Geographical features are the predominant driver of molecular diversification in widely distributed North American whipsnakes.

Abstract

Allopatric divergence following the formation of geographical features has been implicated as a major driver of evolutionary diversification. Widespread species complexes provide opportunities to examine allopatric divergence across varying degrees of isolation in both time and space. In North America, several geographical features may play such a role in diversification, including the Mississippi River, Pecos River, Rocky Mountains, Cochise Filter Barrier, Gulf of California and Isthmus of Tehuantepec. We used thousands of nuclear single nucleotide polymorphisms (SNPs) and mitochondrial DNA from several species of whipsnakes (genera Masticophis and Coluber) distributed across North and Central America to investigate the role that these geographical features have played on lineage divergence. We hypothesize that these features restrict gene flow and separate whipsnakes into diagnosable genomic clusters. We performed genomic clustering and phylogenetic reconstructions at the species and population levels using Bayesian and likelihood analyses and quantified migration levels across geographical features to assess the degree of genetic isolation due to allopatry. Our analyses suggest that (i) major genetic divisions are often consistent with isolation by geographical features, (ii) migration rates between clusters are asymmetrical across major geographical features, and (iii) areas that receive proportionally more migrants possess higher levels of genetic diversity. Collectively, our findings suggest that multiple features of the North American landscape contributed to allopatric divergence in this widely distributed snake group.