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From: mrh@cybvax0.UUCP (Mike Huybensz)
Newsgroups: sci.bio
Subject: Re: Blind cave fish
Message-ID: <1343@cybvax0.UUCP>
Date: Fri, 9-Jan-87 14:51:52 EST
Article-I.D.: cybvax0.1343
Posted: Fri Jan  9 14:51:52 1987
Date-Received: Sat, 10-Jan-87 02:12:42 EST
References: <1340@cybvax0.UUCP> <2526@ecsvax.UUCP>
Reply-To: mrh@cybvax0.UUCP (Mike Huybensz)
Distribution: na
Organization: Cybermation, Inc., Cambridge, MA
Lines: 24

In article <2526@ecsvax.UUCP> emigh@ecsvax.UUCP (Ted Emigh) writes:
> The first explanation -- lack of selection FOR eyes --
> needs to have a small population size in order to be effective (random
> drift).  An example I use in my Human Genetics class is to compute how
> long it takes for an allele to decrease in frequency from 100% to 20%
> BY MUTATION ALONE.  For a mutation rate of 1X10^(-5) (considerably higher
> than most mutation rates in humans) it takes about 150,000 generations.
> The time it needs to operate by mutation alone is too long for the various
> cave animals to lose eyes by mutation alone.

There are two reasons why I don't think you can dismiss a degradation
hypothesis so simply.  First, because some cave populations might be ancient
enough, and second because your counter-example makes the assumption of a
single gene when the actual number of genes necessary to the developmental
pathway might be quite large.

These hypotheses could be distinguished by genetic comparison of cave animals
with closely related outside populations.  If blindness is favored by natural
selection, then we would expect to see a very few alleles responsible for the
blindness.  If it is a result of degradation, then we would expect to see
many alleles.
-- 

Mike Huybensz		...decvax!genrad!mit-eddie!cybvax0!mrh