Regressive Evolution: Testing Hypotheses of Selection and Drift

Loss of eyes and pigmentation are common in cavefishes, but whether their evolution is driven by natural selection or genetic drift remains in dispute. These same convergences, however, present a remarkable opportunity to parse out these disparate evolutionary forces. Each cavefish species is a replicate of an experiment revealing an evolutionary response to an ecological shift to permanent darkness and food scarcity. Thus, hypotheses based upon observations of one species may be tested by their predictions in other species. Application of this approach to independently evolved populations of Astyanax cavefishes show that the principal driver of eye loss is natural selection acting directly upon eye phenotype, while the principal driver of loss of melanophores is genetic drift. Selection also seems to contribute to the evolution of albinism and changes in melanin structure, and epistatic interactions among genes may also drive changes in allelic frequencies through transmission biases.