Evolutionary Dynamics of Pine Squirrels (Tamiasciurus) in Western North America
Chavez, Andreas Shintaro
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The evolution of new phenotypes and species is a population genetic process that is governed by four fundamental forces: natural selection, drift, mutation, and gene flow. Ecological genetics is the reciprocal interaction between population genetic theory and empirical observations from nature and the laboratory. Here I present a study in which I synthesize ecological information with population genetic studies in order to better understand how and why organisms diversify at the genetic, phenotypic, and species level. Pine squirrels (Genus: <italic>Tamiasciurus</italic>) are an important study organism for investigating the early stages of adaptation and speciation in nature because they are comprised of only recently divergent lineages, form narrow hybrid zones, show sharp goegraphic variation in several phenotypic traits of ecological interest, such as fur coloration and cranial morphology associated with bite force, and are a model organism for behavioral and ecological research. Pine squirrels (also known as tree squirrels) are ubiquitous across coniferous forests of North America and are comprised of only two recognized species: the Douglas squirrel (<italic>T. douglasii</italic>) and the North American red squirrel (<italic>T. hudsonicus</italic>). In my first chapter, I show with molecular divergence analyses using multilocus genetic data that these two species split less than a half million years ago. I also use phylogenetic inference and isolation with migration models to resolve the biogeographic puzzle of red squirrels occurring on Vancouver Island despite the closest mainland regions being occupied by Douglas squirrels. A species tree analysis using 15 nuclear loci indicates that the origin of squirrels on the island was likely from <italic>T. hudsonicus</italic> populations that occurred in interior montane regions that apparently persisted south of continental ice during the LGM. Surprisingly, phylogenetic analysis with mtDNA shows that all island squirrels carry the mtDNA of the sister species <italic>T. douglasii</italic>. We found historical migration between <italic>T. douglasii</italic> and island <italic>T. hudsonicus</italic>, but no historical migration between <italic>T. douglasii</italic> and mainland <italic>T. hudsonicus</italic> using IM models. These findings show a complex colonization and migration history between both mainland species and the island population. In my second chapter, I examined hybrid zone dynamics between the two squirrel species along an environmental gradient in the North Cascade Mountains of southern British Columbia and northern Washington. I found that genetic and phenotypic variation had steeper clines than a neutral genetic marker, which suggests that divergent selection is overriding gene flow in maintaining distinction between these species. Furthermore, all phenotypic clines were centered in a forest ecotone, thereby implicating environmental factors as being responsible for the location of the species boundary. Furthermore, I detected hybridization occurring to at least the F2 generation, which supports the notion that hybrid inviability is not as strong as environmental forces in maintaining distinction between species at this hybrid zone. In my third chapter, I show differential patterns of clinal variation in several ecologically important traits within Douglas squirrels (<italic>T. douglasii</italic>) along a forest gradient in Oregon. Ventral fur color shows a relatively sharp clinal transition from deep orange in the coastal region to a whitish-yellow, which coincides with a gradient in tree canopy openness. In contrast, cranial morphology varies continuously and gradually and does not show any sharp transitions, which is surprising given the abrupt changes in size and hardness of their primary food source, the cones from which they extract seeds. Collectively, my dissertation research provides an integrative examination of the contemporary processes of selection and gene flow that have shaped phenotypic variation and the genetic structure of pine squirrels in western North America.
- Biology