase,  and  the r e s u l t w i l l be  r e i n f o r c e m e n t and  strict  habitat selection. But  no matter  how  w e l l developed e c o l o g i c a l i s o l a t i o n  becomes i t would seem to be always l e s s than p e r f e c t . s t i c k l e b a c k s e c o l o g i c a l i s o l a t i o n might become so  In  perfected,  as a r e s u l t of reinforcement, t h a t s t r i c t h a b i t a t s e l e c t i o n and  adaptation  h a b i t a t s may and  would ensure.  Even i f t h i s occurred,  l i e s i d e by s i d e as they do i n B o n s a l l  s t r a y i n g of no more than f e e t  in hybridization. of f i s h  Creek,  (or inches) might r e s u l t  Trautman (1948) r e p o r t s  t h a t two  species  (Schilbeodes) are normally w e l l separated by e c o l o g i c a l  i s o l a t i o n , but h a b i t a t s of each s p e c i e s may o f one  however,  be w i t h i n  another, r e s u l t i n g i n h y b r i d i z a t i o n .  are known i n f i s h  (Trautman, 1957;  Many s i m i l a r example  Hubbs, 1961;  for  Reference to Mayr's c l a s s i f i c a t i o n of mechanisms  (Introduction)  shows t h a t he  Mayr (1963) s t a t e s  fundamental d i f f e r e n c e between the two  types:  mechanisms prevent the wastage of gametes and susceptible  reviews).  isolating  l i s t s ecological  i s o l a t i o n as a premating mechanism and h y b r i d as a postulating mechanism.  inches  inferiority "There i s a premating so are  to improvement by n a t u r a l s e l e c t i o n :  highly  postmating  105  mechanisms do not prevent wastage o f gametes and by n a t u r a l s e l e c t i o n i s d i f f i c u l t and  t h e i r improvement  indirect".  This  implies  t h a t the mode o f o p e r a t i o n o f n a t u r a l s e l e c t i o n w i l l be  very  d i f f e r e n t f o r pre and postmating i s o l a t i n g mechanisms.  It i s  obvious, however, t h a t i n s t i c k l e b a c k s there i s no between some pre and isolation  separation  postmating i s o l a t i n g mechanisms.  Ecological  (a premating mechanism) produces a postmating i s o l a t i n g  mechanism ( h y b r i d i n f e r i o r i t y ) .  The  two  cannot be  separated.  Each race i s w e l l adapted to a d i f f e r e n t h a b i t a t , and adaptation hybrids  ( e c o l o g i c a l i s o l a t i o n ) produces p o o r l y  ( h y b r i d i n f e r i o r i t y ) because they are  this  adapted  intermediate.  There are numerous examples o f h y b r i d zones where i t seems t h a t e c o l o g i c a l i s o l a t i o n and h y b r i d i n f e r i o r i t y go hand i n hand (Dixon, 1955; 1928;  Ingles and  Mayr and  is doubtful,  Gilliard,  B i g l i o n e , 1951; 1952;  Hubbs, 1961;  Hubbs and M i l l e r ,  Meise,  1943).  It  i n a l l cases of h y b r i d zones, t h a t e c o l o g i c a l  i s o l a t i o n and h y b r i d i n f e r i o r i t y can be  separated, and  whether a d i s t i n c t i o n can be made between the  two  hence,  modes of  o p e r a t i o n o f n a t u r a l s e l e c t i o n t h a t Mayr i m p l i e s .  SPECIATION  I t i s the o p i n i o n o f the w r i t e r t h a t there consider  i s a need to  the s u b j e c t of s p e c i a t i o n w i t h s p e c i a l c a u t i o n , f o r the  f o l l o w i n g reasons. in biology  Speciation i s a r e l a t i v e l y youthful  (Mayr, 1957), one  and based on i n f e r e n c e .  The  field  i n which our evidence i s i n d i r e c t second p o i n t i s b e s t expressed i n  106  the words of Mayr  (1959) who  states  "The more I study e v o l u t i o n  the more I am impressed by the uniqueness,  by the u n p r e d i c t a b i l i t y ,  and by the u n r e p e a t a b i l i t y of e v o l u t i o n a r y events.", and goes on to suggest t h a t i t may  be a mistake  to a p p l y a g e n e r a l i z i n g  technique l i k e mathematics to a f i e l d of events so unique evolution.  