Shifting rivers and stationary ground: biogeographic history of slimy salamanders on an isolated bluff in the Mississippi River floodplain

Geographic barriers are integral to the processes of dispersal and vicariance, but some barriers, such as rivers, may vary spatiotemporally, potentially affecting biodiversity patterns in significant ways. Fountain Bluff is an isolated upland in the Mississippi River floodplain along the Illinois‐Missouri border, USA. The Mississippi River presently runs on the west side of Fountain Bluff, but the course ran on the east side prior to the Illinois glacial stage (~150 000 years ago). The northern slimy salamander (Plethodon glutinosus) occurs to the east in Illinois, whereas the morphologically similar western slimy salamander (Plethodon albagula) occurs to the west in Missouri. We used mitochondrial and nuclear DNA sequences to determine the species identity of slimy salamanders on Fountain Bluff and test whether they originated via vicariance (P. albagula) or dispersal (P. glutinosus). We found that slimy salamanders on Fountain Bluff are P. glutinosus, which supports the hypothesis that they originated via dispersal from the river bluffs ~6 km to the east. Divergence time estimates corroborated that dispersal likely occurred after the westward diversion of the Mississippi River. The shifting courses of large rivers have had important impacts on biodiversity through the creation and removal of geographic barriers as well as by facilitating passive dispersal across riverine barriers. Fountain Bluff is an isolated upland in the Mississippi River floodplain (USA). We used DNA sequences to determine whether slimy salamanders on Fountain Bluff originated via vicariance or dispersal when the course of the Mississippi River shifted ~150 000 years ago. We found that Fountain Bluff slimy salamanders originated via dispersal from uplands to the east after the Mississippi River was diverted westward.