Pleistocene glaciation drove shared population coexpansion in eastern North American snakes

Glacial cycles during the Pleistocene had profound impacts on local
environments and climatic conditions. In North America, some regions that
currently support diverse biomes were entirely covered by ice sheets,
while other regions were environmentally unsuitable for the organisms that
live there now. Organisms that occupy these regions in the present day
must have expanded or dispersed into these regions since the last glacial
maximum, leading to the possibility that species with similar geographic
distributions may show temporally concordant population size changes
associated with these warming trends. We examined 17 lineages from 9
eastern North American snake species and species complexes to test for a
signal of temporally concordant coexpansion using a machine learning
approach. We found that the majority of lineages show population size
increases towards the present, with evidence for coexpansion in five out
of fourteen lineages, while expansion in others was idiosyncratic. We also
examined relationships between genetic distance and current environmental
predictors and showed that genomic responses to environmental predictors
are not consistent among species. We therefore conclude that Pleistocene
warming resulted in population size increases in most eastern North
American snake species, but variation in environmental preferences and
other species-specific traits results in variance in the exact timing of
expansion.