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Divergent natural selection and müllerian mimicry in polymorphic heliconius cydno (Lepidoptera: Nymphalidae)

University of British Columbia

Natural selection favours bright colours or bold patterns that advertise unpalatability. In a noxious polymorphic species frequency-dependent selection should lead to fixation of the common morph, because rare morphs suffer relatively higher attack rates by naive predators. This generally leads to warning colouration that is monomorphic within species and shared between species (Müllerian mimicry). However, several unpalatable species of Heliconius butterflies (Lepidoptera: Nymphalidae) exhibit polymorphic warning colouration within a population. One possible explanation is that divergent selection may favour different colour-pattern morphs of a single unpalatable species if each matches a different warningly coloured unpalatable Miillerian mimic species (comodels). In this thesis I explore this hypothesis by investigating the genetic basis and fitness consequences of polymorphism for warning-colour pattern within a single species of Heliconius butterfly, H. cydno. In Western Ecuador, H. cydno is polymorphic for colour (yellow versus white), pattern (triangle versus band), and hind-wing band-width. I find that H. cydno's colour-pattern polymorphism has a simple Mendelian genetic basis. Two alleles at a single locus with complete dominance determine colour differences (white alleles dominate over yellow). Pattern differences are slightly more complex: a single locus with three alleles (and complete dominance) or two epistatic diallelic loci can account for the variation. Relative hind-wing band-width may have a polygenic basis. Using the multi-site transplant experiment, I find that divergent selection favours transferred colour-morphs of H. cydno (yellow or white) that resemble their putative Müllerian comodels (H. eleuchia or H. sapho respectively). This provides unique experimental evidence for the benefit of Müllerian mimicry. Divergent selection generated by the two comodels may promote maintenance of the colour-pattern polymorphism in H. cydno. In support of this hypothesis, I found the frequency of yellow H. cydno correlates with the density of their respective comodels (yellow H. eleuchia or white H. sapho) at different locales. H. cydno is also polymorphic for colour and pattern where it occurs in the local absence of comodels. Polymorphism at these sites indicates that gene-flow, reduced selection, or both helps maintain colour-pattern diversity when comodels are absent. My research suggests that Müllerian mimicry can develop between a single species and more than one comodel taxon. In Western Ecuador, two Heliconius species appear to generate divergent selection favouring polymorphism in a third species H. cydno.

Thesis/Dissertation

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2009-06-03

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http://hdl.handle.net/2429/8665

Zoology

University of British Columbia

UBCV

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