UBC Theses and Dissertations
The evolutionary genomics of adaptation and speciation in the threespine stickleback Samuk, Kieran Mikhail
Speciation and adaptation are key processes in biological evolution. Speciation creates genealogically discrete lineages, whereas adaptation causes organisms to become better matched to their environments. In this thesis, I conducted three studies that advanced our knowledge of speciation or adaptation. All three studies made use of a unique study system: threespine stickleback – small fish found in marine and fresh waters throughout the northern hemisphere. I first explored the potential of a newly-discovered “white” form of threespine stickleback for studying the early phases of speciation. Using a variety of population genomic methods, I showed that white stickleback are genetically distinct from other marine stickleback, and diverged recently in the face of substantial gene flow. These features make white stickleback an excellent system for studying the early phases of speciation. Next, I used white stickleback to examine the role of sexual and trophic divergence in the early phases of speciation. Using morphological and isotopic data, I found evidence for only weak trophic differentiation between white and common stickleback. Instead, genetic differences between the two forms are concentrated on genomic regions that harbour genes with male-biased expression. This suggests that, apart from difference in body size, strong trophic differentiation may not be necessary in the early phases of speciation. The final study explored the role of gene flow in shaping the genomic architecture of adaptation. Theory predicts that when adaptation occurs in the face of gene flow, genomic architectures in which adaptive loci are localized in regions of low recombination will be favored over others. I tested this prediction by quantifying the correlation between recombination rate and the density of adaptive loci in pairs of stickleback populations that varied in their degree of gene flow. In line with theory, we found that adaptive loci were more like to be found in regions of low recombination when divergent selection and gene flow co-occurred. Together, the studies presented in this thesis provide new tools and significant advances in our understanding of speciation and adaptation.
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