DISSERTATION RESEARCH: Investigating the Joint Effects of Phylogeny and Adaptation on Phenotypic Variation at a Continental Scale (Serpentes:Thamnophis)

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Local adaptation is a hallmark of geographically variable species with wide distributions that span geographic barriers to gene flow. Determining the underlying population structure upon which to place these local adaptations is a central objective in evolutionary biology. However, the impact of evolutionary history on inference of contemporary population structure has been largely unexplored. This research aims to examine the effects of evolutionary history on genetic and morphological variation among populations at regional and continental scales. Empirical data and the results from this study will serve as a model for teaching evolution, specifically genetic drift, in both academic and public settings. Extensive phenotypic variation between widespread organisms often complicates determination of taxonomic relationships. The range of the common garter snake Thamnophis sirtalis spans all of North America, excluding Mexico and the desert southwest. Twelve T. sirtalis subspecies are currently recognized and defined by color patterning. One of these subspecies, T.s. pickeringii, co-occurs with two less widely distributed congeners (T. elegans and T. ordinoides) in the Pacific Northwest across the San Juan Archipelago. This region was subject to multiple Pleistocene glaciation events up until ~20,000 years ago, after which flora and fauna recolonized the islands from neighboring mainland refugia. This project aims to investigate the relationship between evolutionary history and contemporary population variation by comparing simulated data of known demographic histories to genomic data from two empirical study systems: 1) the young San Juan Archipelago Thamnophis species assemblage, 2) a continental-scale sampling of all Thamnophis sirtalis subspecies. These two systems occur in proximity to multiple known Pleistocene refugia and across geographic barriers to gene flow for other North American flora and megafauna, but it is unclear whether a generalist ectotherm will show the patterns observed in previous phylogeographic studies. Using genomic data, this project will test whether 1) T. sirtalis subspecies designations (and therefore phenotypes) represent unique evolutionary lineages, 2) distinct evolutionary lineages within T. sirtalis occur along known or novel barriers to gene flow, or 3) the combined effects of historical vicariance and non-uniform selective forces across different subspecies produce a signal discordant with phenotypic variation.