Constraints to speciation despite divergence in an old haplochromine cichlid lineage (Astatoreochromis alluaudi)

Most of the 500 cichlid species of Lake Victoria evolved very rapidly in the wake of an adaptive radiation within the last 15000 years. However, of five old cichlid lineages that colonized the lake four lineages did not speciate within Lake Victoria. Endemic to the Lake Victoria region, Astatoreochromis alluaudi is a member of an old haplochromine lineage that only consists of two species. Even though the species occurs in a wide range of habitats, there were no indications of evolutionary diversification. Here, we tested predictions of several hypothetical mechanisms that might constrain speciation, including high dispersal rates, a generalist life strategy and the lack of behavioral assortative mating. Genomic analyses of thirteen populations revealed several genomically distinct groups, associated with major habitat distinctions, indicating the existence of two ecotypes.