(a) Regressive evolution is a change in a population over time that involves the loss of certain phenotypic characteristics. It is thought to be caused by either genetic drift or natural selection. An example of regressive evolution is the loss of eyes in one form of the Mexican cavefish, Astyanax mexicanus. These eyeless cavefish live in caves that are in total darkness. There are three theories to explain how the loss of eyes in the cavefish has occurred. Theory A There is no advantage to having eyes in a cave that is in total darkness, where energy sources are scarce. Having eyes is a disadvantage as there may be an energy cost. Theory B A mutation has occurred in a single gene. This mutation has two effects: • a lack of eye development • an increase in the number of chemoreceptors on the skin. Theory C Various mutations occurred in the genes responsible for eye development over a period of time. By chance, these mutations increased in frequency in small isolated populations. Eventually this produced a population of eyeless cavefish.