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COPYRIGHT 2005 Natural History Magazine, Inc.
Sometimes, when we lead students or beginning cave explorers into their first "wild" cave, we turn out all our lights at our first rest stop. For many people, the experience of total darkness is stunning, and even a bit overwhelming. Wave a hand in front of your face, and you see nothing. Yet other senses seem to come alive. If the cave is wet, you can hear the drip, drip, drip of water across the chamber. You can smell the earth and damp air as the cave "breathes." As you grope for the comforting switch on your electric headlamp--or the flint lighter of your carbide lamp--your spatial sense, or proprioception, becomes keen and alert. If you leave the lights off for a few minutes, you might even begin to "see" how, with patience, you could learn a lot about your surroundings--and perhaps move about (if there are no deep pits nearby!) without the sense of sight at all.
Many species, of course, have lived in the total darkness of the underground for millennia, functioning perfectly well without vision. In fact, scores of troglobitic (cave-living) animals as diverse as crustaceans, insects, salamanders, and spiders have lost their eyes in the process. More than a hundred species offish living permanently in caves around the world are blind or have some degree of eye degeneration. To a biologist, those facts are fascinating--and deeply perplexing. With eyes and without eyes, a fish can see no more than you can in the perpetual darkness of a cave. But why lose them? What's more, since so many species of cave-dwelling fish, not at all closely related, have lost their eyes, the phenomenon seems to be neither an accident devolution nor an isolated event.
The loss or degradation of a trait through time is known as regressive evolution. But why is loss so fascinating, and so important to understand? Textbooks tend to focus instead on "constructive" evolution and the development of new or modified structures. A student of biology is bound to learn about the development of such novelties as legs in amphibians, hair and mammary glands in mammals, and the large, complex brain in higher primates. What the student probably does not realize is that for every new development, in all likelihood, something was sacrificed. The gills, scales, and tails that were lost by the ancestors of amphibians, mammals, and higher primates are just a few cases in point.
So why do cave fish lose their eyes? We ask the reader to stop for a moment and ponder this riddle. Whenever we pose the question to our students in introductory biology classes,...
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