Coevolutionary theory predicts that differences in the genetic demography of interacting species can influence patterns of local adaptation by affecting the potential of local populations to respond to selection. We conducted a comparative phylogeographic study of venomous rattlesnakes and their venom-resistant ground squirrel prey across California, and assessed how effective population size (N_e) estimates correspond with a previously documented pattern of rattlesnake local adaptation. Using RAD-seq markers, we detected lineage relationships among both the rattlesnakes (Crotalus oreganus ssp.) and ground squirrels (Otospermophilus sp.) that are incongruent with previous phylogenetic hypotheses. Both rattlesnakes and squirrels share a deep divergence at the Sacramento-San Joaquin River Delta. At this broad phylogeographic scale, we found that the locally adapted rattlesnakes had higher N_e than squirrels. At the population scale, snakes also had larger N_e accompanied by larger values of several metrics of population genetic diversity. However, the specific magnitude of local adaptation of venom activity to ground squirrel venom resistance was not significantly correlated with local differences in N_e or other diversity statistics between predator and prey populations, suggesting other factors in the geographic mosaic of coevolution contribute to the specific local-scale outcomes of this interaction. These results suggest an evolutionary mechanism that may explain some (but clearly not all) of rattlesnake local adaptation in this coevolutionary interaction – larger population sizes raise adaptive potential of rattlesnakes compared to ground squirrels.

Double digest RAD-seq data processed using AftrRAD v. 5.0.1.

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