Another common form of animal coloration is the lack of pigmentation–or albinism. This condition is frequently observed in natural populations of cave-dwelling animals, including fish, crayfish, insects, spiders and other species. The common occurrence of albinism in cave animals is thought to represent the flip side of evolution under natural selection. That is, with little or no light, natural or sexual selection on pigment color and pattern is relaxed. Mutations that abolish pigmentation, and that would generally be harmful to animals in other habitats, are tolerated in the darkness of these caves.
Albinism, too, appears to have a simple genetic basis that makes it “easy” to evolve. Recently, Meredith Protas and Cliff Tabin at Harvard Medical School, Bill Jeffery at the University of Maryland, and their collaborators pinpointed the genetic basis of albinism in the Mexican blind cavefish. These albino fish are found in about 30 caves in the Sierra de El Abra region in northeastern Mexico. Each population is derived from a pigmented, fully sighted surface- or river-dwelling form. The researchers have investigated the genetic basis of albinism in populations from the Pachón and Molino caves and found that albinism in each population was caused by mutations in the same pigmentation gene, but different specific mutations in each case. Here again, in these fish, evolution has repeated itself twice in the origin of the same trait. Furthermore, the specific gene mutated in these fish is also the same gene responsible for albinism in humans, pigs, mice and other fish species.
The natural histories of the rock pocket mice and cavefish vividly demonstrate how animals have adapted to new surroundings; no matter how alien those habitats once were to their ancestors. These obscure animals have also provided the concrete links between specific genes, natural selection and evolution in the wild that have long been sought by biologists. While not as majestic as the game animals of the African savanna, these animals illustrate larger lessons that would have been appreciated by Roosevelt, and perhaps even warranted their own, albeit small, trophy case for displaying the continuing progress in understanding how evolution works.
Sean B. Carroll is an evolutionary biologist at the University of Wisconsin. His new book, Remarkable Creatures: Epic Adventures in the Search for the Origins of Species (Houghton Mifflin Harcourt), chronicles the experiences and discoveries of intrepid naturalists who developed and advanced the theory of evolution.