Linking fluvial barriers and habitat specialization in the evolution of microendemism in riverine fishes.

Species with highly restricted geographic distributions are key components of biodiversity hotspots. In southeastern North America, a global hotspot of temperate freshwater biodiversity, such microendemic distributions exhibited by freshwater fishes have been attributed to geographic isolation related to spatial shifts in suitable habitat associated with climatic and geologic processes. Here, through examination of phylogenetic relationships and analysis of abiotic characteristics of the environment, we test hypotheses for the origin of microendemism within the Holiday Darter Etheostoma brevirostrum (Suttkus and Etnier) species complex. Phylogenomic analyses of double digest restriction-site-associated DNA (ddRAD) sequence data find four geographically disjunct and highly diverged lineages each of which exhibit remarkably narrow geographic distributions in the Coosa River system in Alabama, Georgia, and Tennessee, USA. We employ comparative analyses of environmental variables to quantify the degree of ecological specialization exhibited by E. brevirostrum as compared to other darter lineages that have also diversified at small scales within other river basins. We propose that ecological specialization to high-elevation streams underlain by metamorphic rock as well as the presence of knickpoints, steep sections of river channels that act as barriers to dispersal between tributaries, generated opportunities for geographic isolation and diversification in E. brevirostrum.