UBC Theses and Dissertations
The role of insularity in promoting intraspecific differentiation in Song Sparrows Wilson, Amy
Islands are valuable research systems for evolution and conservation, but most work has focused on oceanic islands. Far less study has occurred on near-shore islands where inter-island and island-mainland dispersal is an important microevolutionary process. Further studies in near-shore systems would aid the expansion of island evolutionary theory and conservation initiatives. In this thesis, I studied populations of Song Sparrows (Melospiza melodia) on near-shore islands along the Pacific coast of North America to examine the causes and consequences of dispersal for microevolutionary and ecological processes. Within an island metapopulation, where inter-island distances ranged from 200m to 2km, male and female immigration rates were influenced by adult density and sex ratio respectively, suggesting that intrasexual territoriality influences immigration. Islands differed in immigration levels, with low immigration and high resident recruitment on more isolated islands. I next examine genetic structuring at a larger spatial scale (0-300km). I found that the scale of genetic structuring within continuously distributed populations was less than 10km, suggesting that Song Sparrows are a sedentary passerine. Regional comparisons revealed that holding geographic distance constant, larger genetic distances occur in areas located at subspecific boundaries or across water barriers. The apparent reduction in dispersal to islands had broad-scale consequences. Across Pacific Coast islands, island populations consistently had lower genetic variation than mainland populations. Small and remote island populations tended to have the lowest genetic variation. From an in situ conservation stance, populations on large, remote islands could be important contributors to intraspecific genetic diversity because of high genetic differentiation. Finally, I link genetic structuring with contemporary dispersal and show that migration rates among the Channel Islands are low, suggesting that these islands are demographically independent. The absence of shared mtDNA haplotypes between extant and extinct populations suggests that inter-island migration was historically low, potentially explaining why the two extirpated islands have not been recolonized. Collectively, my thesis results increase our understanding of the mechanisms of divergence on insular populations by examining factors affecting dispersal, the spatial scale of divergence and estimating the consequences of reduced gene flow on islands for broad-scale patterns of genetic variation, microevolution and demographic stability.
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