Cavefish (without eyes) and surface dwelling brethren. (Image courtesy of New York University) (Credit: Image courtesy of New York University)
Regressive evolution is a phenomenon by which a species loses features through evolution. It is especiall y evident in many cave-dwelling species, the majority of which actually descended from species that originally lived above ground. Some of these organisms happened to have traits that were beneficial in a cave environment, prompting part of the population to move underground. Over time some features, like eyes or skin pigmentation, for example, became unnecessary and eventually disappeared.
Example[]
Astyanax cavefish have adapted to life in caves by loss of eyes and pigmentation.
References[]
Retrieved from http://curiosity.discovery.com/question/what-is-regressive-evolution on November 28, 2013.
New York University (2007, February 16). 'Regressive Evolution' In Cavefish: Natural Selection Or Genetic Drift. ScienceDaily