Why do cave fish lose their eyes? A Darwinian mystery unfolds in the dark

Natural History, June, 2005 by Luis Espinasa, Monika Espinasa

The second hypothesis is that genes controlling the development of unnecessary structures become effectively neutral. Once the genes neither enhance nor hinder the organism's survival, the forces of natural selection that once maintained those genes in good working order no longer operate. The genes accumulate mutations that impair their function, and so the unnecessary structures governed by the genes degenerate. That view is summed up in the phrase "neutral-mutation theory."

Biologists favoring the first hypothesis face the problem of having to come up with an advantage for losing one's eyes. One of the best ways to address this kind of puzzle is to look for instances of pleiotropy, a known and common phenomenon of genetics. Pleiotropic effects are the multiple, often seemingly unrelated characters caused by a single gene.

One of the best-documented cases of pleiotropy is the gene associated with sickle-cell disease. In the red blood cells of people carrying the gene, mutant hemoglobin molecules stick together to form rodlike bundles that deform the red blood cells into the shape of a sickle. People with sickle cells often suffer chronic anemia, extreme pain, and organ damage. At the same time, however, the parasite that causes malaria cannot thrive inside red blood cells that carry the altered hemoglobin, and so people with the mutant gene show enhanced resistance to malaria. The malaria resistance conferred by the sickle-cell gene is a pleiotropic effect. In spite of its harmful effects, the gene persists among human populations in parts of Africa with a high incidence of malaria. Similarly, if the genes that control eye degeneration in cave fish also control other, unrelated structures that could confer an advantage to fish living in darkness, pleiotropy could explain the loss of eyes.

What about the second hypothesis, neutral-mutation theory? Mutations occur in all living organisms and in all genes. Natural selection, though, exercises quality control over genes. If a fish living in a surface stream suffers a mutation harming its vision, it is more likely than its sighted fishmates to die of hunger or to be eaten by predators. The decreased fitness often leads to early death or a low rate of reproduction, thus eliminating the harmful mutation.

According to the neutral-mutation theory, however, if a genetic mutation leads to blindness of an eyed fish living in a cave, the fish will have no advantage or disadvantage over other eyed fish in the cave. The mutation will simply remain in the population. As more and more such mutations accumulate, the entire population will become blind.

A new generation of molecular and developmental biologists have been tackling the issues raised by such questions for several years. Horst Wilkens of the Zoological Institute and Zoological Museum of the University of Hamburg in Germany and Richard L. Borowsky of New York University have been working with both cave and surface varieties of the tetra fish, Astyanax mexicanus. They have mapped the genes in both cave and surface fish that control eye size, pigment loss, and condition factor (a polite term for "fatness"). Caves, in general, are food-poor environments, and cave fish have evolved a highly efficient metabolism that enables them to survive on fewer calories than their surface counterparts need.