Reconciling Western toad phylogeography with Great Basin prehistory

This study marks the first attempt to study historical processes that may be responsible for the contemporary geographic distribution and phylogeny of Great Basin Western toads (Anaxyrus boreas spp.). The dynamic aquatic history of the Great Basin was evaluated as a potential model for early toad dispersal into regions which would later become arid and isolated, affecting gene flow and eventually promoting allopatric speciation. This research was accomplished by exploring the spatial and historical relationships among Western toad clades and Great Basin drainages. Toad clades that are distributed over large areas, and are composed of many populations, tend to fall within the confines of regional, riverine drainages. Smaller, more genetically distant clades are generally harbored in small, riverless drainage basins. Dates of estimated evolutionary divergence among Western toad clades varied considerably using rates of molecular substitution that are reasonable for this organism. In addition, aquatic histories contain last known dates of interbasin connectivity that are well within the range of toad evolutionary divergence times reported here. Although the specific dates presented in this study encourage further refinement, this study suggests that the relative ages of Western toad clades are positively related with their geographic isolation. These results suggest that the evolutionary history of the Western toad may have been affected by prehistoric environments dominated by glacial cycles. This information can be used to inform strategies used by wildlife managers to catalog and protect unique biodiversity.