Following up PZ’s previous post about blind fish, I want to quote a section of R. A. Fisher’s “The Genetical Theory of Natural Selection”. Fisher is credited as being one of the founders of modern evolutionary biology, which drove him to create modern statistics.
This quote begins on page 128 of the 1999 complete variorum edition. I think it was originally written in 1930.
The power of the means of dispersal alone, without the necessity for selective discrimination in either region, is excellently illustrated by the theory, due to Ray Lankester, which satisfactorily accounts for the diminution or loss of functional eyes by the inhabitants of dark caverns. Ray Lankester pointed out that the possession of the visual apparatus is not merely useless to such animals but, by favoring their migration towards sources of light, will constantly eliminate them from the body of cave inhabitants, equally effectively whether they survive or perish in their new environment. Those which remain therefore to breed in the cavern are liable to selection in each generation for their insensibility to visual stimuli. It should be noted that with such very restricted habitats migrational selection of this sort might attain to very high intensity in consequence produce correspondingly rapid evolutionary effects.
Very interesting perspective isn’t it?