as  In view o f the youth of the study o f s p e c i a t i o n /  the i n d i r e c t nature o f the evidence, and the uniqueness e v o l u t i o n a r y events, the time has not come when we  of  can  g e n e r a l i z e from a few examples so f a r as to e x c l a i m t h a t a l l s p e c i a t i o n occurs i n t h i s or t h a t manner.  Herein l i e s  need f o r s p e c i a l c a u t i o n , f o r there i s a tendency  the  to assume  the c o n c l u s i o n f o r a g i v e n case of s p e c i a t i o n ; t h a t i t i s geographic. Geographic  s p e c i a t i o n has been so thoroughly  expounded upon (see Mayr 1963 to t r e a t i t i n d e t a i l h e r e .  f o r a review) t h a t there i s no need I t i s s u f f i c i e n t to i n d i c a t e  that  i s o l a t e d p o p u l a t i o n s o f marine s t i c k l e b a c k s can be found i n freshwater, and i f b a r r i e r s l a s t long enough f o r i s o l a t i n g mechanisms to develop, r e p r o d u c t i v e i s o l a t i o n w i l l come about, and s p e c i e s formed.  Provided o t h e r p l a u s i b l e a l t e r n a t i v e s  be reasonable excluded, geographic s p e c i a t i o n may  be  can  accepted  as most l i k e l y . However, there are other a l t e r n a t i v e s t h a t may to s t i c k l e b a c k s .  Sympatric  speciation resulting  d i s r u p t i v e s e l e c t i o n i s p o s s i b l e , and furthermore s p e c i a t i o n i s not without  apply  from geographic  difficulties.  S t i c k l e b a c k s have a v e r y widespread but  interrupted  107  distribution is  almost  i n boreal  regions.  The d i s t r i b u t i o n o f  continuous around the hemisphere b e i n g  only i n northeastern  Russia  (Berg,  1965).  interrupted  L e i u r u s , however,  a v e r y d i s r u p t e d d i s t r i b u t i o n o c c u r r i n g i n Europe, Iceland, Pacific  Baffin  Island,  Matthew,  Pribolofs,  Queen C h a r l o t t e  remote i s l a n d s  Bering Island,  (Evermann a n d G o l d s b o r o u g h , 1907; data  Greenland,  Island,  like Agattu,  Attu, Berg,  Shemya,  and f i n a l l y 1965;  U.B.C.).  I f s p e c i a t i o n were  freshwater  race arose  1963;  so v a s t an a r e a on s e v e r a l  remote i s l a n d s ?  L e i u r u s i s adapted  from the s e a .  Tolerance  1947),  it  so t h a t  on one c o n t i n e n t  of  and the  i n i s o l a t i o n a t a g i v e n p l a c e , how  c o u l d i t then expand i t s d i s t r i b u t i o n i n the throughout  St.  Japan.  Munzing,  geographic  the  Kodiak,  f r o m t h e N a t i o n a l Museum o f Canada a n d t h e I n s t i t u t e  Fisheries,  has  the A t l a n t i c Coast o f North America,  Coast o f North America,  the A l e u t i a n s i n c l u d i n g  trachurus  for salt  freshwaters  continents  a n d o n many  to f r e s h w a t e r  water  i s not great  i s implausible to-suggest and spread to o t h e r s  a n d i s unknown  that i t  (Heuts, arose  a n d t o many remote  islands. However s p e c i a t i o n o c c u r r e d  i t seems i n e s c a p a b l e  trachurus,  w h i c h i s known f r o m t h e open o c e a n  Schroeder,  1953),  many t i m e s . (divergence trachurus remarks  It  gave r i s e  t o l e i u r u s more  is difficult  i f you l i k e )  to b e l e i v e  could occur  speciation w i l l  the p r o b a b i l i t y t h a t  occur  once  such  Mayr's  successful  a r e as f o l l o w s ,  probably  geographic  many t i m e s  p r o d u c e s many l e i u r u s p o p u l a t i o n s .  concerning  (Bigelow and  than once,  that  that  speciation  that (1963)  geographic  "Most s p e c i e s b u d  108  off peripheral isolates at regular intervals.  N e a r l y a l l o f them  e i t h e r r e e s t a b l i s h c o n t a c t w i t h the p a r e n t a l s p e c i e s o r e l s e d i e out."  