Amphipods, which resemble small shrimp, are laterally compressed. Reduced pigment and degenerate eyes of cave dwelling animals are called regressive characteristics. The surface dwelling ancestors of these animals had sight and pigment, but these traits were not essential to survival in a cave. Many species in caves including fish, salamanders, spiders, and various insects have adapted to have reduced pigment and degenerate eyes. Both pigment and eyes have degenerated in thousand of species during their adjustment to cave life.
Mutations tend to be more often destructive than constructive.
Geneticists have estimated that only one mutation in 500 are beneficial. Over evolutionary time, the population of mutations for no eyes and pigment increased in number until they began to dominate many species of cave life. During the evolution of most groups of cave animals, the pigment becomes reduced before the eyes degenerate. Because no direct mutations from pigmented to albino forms have been observed by scientists, it is believed that pigment is lost in a series of stages. Animals and insects that already have weak eye sight are most likely the next animals that will be incorporated into cave life.