Eyes are good things to have in the light. But if you lived in the dark... all the time... would those eyes become so much a nuisance that you might lose them?
It’s not that there’s an advantage for blindess... it’s just that there’s no disadavantage to it. And eyes are complex things to make, so lots of mutations could interfere with making eyes.
A recent paper by Klaus and colleagues suggests that sometimes, blindness in a cave-dweller is an advantage, not just neutral. They examined a group of crabs (genus Sundathelphusa; pictured, showing most cave adapted at bottom) in the Philippines. These are freshwater crabs, and some live in lakes and rivers and such above ground, and some live in caves. In fact, these crabs invaded caves over half a dozen times in the genus. The repeated examples make for nice natural experiments.
Using a combination of genetics plus the shape of the animal, they found that the eyes of the cave crabs had evolved just as fast as other features. Klaus and company argue that if the loss of eyes was genuinely neutral, you would expect it to be happening more slowly than other features, which are presumably under selection. Instead, the eyes were evolving just as quickly as the other featured, which suggests there is some sort of advantage to being blind.
What the advantage might be... the authors don’t say, surprising. In the introduction, Klaus and colleagues mention the idea that losing eyes “frees up” compuational power for other sensory organs. But they don’t follow that up in the discussion. They don’t even speculate a tiny little bit in the discussion. Other papers have also suggested some sort of advantage to blindness, but as far as I know, nobody has yet come up with a testable hypothesis. That it seems to be the case with both vertebrates and invertebrates suggests that whatever that selective factor is, it is very general.
Additional, 17 April 2013: Another take on this paper is at Mostly Open Oceans.
Klaus S, Mendoza JCE, Liew JH, Plath M, Meier R, Yeo DCJ. 2013. Rapid evolution of troglomorphic characters suggests selection rather than neutral mutation as a driver of eye reduction in cave crabs, Biology Letters 9(2) 20121098. DOI: 10.1098/rsbl.2012.1098
Turning light and going blind: A tale of caves and genes
Once more into the cave