Erosion of heterogeneous rock drives diversification of Appalachian fishes

We show that river incision into different rock types drives allopatric lineage divergence in the Greenfin Darter (Nothonotus chlorobranchius), a fish species endemic to the Tennessee River, USA. Using species occurrence data and DNA sequences sampled across the genome, we find that populations of N. chlorobranchius are restricted to tributaries flowing over metamorphic rocks of the Blue Ridge geologic province. We propose that as the contact between metamorphic and sedimentary rock was exhumed by erosion, it migrated in an upstream direction across the landscape, progressively isolating populations in Blue Ridge tributaries. We find agreement between the geologic model and genomic estimates for the geographic sequence and absolute timing of divergence between clades of N. chlorobranchius.