And then,  "Speciation i s a r i s k y process.  The improver-  ishment o f the gene p o o l and the g e n e t i c i n s t a b i l i t y  that  accompanies i t are f a r more l i k e l y to l e a d t o d i s a s t e r than t o success."  However, Mayr shows there i s ample time i n the h i s t o r y  of the e a r t h t o account f o r a l l s p e c i e s as a r e s u l t o f geographic s p e c i a t i o n ; the p o i n t a t i s s u e i s t h a t i n any given s p e c i e s the chances t h a t a geographic i s o l a t e would s u c c e s s f u l l y s p e c i a t e are very small.  I f , as Mayr contends, t h i s i s so i t seems even more  i m p l a u s i b l e t h a t trachurus  gave r i s e to l e i u r u s many times.  Furthermore, i t seems u n l i k e l y t h a t permanent b a r r i e r s necessary f o r geographic s p e c i a t i o n would be a t a l l common on extremely s m a l l i s l a n d s l i k e A t t u and o t h e r s . few  Indeed, there a r e v e r y  streams present on such i s l a n d s t h a t trachurus c o u l d use  to breed  i n , and thus become i s o l a t e d .  Even i f they d i d become  i s o l a t e d b a r r i e r s on such i s l a n d s would probably be v e r y temporary. As mentioned, sympatric i s a p o s s i b l e a l t e r n a t i v e . Mather  s p e c i a t i o n by d i s r u p t i v e s e l e c t i o n (1955) d i s c u s s e d the expected  consequences o f d i s r u p t i v e s e l e c t i o n and p o i n t e d o u t t h a t i t c o u l d e i t h e r e s t a b l i s h a polymorphism i n a p o p u l a t i o n o r i t c o u l d promote g e n e t i c i s o l a t i o n . are favoured  When two o p t i m a l  phenotypes  i n a p o p u l a t i o n and each i s dependent on the other,  as i n sex o r B a t e s i a n mimicry, polymorphism w i l l r e s u l t . the two favoured optima are independent o f one another and  isolation will  result.  If  divergence  109  Obviously d i s r u p t i v e s e l e c t i o n has implications  in evolution  experiments has  and  extremely important  Thoday i n a long s e r i e s of  v e r i f i e d Mather's p r e d i c t i o n s  (see Thoday, 1963).  Thoday, u s i n g D r o s o p h i l a melanogaster demonstrated t h a t s e l e c t i o n has  following e f f e c t s :  1.  promotes g e n e t i c  2.  can  3.  produces g e n e t i c  4.  produces c o u p l i n g  His r e s e a r c h repulsion  the  has  disruptive  variability  produce polymorphism divergence and  isolation  linkages  a l s o shown t h a t s t a b i l i z i n g s e l e c t i o n produces  linkages.  Furthermore, Clarke  have presented s t r o n g  and  Sheppard  (1962)  evidence t h a t d i s r u p t i v e s e l e c t i o n  operates i n n a t u r a l p o p u l a t i o n s to p e r f e c t mimicry i n  the  polymorphic females of P a p i l i o dardanus by p e r f e c t i n g dominance of the s w i t c h i n g  genes t h a t c o n t r o l the  With r e s p e c t above i s not  to sympatric s p e c i a t i o n ,  2)  listed  important and w i l l not be c o n s i d e r e d , but  remaining f a c t s are sticklebacks  polymorphism.  is  Mather  important when sympatric s p e c i a t i o n i n  discussed. (1955) has  made i t c l e a r t h a t sympatric  as a r e s u l t of d i s r u p t i v e s e l e c t i o n depends on three 1.  The  the  speciation  criteria:  h a b i t a t to which each optimum phenotype i s  adapted must p e r s i s t . 2.  3.  Each optimum phenotype must be  independent of  the  other.  The  phenotypes i n each h a b i t a t must be  sufficiently  d i s t i n c t t h a t d i f f e r e n t s e l e c t i v e f o r c e s can  operate  110  on each. All  the c r i t e r i a are s a t i s f i e d i n s t i c k l e b a c k s .  