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

Abstract

Intensive field observations were conducted on a small British Columbia coastal stream for two years, supplemented by laboratory rearing and experiment. Stress was placed upon possible isolating mechanisms, as outlined by Mayr, that might serve to keep the marine stickleback (trachurus) and the freshwater form (leiurus)distinct. Lelurus permanently occupies the upper reaches of the stream; trachurus is anadromous, but enters the lower reaches to breed in "pure" freshwater. Between the breeding ground of the two, where some coexistence occurs, hybridization between leiurus and trachurus is extensive, but it is restricted to very narrow zones both in Little Campbell River near Vancouver and in certain Vancouver Island streams. Morphological analysis provides firm circumstantial evidence that hybrids are plentiful and that back-crossing occurs. Hybridization is confirmed by rearing experiments in the laboratory with crosses in all combinations. Most backcrossing is to the freshwater race and this is considered in detail. Behavioural experiments demonstrated the absence of mate preference; random mating occurs with respect to ethological isolation and hybrids perform courtship and parental care activities normally. Rearing hybrids uniformly and in all conbinations demonstrated the absence of genetic incompatibility, and vigorous offspring were produced. Fertile hybrids are abundant in the wild, but the sexes are seasonally isolated from one another. Behavoiral and genetic blocks to hybridization are absent, and thus there is no means to prevent hybridization where leiurus and trachurus come together. But evidence from the field and from preference tests demonstrated that ecological isolating mechanisms are very powerful barriers to hybridization. This is enforced to some extent by partial seasonal isolation, but early spawning migrants make a major contribution to hybridization in the Little Campbell River. Several factors probably operate to cause very narrow hybrid zones in stickle-backs, including a sedentary habit, habitat preference, and hybrid inferiority outside the hybrid zone. The latter could not be detected inside the hybrid zone. The complex of Isolating mechanisms found may well produce stable hybrid zones over long periods of time, and is discussed. Reinforcement of ecological isolating mechanisms probably occurs, and Moore's criticism of the spread of reinforced genotypes would not apply in this instance. Adaptation and consequent ecological isolating mechanisms are the cause of hybrid inferiority. In this instance pre and postmating mechanisms cannot be separated, for they go hand in hand. Geographic speciation is possible but problematical, and sympatric speciation cannot be excluded as a plausible explanation of divergence between sticklebacks as a result of disruptive selection. Distributional patterns of leiurus can best be explained by repeated and independent origins from trachurus. The two forms fulfill the species definition by Mayr and remain reproductively isolated, have well developed isolating mechanisms, and show considerable genetic divergence, much of it polygenic. There is no evidence that, introgression occurs, but a reverse cline in leiurus as well as very narrow hybrid zones demonstrates that selection severely restricts gene flow. Reproductive isolation between marine and freshwater forms seems to be widespread around the hemisphere.

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