River drainages and phylogeography: An evolutionary signiWcant lineage of shovel-nosed salamander (Desmognathus marmoratus) in the southern Appalachians

All models of speciation involve some mechanism for the separation of biological populations that eventually leads to the formation of new species. In the case of aquatic species, biological evolution is often coincident with geological evolution, because populations can become geographically isolated among river systems as drainage morphologies evolve. Thus, for species that are completely restricted to aquatic habitats (e.g., Wshes), river drainages provide a powerful framework for reconstructing phylogeny and historical biogeography (Avise, 1994). The southern region of the Appalachian mountains is a model setting for investigating the historical diVerentiation of populations in aquatic environments. The region’s communities and topography are very old as they were well south of the maximum extent of Pleistocene glaciation. The result has been the persistence of biota in the region and the opportunity for diversiWcation with time. For example, the Tennessee River system, which drains the western slopes and interior of the southern Appalachians, has the highest species richness and greatest number of endemic species of Wshes in North America (Jenkins et al., 1972; Starnes and Etnier, 1986), presumably due to the isolation of lineages in drainages separated by long river distances.