On Mon, 5 Apr 2004 16:10:30 +0000 (UTC),
lamoran{at}bioinfo.med.utoronto.ca (Larry Moran) wrote:

>On Mon, 5 Apr 2004 05:22:05 +0000 (UTC), 
>Jeffrey Turner  wrote:
>> Larry Moran wrote:
>>> Jeffrey Turner  wrote:
>
>[snip]
>
>>> I suspect the real problem is yours. You are confused about the 
>>> difference between "spandrels" and random genetic drift as a 
>>> mechanism of evolution.
>> 
>> Random genetic drift doesn't occur fast enough to account for very
>> much in evolution.  You have yet to seriously challenge my assertion
>> on that.
>
>Let's consider the simple case of neutral alleles becoming fixed in
>a population by random genetic drift. The rate of substitution (K) of 
>neutral alleles in a population is equal to the mutation rate (u).
>
>                        K = u
>
>Note that the rate of substitution is independent of population size.
>This result has been discussed many times on sci.bio.evolution and it's
>well-described in the textbooks.
>
>Let's see what this means for mammals with a genome size of 3 billion
>base pairs per haploid genome. If the mutation rate is 10^-9 per base 
>pair in each generation then the overall mutation rate per diploid 
>genome is about 6 mutations per generation. Thus, in a typical mammalian
>species there are about 6 new alleles becoming fixed in the population
>by random genetic drift every single generation. 
>
>Is that fast enough for you? How does it compare with the overall rate
>of fixation of alleles under natural selection? I assume you know that
>number or you wouldn't have made your claim.
>
>(Note, there are many simplifications in such population genetics models
>but the basic point is valid. It may take a long time for any one mutation
>to become fixed but there are so many alleles that the net overall effect 
>of chance processes is quite remarkable. The quoted rate includes 
>mutations in junk DNA - we could adjust for mutations in codong regions 
>genes by dividing by 100.) 
>


All this is true, according to what I can recall of population
genetics.  However the time to fixation is enormous for a population
of any size.  Since the number of generations that have occurred in
the human species since its origin as H. sapiens sapiens is much
smaller than the (current) population size, wouldn't that mean that
probably none of the mutations are actually fixed yet?  A lot of pop
gen talks about equilibrium situations but how often do populations
actually have enough time to reach equilibrium?

My questions are certainly irrelevant to the original question.
Whether fixed or not, there are definitely a lot of mutant alleles
floating around in humans, all either absolutely neutral or so close
to neutral (such a small fitness difference) that you might as well
call it neutral. And there are certainly enough instances of
bottlenecks and founding populations in species where genetic drift
can work much more rapidly.

I don't understand why there should be any controversy about the fact
of drift (Edser's epistemology notwithstanding).  Just how significant
it is in any particular situation compared with selection is another
story where I can understand disagreement.
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