which trachurus and l e i u r u s a r e adapted p e r s i s t i n time,  The h a b i t a t s to  (sea and freshwater) do  independent phenotypes may a r i s e from t r a c h u r u s  as a r e s u l t o f recombination  and c r o s s i n g - o v e r , and the phenotypes  i n each h a b i t a t may be s u f f i c i e n t l y d i s t i n c t as a r e s u l t , of recombination  again,  and c r o s s i n g - o v e r i n trachurus p o p u l a t i o n s .  In Thoday's experiments d i s r u p t i v e s e l e c t i o n was a p p l i e d to both ends o f the normal curve f o r s t e r n o p l e u r a l chetae  (a p o l y g e n i c c h a r a c t e r ) .  i n d i v i d u a l s w i t h h i g h chetae  That i s , he s e l e c t e d f o r  number and low chetae  number.  These highs and lows were put i n t o v i a l s together and l e f t t o mate e i t h e r a s s o r t a t i v e l y , d i s a s s o r t a t i v e l y , o r randomly.  Within  12 t o 16 generations divergence between highs and lows had o c c u r r e d and r e p r o d u c t i v e i s o l a t i o n , as a r e s u l t o f mate p r e f e r e n c e and probably h y b r i d i n f e r i o r i t y , was w e l l developed.  I t i s important  to r e a l i z e t h a t the aspect, o r a s p e c t s , o f the phenotype t h a t d i s r u p t i v e s e l e c t i o n operates on i s i r r e v e l a n t . important  t h a t the phenotypic  such as p l a t e s , g i l l  I t i s only  c h a r a c t e r have a p o l y g e n i c b a s i s ,  r a k e r s , and chetae.  I t i s obvious  that  t r a c h u r u s p o p u l a t i o n s c a r r y l a r g e s t o r e s o f concealed g e n e t i c v a r i a b i l i t y as a l l p o p u l a t i o n s o f outbreeding animals do (Dobzhansky, 1955; Dempster, 1955; Mayr, 1963). e s p e c i a l l y obvious  i n t r a c h u r u s because o f much  This i s geographic  v a r i a t i o n , and r e g a r d l e s s o f how i t gave r i s e to l e i u r u s , i f s p e c i a t i o n o c c u r r e d many times  t h i s demonstrates the l a r g e  s t o r e s o f v a r i a b i l i t y trachurus must c a r r y .  Ill  Polygenic v a r i a t i o n i s most l i k e l y s t o r e d as balanced p o l y g e n i c systems w i t h i n the chromosomes (Mather and H a r r i s o n , 1949;  Mather, 1956;  Thoday, 1963).  w i l l mostly be s t o r e d i n the heterozygous  This v a r i a t i o n  s t a t e and  balanced.  T h i s p o t e n t i a l g e n e t i c v a r i a b i l i t y s t o r e d i n balanced p o l y g e n i c systems g i v e s g e n e t i c f l e x i b i l i t y and the a b i l i t y to adapt  to  change i n environment because i t can be r e l e a s e d s l o w l y over many generations as a r e s u l t o f c r o s s i n g - o v e r and between l i n k e d polygenes Sympatric the f o l l o w i n g way.  (Mather,  recombination  1943).  s p e c i a t i o n i n s t i c k l e b a c k s c o u l d occur i n Trachurus  spends most of i t s l i f e  but moves i n t o freshwater to breed. i n b a l a n c e d p o l y g e n i c systems may  i n the sea  Genetic v a r i a t i o n s t o r e d  o c c a s i o n a l l y , and over long  p e r i o d s , g i v e r i s e to i n d i v i d u a l s b e t t e r adapted  to freshwater  as a r e s u l t of c r o s s i n g - o v e r and recombination.  Such i n d i v i d u a l s  may  remain i n freshwater f o r longer p e r i o d s because they are b e t t e r  adapted.  A t t h i s p o i n t two o p t i m a l phenotypes are present;  n a t u r a l s e l e c t i o n w i l l continue to i n c r e a s e a d a p t a t i o n to freshwater, and d i s r u p t i v e s e l e c t i o n w i l l promote divergence and i s o l a t i o n d e s p i t e sympatry  (Thoday, 1963).  A . s p e c i f i c example makes t h i s c l e a r and g i l l (a p o l y g e n i c c h a r a c t e r ) may  be used.  