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

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

"We desperately need our exercise and beauty treatments, so we can have bodies like Brad Pitt and Salma Hayek," we joke with our students. We're certain they will greatly enhance our fitness. Yet regardless of our needs and all our efforts, our children have not inherited those characteristics. Please." we urge the students, "see if you can come up with a Darwinian mechanism to explain why cave fish lose their eyes--one that involves natural selection."

So how did Darwin slip into Lamarckism? Cave organisms are ideal models to support Darwin's idea of natural selection. In the dark, organisms with the more acute sensory organs should survive and reproduce preferentially. Hence, with time, the population should evolve such characteristics as longer antennae, longer legs, and additional organs rich in sensory receptors--just as, in fact, they do. Nevertheless, Darwin was stumped over just how the same kind of mechanism could work for regressive evolution. Reluctantly, he invoked disuse:

By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennae or palpi, as a compensation for blindness.

But that conclusion irked Darwin. He continued, no doubt in frustration, and fell back on a Lamarckian explanation: "It appears probable that disuse has been the main agent in rendering organs rudimentary. It would at first lead by slow steps to the more and more complete reduction of a part,... as in the case of the eyes of animals inhabiting dark caverns." Although Darwin found that conclusion unsatisfactory, in the end, he admitted defeat:

It is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless. Some additional explanation is here requisite which I cannot give.

Can biologists today do any better? In a cave, what advantage, if any, does a fish without eyes have over a fish with eyes? Our students typically bring up efficiency and the idea of conserving energy. Maybe blind fish do not have to waste valuable energy on making useless eyes.

Yet that explanation won't work either. The energy needed to make an eyeball, which is basically a globule of protein that encapsulates some water, is probably much less than what is needed to make the plug of fat and bone that covers the same space in blind cave fish. As every dieter knows, there are more calories in a gram of fat than in a gram of protein, and even more calories in fat than in water. Growing a complex structure such as the vertebrate eye seems to require less energy than growing a fatty plug over a degenerated eye. Complexity is not always synonymous with energy expenditure.

Biologists today endorse two principal but competing hypotheses to account for the evolution of degenerated eyes. The first hypothesis assumes that the loss of eyes somehow enhances the efficiency of neural processing or reshapes the fish's morphology and physiology to better suit a life of total darkness. As a consequence, natural selection drives the regressive evolution.