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UBC Theses and Dissertations

Components of premating reproductive isolation in threespine stickleback Southcott, Laura


Reproductive isolation, the reduction in gene flow between two species, is central to the study of speciation. Many so-called isolating barriers may be involved in preventing two species from successfully hybridizing, but the importance of each barrier and the presence of interactions between them have not often been measured. I investigated habitat and mating isolation in the recently diverged benthic-limnetic species pairs of threespine stickleback (Gasterosteus aculeatus species complex). In these species, the males of which build nests and guard territories during the breeding season, there is a strong pattern of male habitat choice, with limnetics nesting in open habitats and benthics nesting under vegetation. When males were given a choice of nesting habitats in enclosures in an artificial pond, they almost always chose according to this pattern. However, females displayed no difference in probability of spawning with conspecifics in different habitats in no-choice mating trials, making habitat isolation at best a weak barrier to hybridization in the absence of interactions with other isolating mechanisms. From an existing dataset of mate choice trials, I calculated the contributions of body size differences and male nuptial colour to mating isolation. Isolation due to body size differences was strong in both species, but stronger in benthics. Isolation due to colour preference was negligible in benthics and moderately strong in limnetics. Generalized linear models indicated that interactions with other, unspecified species-specific traits increases isolation due to body size and, in benthics, due to colour. Together, these traits provide strong but incomplete premating isolation, resulting in an expected hybridization rate higher than that observed in nature, and thus are not sufficient in themselves to maintain the species pairs.

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