In trachurus c r o s s i n g - o v e r  and recombination w i t h i n or between b a l a n c e d polygenes r a k e r number may  rakers  that control  o c c a s i o n a l l y occur, producing i n d i v i d u a l s w i t h  fewer r a k e r s t h a t are b e t t e r adapted  to f e e d i n g i n freshwater.  These i n d i v i d u a l s c o u l d then remain i n freshwater sometimes do i n w i n t e r ) .  (as trachurus  These o r i g i n a l recombinants  may  112  o c c a s i o n a l l y i n t e r b r e e d w i t h trachurus but lower  r a k e r numbers would  s t i l l be preserved because d i s r u p t i v e s e l e c t i o n promotes c o u p l i n g l i n k a g e s , h o l d i n g the genotypes c o n t r o l l i n g lower raker numbers together once they are formed t h a t may  (Thoday, 1963).  The  intermediates  be formed from i n t e r b r e e d i n g would be s e l e c t e d a g a i n s t  because they are l e s s w e l l adapted than trachurus i n the sea  and  l e s s w e l l adapted than i n d i v i d u a l s w i t h lower r a k e r number i n freshwater. preserved  Since i n d i v i d u a l s w i t h lower raker numbers w i l l  ( c o u p l i n g l i n k a g e s ) , s e l e c t i o n w i l l continue  be  to  p e r f e c t t h e i r a d a p t a t i o n , and s e l e c t i o n w i l l be i n t e n s e i n the d i v e r g e n t but s t a b l e freshwater environment. adaptation w i l l  S e l e c t i o n and  continue to produce i n d i v i d u a l s w i t h  lower  r a k e r numbers t h a t are s t i l l b e t t e r adapted, and w i l l  continue  to e l i m i n a t e l e s s w e l l adapted i n t e r m e d i a t e s i f they o c c u r . But i n t e r b r e e d i n g w i l l be p r o g r e s s i v e l y l e s s because a l l the while d i s r u p t i v e s e l e c t i o n i s operating. (or  d i v e r g e n t p o p u l a t i o n s ) are sympatric,  will  favour two  o p t i m a l phenotypes  Where the two disruptive  (lows and highs)  races  selection and  i n t e r m e d i a t e s w i l l be s e l e c t e d a g a i n s t (Thoday, 1963).  The  newly a r i s e n race w i l l e v e n t u a l l y become so w e l l adapted to the new  h a b i t a t t h a t e c o l o g i c a l i s o l a t i o n between races w i l l  In t h i s example a s i n g l e p o l y g e n i c c h a r a c t e r s , g i l l used.  result.  rakers, i s  In f a c t s e l e c t i o n operates on the t o t a l phenotype  (Mayr, 1963).  I f o t h e r p o l y g e n i c c h a r a c t e r s t h a t are  i n freshwater are c o n s i d e r e d  (body shape, b e h a v i o u r a l  adaptive traits,  p h y s i o l o g i c a l p r o p e r t i e s ) and d i s r u p t i v e s e l e c t i o n operates all,  p r e s e r v i n g these c h a r a c t e r s  ( c o u p l i n g l i n k a g e s ) and  on  selecting  113  a g a i n s t l e s s w e l l adapted i n t e r m e d i a t e s then divergence  and  In f a c t ,  isolation will  i n any or a l l c h a r a c t e r s ,  ensue.  t h i s process o f d i s r u p t i v e s e l e c t i o n h e l p s  e x p l a i n the d i f f i c u l t  problem of why  h y b r i d zones o f s t i c k l e b a c k s .  to  swamping does not occur i n  D i s r u p t i v e s e l e c t i o n would  swamping because i t promotes divergence  and  isolation,  prevent  not swamping.  Moreover, i f d i s r u p t i v e s e l e c t i o n were o p e r a t i n g i n h y b r i d zones i t s e f f e c t s would be most i n t e n s e i n or near the h y b r i d zone where interbreeding occurs.  And  i t would produce divergence  polygenic characters, l i k e p l a t e s . i s i n f a c t found, and again, difficult  problem.  This i n c r e a s e d  in  divergence  disruptive s e l e c t i o n explains a  This d i f f i c u l t  problem concerns the  fact  t h a t l e i u r u s n e a r e s t the h y b r i d zone have lower p l a t e counts than normal, and  trachurus  counts than normal.  