The evolutionary genetics of cave fishes: Convergence, adaptation and pleiotropy

Cave animals have converged repeatedly on a troglomorphic suite of characters including regression of eyes and pigmentation, augmentation of other senses, and changes in morphology, metabolism and behavior. Convergences are evolution’s “experimental replicates” and their study permits the testing of hypotheses about the evolutionary process. The genetic bases for these changes are the focus of this review. We cover the Mendelian, quantitative and molecular genetics of troglomorphic evolution, and the population genetics of cave forms, with an emphasis on the Mexican tetra, Astyanax mexicanus. Molecular data is useful for reconstructing the relationships of cave populations, but gene flow into and out of cave populations can obscure relationships. We examine the roles of selection and drift in troglomorphic evolution and conclude that both processes are important. Direct selection against eyes in the cave probably reflects energy conservation and the high metabolic cost of maintaining the retina. Indirect selection through pleiotropy is probably a major factor in all evolutionary adaptation to new environments, including the cave.