Biogeographic barriers and historical climate affect phylogeographic structure and demographic history of the common gartersnake

Aim: Current distributions of widespread North American (NA) species have
been shaped by Pleistocene glacial cycles, latitudinal temperature
gradients, sharp longitudinal habitat transitions, and the vicariant
effects of major mountain and river systems that subdivide the continent.
Within these transcontinental species, genetic diversity patterns might
not conform to established biogeographic breaks compared to more spatially
restricted taxa due to intrinsic differences (e.g., greater dispersal
ability in generalist species) or spatiotemporal differences (e.g.,
wide-ranging species predate younger barriers). In this study, we
highlight the effects of these extrinsic variables on genetic structuring
by investigating the phylogeographic history of a widespread generalist
squamate found throughout NA.  Location: North America
Taxon: Common gartersnake, Thamnophis sirtalis Methods: We
evaluate the effects of major river basins and the forest-grassland
transition into the Interior Plains on genetic structure patterns using
phylogenetic, spatially informed population structure, and demographic
analyses of SNP data, and address range expansion history with ecological
niche modeling using locality and historic climate data. Results: We
identify four phylogeographic lineages with varying degrees of
connectivity between them. We find discordant population structure
patterns between sex-linked and autosomal loci with respect to the
relationship between the central NA lineage relative to coastal lineages.
We find support for southeast Pleistocene refugia where recent secondary
contact occurred during the Last Glacial Maximum and evidence for both
northern and southern refugia in western NA. Main Conclusion: Our results
provide strong evidence for a Middle Pliocene origin for Thamnophis
sirtalis in central-southeastern NA preceding its rapid expansion across
the continent prior to middle Pleistocene climate-mediated lineage
formation. We implicate major riverine networks within the Mississippi
watershed in likely repeated westward expansion events across the Interior
Plains. Finally, we corroborate prior conclusions that phenotypic
differences between subspecies do not reflect shared evolutionary history
and note that the degree of separation between inferred lineages warrants
further investigation before any taxonomic revisions are proposed.