This i s the r e v e r s e to be expected i f  i n t r o g r e s s i o n were  occurring.  selection i s occurring. the divergence preserve  i n the h y b r i d zone have h i g h e r p l a t e  I t i s expected i f d i s r u p t i v e  D i s r u p t i v e s e l e c t i o n c o u l d promote  i n t h i s p o l y g e n i c c h a r a c t e r , and  the divergence  i t would  (coupling l i n k a g e s ) .  I t i s i n t e r e s t i n g to note t h a t i n another case of r e p o r t e d sympatric  s p e c i a t i o n (Ford, 1964)  a reverse  was  found w i t h d i s t a n c e from the h y b r i d zone.  was  concerned w i t h two  populations  (Maniola  cline  A l s o t h i s case  j u r t i n a ) meeting  across two h a b i t a t s . In summary, sympatric s e l e c t i o n cannot be excluded sticklebacks speciation.  s p e c i a t i o n by d i s r u p t i v e  as a p l a u s i b l e e x p l a n a t i o n  of  I t i s c o n s i s t e n t with the widespread  114  but d i s j u n c t d i s t r i b u t i o n o f l e i u r u s , and e s p e c i a l l y so i f t h a t race has a r i s e n from trachurus more than once, because t h i s mode of  s p e c i a t i o n does not c o n t a i n the same element o f r i s k t h a t  geographic  s p e c i a t i o n does.  There i s no homozygosity and  impoverishment o f the gene pool t h a t makes geographic so r i s k y  (Mayr, 1963).  speciation  On the c o n t r a r y , d i s r u p t i v e s e l e c t i o n  promotes g e n e t i c f l e x i b i l i t y  (Thoday, 1963) p r o v i d i n g f o r  raw m a t e r i a l on which n a t u r a l s e l e c t i o n can operate and d r a s t i c a l l y d i f f e r e n t environment.  i n a new  D i s r u p t i v e s e l e c t i o n seems  t h e r e f o r e a more p l a u s i b l e e x p l a n a t i o n f o r repeated  development  of  invoking  freshwater  forms from a marine ancestor, without  a remarkable s e r i e s o f c o i n c i d e n c e s i n v o l v i n g temporary geographic both  b a r r i e r s i n each i s o l a t e d stream now o c c u p i e d by  types.  TAXONOMIC STATUS OF.LEIURUS AND TRACHURUS  Mayr (1963) d e f i n e s s p e c i e s as "Groups o f a c t u a l l y or p o t e n t i a l l y i n t e r b r e e d i n g p o p u l a t i o n s which a r e r e p r o d u c t i v e l y i s o l a t e d from o t h e r such groups."  S i b l e y (1961) c l a r i f i e d the  d e f i n i t i o n w i t h r e s p e c t to i n s t a n c e s where h y b r i d i z a t i o n occurs, p o i n t i n g o u t t h a t one must attempt to determine whether o r not h y b r i d s are a t a s e l e c t i v e disadvantage types.  r e l a t i v e to p a r e n t a l  I f h y b r i d s a r e a t a s e l e c t i v e disadvantage  the s p e c i f i c  s t a t u s o f the h y b r i d i z i n g p o p u l a t i o n s i s i n s u r e d - t h a t i s , gene flow w i l l be more o r l e s s completely b l o c k e d between p o p u l a t i o n s by n a t u r a l s e l e c t i o n .  115  L e i u r u s and trachurus p o p u l a t i o n s s t u d i e d i n the Campbell R i v e r and on Vancouver I s l a n d f u l f i l l definition. the two,  the s p e c i e s  There i s c o n s i d e r a b l e g e n e t i c divergence  i s o l a t i n g mechanisms are w e l l developed,  reproductively isolated.  Little  There i s c o n s i d e r a b l e  between  and they  are  morphological  divergence between the s p e c i e s , and most characters i n v e s t i g a t e d have a p o l y g e n i c b a s i s . isolation,  Other d i s t i n c t i o n s such as  seasonal i s o l a t i o n , p h y s i o l o g i c a l and  ecological  behavioural  a t t r i b u t e s l i k e anadromy, a l l exemplify c o n s i d e r a b l e g e n e t i c divergence. I s o l a t i n g mechanisms have a l r e a d y been d i s c u s s e d . I t remains o n l y to emphasize the e f f i c i e n c y w i t h which n a t u r a l s e l e c t i o n b l o c k s gene flow between the s p e c i e s .  