Strong Selection is Necessary for Evolution of Blindness in Cave Dwellers
Blindness has evolved repeatedly in cave-dwelling organisms, and investigating loss of sight presents an opportunity to understand the operation of fundamental evolutionary processes, including drift, selection, mutation, and migration. The observation of blind organisms has prompted many theories for their blindness, including loss-by-disuse and selection against eye development when eyes are not used. Here we have developed a model that shows just how strong selection must be for blind populations of a cave-dwelling species to evolve. We used approximations to determine levels of selection that would result in caves containing only sighted individuals, only blind individuals, or a stable population of both. We then incorporated drift into the model using simulations. Based on our model, strong selection is necessary for the evolution of blindness unless immigration rates are extremely low. Drift decreased the fixation of blindness in populations, although for intermediate levels of migration the level of selection required to fix blindness decreased substantially. We hypothesize that this degree of selection may be due to phototaxis in sighted individuals, who move toward the light leaving only blind individuals in the cave.