Biogeographic Barriers, Pleistocene Refugia, and Climatic Gradients in the Southeastern Nearctic Drive Diversification in Cornsnakes (Pantherophis guttatus complex)

The southeastern Nearctic is a biodiversity hotspot that is also rich in cryptic species. This richness can be explained by numerous hypotheses affecting divergence, which include biogeographic barriers, adaptation to climatic gradients across this region, and Pleistocene speciation in glacial refugia. However, previous phylogeographic studies have both supported and refuted these hypotheses. Therefore, while one or more of these hypotheses may explain diversification, it is likely that taxa are forming within this region in species-specific ways. Here we generate a genomic dataset for the cornsnakes (P. guttatus complex), which are widespread across this region, spanning both biogeographic barriers and climatic gradients. We use phylogeographic model selection combined with hindcast ecological niche models to determine regions of habitat stability through time. This combined approach suggests that numerous hypotheses are required to explain the current diversity of this group of snakes. The Mississippi River caused initial speciation in this species complex, with more recent divergence events linked to adaptations to ecological heterogeneity and allopatric Pleistocene refugia. Lastly, we discuss the taxonomy of this group and suggest there may be additional cryptic species in need of formal recognition.