In the  Little  Campbell R i v e r where h y b r i d i z a t i o n i s e x t e n s i v e , gene glow i s b l o c k e d withone one m i l e , and each s p e c i e s remains d i s t i n c t on e i t h e r s i d e o f t h i s b r i e f zone.  There i s no  indication  t h a t i n t r o g r e s s i o n occurs, i n f a c t the r e v e r s e d c l i n e i n l e i u r u s demonstrates t h a t s t r i n g e n t s e l e c t i o n counters  g e n e t i c exchange.  In B o n s a l l Creek gene flow i s prevented w i t h i n 200 evidence  of v e r y s t r i n g e n t s e l e c t i o n .  contiguous  feet, also .  Here too, where  p o p u l a t i o n s occur over an i n t e r f a c e between  h a b i t a t s , r e p r o d u c t i v e i s o l a t i o n o f each s p e c i e s i s p r e s e r v e d . Farther a f i e l d ,  an examination  o f specimens from  A l a s k a to Oregon ( i n s t i t u t e of F i s h e r i e s , U.B.C.) r e v e a l s t h a t the m o r p h o l o g i c a l  c h a r a c t e r i s t i c s t h a t d i s t i n g u i s h the  two  s p e c i e s i n the L i t t l e Campbell R i v e r and Vancouver I s l a n d a l s o d i s t i n g u i s h other p o p u l a t i o n s .  An examination  o f the  extensive  116  l i t e r a t u r e p r o v i d e s c o n f i r m a t i o n , and f u r t h e r i n d i c a t e s the b i o l o g i c a l a t t r i b u t e s o f the two remain d i s t i n c t Europe (Heuts, 1947; Munzing, 1963), to the P a c i f i c .  that  from  and Russia (Berg,  1965),  C l e a r l y although l e i u r u s and t r a c h u r u s are  sympatric over a v a s t sphere they remain d i s t i n c t and reproductively isolated;  they are "good" s p e c i e s .  However, the taxonomy may be r e s o l v e d ,  sticklebacks  furnish exciting material for evolutionary studies.  Evidence  i s accumulating t h a t there are s i b l i n g s p e c i e s w i t h i n the complex.  Gasterosteus wheatlandi on the A t l a n t i c Coast i s one  such s p e c i e s (Hubbs, 1929; S c o t t and Crossman, 1964), and another p o s s i b l e s p e c i e s has been uncovered by J . D. McPhail (personal communication) on the P a c i f i c Coast.  A possible  t h i r d s p e c i e s on the Queen C h a r l o t t e I s l a n d s i s c u r r e n t l y b e i n g i n v e s t i g a t e d by E. E. Moodie and myself.  The p r o b a b i l i t y  of an independent o r i g i n o f l e i u r u s from t r a c h u r u s many times g i v e s one cause to pause.  The morphology o f l e i u r u s i s q u i t e  uniform throughout i t s range, r e p r o d u c t i v e i s o l a t i o n i s maintained, and so one wonders i f the i s o l a t i n g mechanisms t h a t separate l e i u r u s and t r a c h u r u s are everywhere the same. Are the freshwater forms r e p r o d u c t i v e l y i s o l a t e d from one another?  In a word, has s e l e c t i o n independently and on many  o c c a s i o n s produced as uniform a genotype  as i t has a phenotype?  S t i c k l e b a c k s o f f e r wide scope f o r a b e t t e r understanding o f s p e c i a t i o n and the mechanisms o f e v o l u t i o n .  117  CONCLUSIONS  H y b r i d i z a t i o n i s e x t e n s i v e between the marine stickleback  (trachurus)and the freshwater form  (leiurus) but i s  r e s t r i c t e d to v e r y narrow h y b r i d zones i n the L i t t l e R i v e r and on Vancouver I s l a n d .  A morphological a n a l y s i s  p r o v i d e s f i r m c i r c u m s t a n t i a l evidence and t h a t b a c k c r o s s i n g o c c u r s .  