The influence of history, geography and environment on patterns of diversification in the western terrestrial garter snake

Aim: A central aim of biogeography is to understand how biodiversity is
generated and maintained across landscapes. Here, we establish
phylogenetic and population genetic patterns in a widespread reptile to
quantify the influence of historical biogeography and current
environmental variation on patterns of genetic diversity. Location:
Western North America. Taxon: Western terrestrial garter snake, Thamnophis
elegans. Methods: We used double-digest RADseq to estimate phylogenetic
relationships and characterize population genetic structure across the
three widespread subspecies of T. elegans: T. e. vagrans (wandering garter
snake), T. e. elegans (mountain garter snake) and T. e. terrestris (coast
garter snake). We assessed patterns of dispersal and vicariance across
biogeographic regions using ancestral area reconstruction (AAR) and
deviations from isolation-by-distance across the landscape using estimated
effective migration surfaces (EEMS). We identified environmental variables
potentially shaping local adaptation in regional lineages using
genetic-environment association (GEA) analyses. Results: We recovered
three well-differentiated genetic groups that correspond to the three
subspecies. AAR analyses inferred the eastern Cascade Range as the
ancestral area, with dispersal to both the east and west across western
North America. Populations of T. e. elegans displayed a latitudinal
gradient in genetic variation across the Sierra Nevada and northern
California, while populations of T. e. terrestris show discrete genetic
breaks consistent with well-known biogeographic barriers. Lastly, GEA
analyses identified allele frequency shifts at loci associated with a
common set of environmental variables in both T. e. elegans and T. e.
terrestris. Main Conclusion: T. elegans is composed of distinct
evolutionary lineages, each with its own geographic range and history of
diversification. T. e. elegans and T. e. terrestris show unique patterns
of diversification as populations dispersed from east to west and while
adapting to the new environments they colonized. Historical events,
landscape features and environmental variation have all contributed to
patterns of differentiation in T. elegans.