| Summary: | Our understanding of the mechanisms driving population divergence is dependent on our ability to effectively detect and interpret patterns of genetic differentiation. In natural populations, the evolutionary processes influencing genetic differentiation can be difficult to ascertain. Seabirds are highly mobile, yet often display population differentiation, suggesting intrinsic barriers to gene flow are important in driving their diversification. The northern gannet (Morus bassanus), a piscivorous, colonial seabird, is widely distributed across the North Atlantic Ocean. Despite strong dispersal abilities, decades of banding studies indicate that the North Atlantic Ocean acts as an effective barrier for migration in northern gannets. However, recent telemetry revealed that some northern gannets migrate across the ocean each year, suggesting trans-Atlantic gene flow is possible. To investigate population differentiation in northern gannets, I analyzed genomic patterns of diversity among eight North American and European colonies. Genetic structure was characterized using indices of genetic differentiation, principal component analysis, a Bayesian clustering method, and discriminant analysis of principal components. I found significant population genetic structure across the breeding range, corresponding primarily to the division between North American and European gannet populations. My analyses did not find significant differentiation among colonies on either side of the Atlantic. I employed Bayesian approaches to estimate contemporary gene flow and divergence between North American and European gannet populations. I found low but significant rates of migration from Europe into North America, but no migration from North America into Europe. My analyses suggest that North American and European populations diverged recently, potentially occupying separate refugia during the Last Glacial Maximum. Finally, I used outlier analysis and random forest feature selection to investigate fine-scale genetic structure, and identified loci that allowed population assignment of 85% of sampled northern gannets. Low genetic differentiation in northern gannets may be due to shared variation following recent range expansion or to admixture resulting from overlap in nonbreeding distribution. My results suggest that the North Atlantic Ocean acts as a semi-permeable barrier to gene flow in northern gannets. The informative genetic markers I identified may allow colony-specific impact assessment and the development of targeted management strategies in northern gannets. M.Sc.
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