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UBC Theses and Dissertations

The parallel evolution of reproductive isolation in threespine sticklebacks Nagel, Laura Maria


Reproductive isolation within and between sympatric populations of threespine stickleback was investigated to find evidence for parallel speciation, which is the independent evolution of the same reproductive isolating mechanisms. One of the requirements of parallel speciation is reproductive isolation between descendant populations of sympatric species. Mate choice tests between benthic and limnetic sticklebacks in two lakes were therefore conducted to examine reproductive isolation between them. Strong reproductive isolation was found. The probability of hybridization between limnetic females and benthic males was affected by the size of both the male and the female. I suggest that reproductive isolation based on body size may have evolved as a by-product of natural selection on body size. Mate choice tests within benthic and limnetic forms were conducted to determine if there was reproductive isolation among benthic and limnetic forms from different lakes. I hypothesized that because each form evolved under similar selective regimes, they would not be reproductively isolated. The results supported this hypothesis, although there was a trend in limnetics to prefer mates from their own population. There was no reproductive isolation between benthics from different lakes. Similar reproductive isolating mechanisms therefore arose independently in several lineages, probably as a by-product of natural selection.

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