Evolutionary outcomes of interactions among phenotypes in post-glacial lakes

Competitive interactions among sympatric phenotypes shape forms of natural selection and evolution of phenotype distributions. Populations of fish in post-glacial lakes are a model system for studying the evolutionary outcomes of competition. Fish in these lakes tend to exhibit phenotypic variability corresponding to a nearshore (benthic/littoral) to open water (limnetic/pelagic) resource use axis. In several lakes, populations have repeatedly diversified into sets of sympatric ecotypes that are phenotypically distinct from each other. Competition is predicted to drive frequency-dependent selection within and among populations of post-glacial fish. We tested whether this form of selection acts within the range of phenotypes found in generalist, single-species populations of threespine stickleback. To do this, we used experimental mesocosms with differing frequency distributions of phenotypes. We found evidence for only weak frequency-dependent survival but not frequency-dependent growth. To evaluate evidence for a possible evolutionary outcome of competitive interactions, we tested the hypothesis that levels of ecological sexual dimorphism should be higher in populations that do not co-occur with competitor species. We found similar levels of sexual dimorphism in allopatric threespine stickleback populations compared to populations sympatric with another stickleback species or prickly sculpin (an intraguild predator). Instead, we found some evidence for an alternative hypothesis that sexual dimorphism might decline with divergence from the ancestral marine stickleback phenotype. Competitive interactions affect partitioning of resources among different ecotypes and might lead to repeatability in resource use and phenotype distributions across different ecotype assemblages. Two Salmonid genera – Salvelinus and Coregonus – frequently diversify into assemblages of two to seven sympatric ecotypes in post-glacial lakes. We evaluated evidence for repeatability of niche occupation, phenotype distributions, and niche by phenotype relationships across assemblages in these genera. We found evidence for repeatable niche occupation and niche by phenotype relationships in Coregonus, but only repeatable niche by phenotype relationships in Salvelinus. While these studies do provide evidence for a role of competitive interactions in shaping natural selection and evolution, they also suggest that predictions from theory for evolutionary consequences of competition may not always be met in fish populations in post-glacial lakes.Science, Faculty ofZoology, Department ofGraduat