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.