Xref: utzoo sci.bio:1996 talk.origins:4930
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!ogil
From: ogil@tank.uchicago.edu (Brian W. Ogilvie)
Newsgroups: sci.bio,talk.origins
Subject: Re: The birds and the beaks
Keywords: evolution, birds, beaks
Message-ID: <2714@tank.uchicago.edu>
Date: 13 Apr 89 18:16:07 GMT
References: <28330@apple.Apple.COM> <3561@aplcomm.jhuapl.edu> <28379@apple.Apple.COM> <3566@aplcomm.jhuapl.edu> <28434@apple.Apple.COM> <3576@aplcomm.jhuapl.edu> <273@acates.UUCP>
Reply-To: ogil@tank.uchicago.edu (Brian W. Ogilvie)
Organization: History of Science, University of Chicago
Lines: 34

In article <273@acates.UUCP> dan@acates.UUCP (Dan Ford) writes:

>What might be going on: In the above-ground population there is a constant
>(but small) mutation rate of albinos.  On the surface this is very bad, and
>is quickly selected against.  Underground, however, it is not a disadvantage
>and may continue in the gene pool.  But, not only does it continue, but it
>apparently is selected FOR since you end up with all these albino animals.
>How can coloring possibly be selected for in a dark environment?  Why a
>repeated selection of albino in an environment where color is meaningless?

The situation could arise from mutation pressure without selection. The
biochemical pathway involved in producing color (skin color, hair color,
chitin color, etc.) is likely to be very complicated. If, say, twenty
genes are involved in producing skin color (this is roughly the number
involved in guinea pigs, I think), then a mutation in any one of these genes
resulting in a nonfunctional protein would produce albinism (or a noticeable
paling, or some other effect). Since a mutation from functional to non-
functional is more likely than the other way, the net effect of unselected
mutation would be partial to complete albinism.

Assuming that the production of pigments takes some amount of energy (a 
quite reasonable assumption), then selection would be active in a dark
environment, since those mutations which prevented the pigmentation
system from operating would conserve the organism's energy. Such mutations
would probably occur in the operator or promoter regions of the various
genes involved, although large deletions would also reduce the energy
devoted to pigment production. It would be interesting to compare the
pigmentation systems of cave-dwellers and closely related surface-dwellers,
though I'm not certain that it would be worth the genetic work involved.

-- 
Brian W. Ogilvie  /  ogil@tank.uchicago.edu
"Cartesianism is the most popular 'popular science' ever invented."
					--Noel Swerdlow