Extreme isolation by distance in a montane frog Rana cascadae

Given the recent interest in declining amphibian populations, it is surprising that there are so few data on genetic drift and gene flow in anuran species. We used seven microsatellite loci to investigate genetic structure and diversity at both large and small geographic scales, and to estimate gene flow in the Cascades frog, Rana cascadae. We sampled 18 sites in a hierarchical design (inter-population distances ranging from 1–670 km) to test for isolation by distance and to determine the geographic scale over which substantial gene flow occurs. Eleven of these sites were sampled as three fine-scale clusters of three, three, and five sites separated by pairwise distances of 1–23 km to estimate number of migrants exchanged per generation via F_ST and by a coalescent approach. We found R. cascadae exhibits a strong pattern of isolation by distance over the entire species range, and that there is a sharp drop in migrants exchanged between sites separated by greater than 10 km. These data, in conjunction with results of other recent studies, suggest that montane habitats promote unusually strong genetic isolation among frog populations. We discuss our results in light of future management and conservation of R. cascadae.