t h a t h y b r i d s are p l e n t i f u l  T h i s i s confirmed by uniform  r e a r i n g experiments i n a l l combinations. to  the freshwater An  Campbell  Most b a c k c r o s s i n g i s  race.  i n v e s t i g a t i o n o f i s o l a t i n g mechanisms as o u t l i n e d  by Mayr r e v e a l s t h a t n e i t h e r b e h a v i o u r a l nor g e n e t i c b a r r i e r s are e f f e c t i v e b l o c k s to h y b r i d i z a t i o n , and thus there i s no means to  prevent h y b r i d i z a t i o n where they come t o g e t h e r .  But  e c o l o g i c a l i s o l a t i n g mechanisms p r o v i d e a v e r y powerful t h a t g r e a t l y reduces  o p p o r t u n i t y to h y b r i d i z e .  barrier  This i s  e n f o r c e d to same e x t e n t by p a r t i a l seasonal i s o l a t i o n , b u t e a r l y b r e e d i n g migrants  make a major c o n t r i b u t i o n to i n t e r b r e e d i n g  i n the L i t t l e Campbell R i v e r .  S e v e r a l f a c t o r s probably  operate  to cause v e r y narrow h y b r i d zones i n s t i c k l e b a c k s , i n c l u d i n g a sedentary h a b i t , h a b i t a t p r e f e r e n c e , and h y b r i d i n f e r i o r i t y o u t s i d e the h y b r i d zone the zone).  (although none c o u l d be d e t e c t e d i n s i d e  The complex o f i s o l a t i n g mechanisms found c o u l d  w e l l produce s t a b l e h y b r i d zones f o r l o n g p e r i o d s , and i s discussed.  Reinforcement  o f e c o l o g i c a l i s o l a t i n g mechanisms  p r o b a b l y occurs, and Moore's c r i t i c i s m o f the spread o f r e i n f o r c e d genotypes o u t s i d e the h y b r i d zone would not apply  118  i n this instance.  Reinforcement o f e c o l o g i c a l i s o l a t i o n  would r e s u l t i n e x c l u s i o n between races, not b i o t i c Adaptation  sympatry.  and consequent e c o l o g i c a l i s o l a t i n g  mechanisms are the cause o f h y b r i d i n f e r i o r i t y .  In t h i s  i n s t a n c e pre and postmating i s o l a t i n g mechanisms cannot be separated,  f o r they go hand i n hand.  This i s discussed i n  r e a l t i o n to Mayr's c l a s s i f i c a t i o n of i s o l a t i n g mechanisms. Geographic s p e c i a t i o n i s p o s s i b l e but sympatric  s p e c i a t i o n cannot be excluded  problematical;  as a p l a u s i b l e  e x p l a n a t i o n and c o u l d have o c c u r r e d as a r e s u l t o f d i s r u p t i v e selection.  Furthermore, d i s t r i b u t i o n a l p a t t e r n s of l e i u r u s  can be e x p l a i n e d most reasonably  as a r e s u l t of s e v e r a l ,  perhaps many, independent o r i g i n s from The  two  forms f u l f i l l  trachurus.  the s p e c i e s d e f i n i t i o n of  Mayr and remain r e p r o d u c t i v e l y i s o l a t e d , have w e l l developed i s o l a t i n g mechanisms, and show c o n s i d e r a b l e g e n e t i c much o f i t p o l y g e n i c .  There i s no evidence  divergence,  that i n t r o g r e s s i o n  occurs, but a r e v e r s e c l i n e i n l e i u r u s as w e l l as v e r y narrow h y b r i d zones demonstrates t h a t s e l e c t i o n s e v e r e l y r e s t r i c t s gene flow. freshwater  Reproductive  i s o l a t i o n between marine  and  forms seems to be widespread around the hemisphere.  119  LITERATURE  Anderson,  E.  1949.  CITED  Introgressive  hybridization.  (Wiley,  New Y o r k ) . Anderson,  E.  1956.  Character  association  for  the p l a n t b r e e d e r .  Genetics  Brookhaven Symposia i n B i o l . , Baggerman,  B.  1957.  An experimental  b r e e d i n g and m i g r a t i o n  analysis  as a  i n plant  breeding.  9:123-140.  s t u d y on the t i m i n g o f  i n the t h r e e - s p i n e d  (Gasterosteus aculeatus  tool  L.).  Arch.  Neerl.  stickleback Zool.,  7:1-213. Bailey,  N.  1959.  Statistical  methods  i n biology.  (Wiley,  New Y o r k ) . Berg,  L.  1965.  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