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From: emigh@ecsvax.UUCP (Ted Emigh)
Newsgroups: sci.bio
Subject: Re: Blind cave fish
Message-ID: <2526@ecsvax.UUCP>
Date: Thu, 8-Jan-87 15:16:40 EST
Article-I.D.: ecsvax.2526
Posted: Thu Jan  8 15:16:40 1987
Date-Received: Thu, 8-Jan-87 22:48:45 EST
References: <741@aecom.UUCP> <927@husc6.UUCP> <124@bcsaic.UUCP> <1340@cybvax0.UUCP>
Reply-To: emigh@ecsvax.UUCP (Ted Emigh)
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Organization: NC State University
Lines: 34

In article <1340@cybvax0.UUCP> mrh@cybvax0.UUCP (Mike Huybensz) writes:
>
>I would attribute blindness of cave animals (something which has arisen many
>times independently) to accumulation of mutations which normally would be
>selected against.  When selection pressure is eliminated by a dark habitat,
>developmental mutations that cause blindness can accumulate, where normally
>they would be immediately eliminated.  Without selection pressure, there is
>no limit to the number of mutations that can accumulate.
>
>It is also possible that there is some advantage to eliminating eyes, in
>terms of vulnerability to injury, disease, etc., or simply in reduced
>energy costs.
>

So far, OK.

>Both of these are independent of population size, and thus not the same as
>genetic drift.
 
The second explanation -- selection for eyelessness -- works independently of
population size.  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.

-- 
Ted H. Emigh     Genetics and Statistics, North Carolina State U, Raleigh  NC
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