Blind Cave Fish and Regressive Evolution

I remember learning about blind and pale cave creatures when I was very young. And I also remember teachers and TV shows explaining that the creatures lost their eyes and skin pigment due to disuse in the lightless environment of the cave. This exlanation however, cannot be correct, because evolution doesn't work that way.

A more likely scenario for how such "unused" traits (eyes and skin pigment) degenerate, is that they are being actively selected against. In the case of eyes, we can imaging how they might be selected against because they are fragile structures which could easily get damaged and infected by bumping against things in the dark (thus killing the organism). However, skin pigment is less easy to explain, unless it also imparts some disadvantage in the dark.

The following article suggest some other interesting possibilities.

http://www.upi.com/view.cfm?StoryID=31052002-032546-7591r

In particular, I found the idea of MultiGenic Traits very interesting:

"The researchers found two things of particular interest. First, each of the traits they studied had more than one genetic cause. They found three genes on different chromosomes -- "unlinked" genes, in genetic terminology -- that affect eye development. They found two other genes causing pigmentation loss and two more that change metabolism.

Perhaps just as important, these genes turned up in a search of only half the chromosomes, so it is likely even more remain to be discovered. That means all of the cave-related traits studied are multigenic, or caused by multiple genes. Multigenic traits, though poorly understood, are of great interest. They include many human conditions of medical significance, such as late-onset diabetes, various cancers, heart disease, and obesity.

Borowsky and Wilkens also discovered one of the genes affecting eye size is closely linked on its chromosome with one of the metabolism genes, and one of the pigmentation genes is closely linked to the other gene affecting metabolism.

This strong association between neutral and selected traits suggests a solution to the puzzle of regressive evolution. Neutral traits, such as blindness in cave-dwelling populations, could become rapidly established by being pulled along by selection for valuable traits, such as efficient metabolism, if the genes are closely linked or identical.

An intriguing possibility, suggested by this study, is each pair of closely linked genes could be a single gene affecting both metabolism and a neutral trait. The researchers have not yet determined whether the pairs of traits are caused by two different genes very near to each other on the chromosome or are multiple effects of a single gene."