Erosional exhumation of carbonate rock facilitates dispersal-mediated allopatric speciation in freshwater fishes

A fundamental goal of evolutionary biology is to understand the mechanisms that generate and maintain biodiversity. Discovery and delimitation of species represent essential prerequisites for such investigations. We investigate a freshwater fish species complex comprising Etheostoma bellator and the endangered E. chermocki, which is endemic to the Black Warrior River system in Alabama, USA, a global hotspot of temperate freshwater biodiversity. Phylogenomic analyses delimit five geographically disjunct species masquerading as E. bellator. Three of these new species exhibit microendemic distributions comparable to that of E. chermocki raising the possibility that they also require protection. The species of the complex are found in streams flowing over carbonate rock and they are separated by waterways flowing over siliciclastic rock, a geographic pattern dictated by the underlying stratigraphy and structural geology. Over time, rivers have eroded downward through layers of siliciclastic rocks in the basin, gradually exposing underlying carbonate rock, the substrate of suitable habitat today. Our results suggest that episodic dispersal to patches of suitable habitat set the stage for allopatric speciation in the species complex. Our study suggests that the presence of heterogeneous rock can facilitate dispersal-mediated allopatric speciation in freshwater organisms in the absence of external tectonic or climatic perturbations.