phillip smith wrote:
> in article bukvq6$16ja$1{at}darwin.ediacara.org, Ron Okimoto at
> rokimoto{at}uark.edu wrote on 21/1/04 5:39 PM:
> 


>>The reservations of the first poster seems to be about the accuracy of
>>determination of allele effects.  Epistasis will often reduce the rate of
>>change
>>of the allele frequency due to selection, and random events can dramatically
>>change allele frequencies in small populations.  Couple them
together and they
>>will likely increase the frequency where the less advantagous allele is fixed
>>by
>>"accident" in a population, but it would still have to
occur by accident and
>>against selection pressure.
>>
>>Ron Okimoto
>>
> 
> The question is. Is the definition of evolution as changes in gene
> frequencies  reasonable. I have no question that the frequency of phenotypic
> traits can change under selection. This not the same thing.
> 
> In small populations it seems that drift is a factor in gene frequencies so
> we can say frequencies change but we don't know why
> In large populations we can say that drift can be discounted. Providing
> epistasis is ignored.
> If you take an infinite population then it is impossible to discount
> epistasis as we have an infinite number of alleles at each locus.  So this
> is not terriblu help full.
> WE might say that the probability of epistasis is a function of population
> size and genome size.
> This seems to suggest that changes in allele in frequencies under selection
> may be a rare event
> 
But you need linkage disequilibrium (LD) as well as epistasis to change 
the effects of a single gene.  LD is reduced by recombination, so you'd 
need the interacting genes to be close to each other, or for the 
epistasis to be large, for the affect to be appreciable.

Even if you do have evolution due to epistasis, this will probably 
entail changes in gene frequencies anyway.  It's just that you have to 
consider more than one gene to understand the dynamics correctly.

Bob

-- 
Bob O'Hara

Dept. of Mathematics and Statistics
P.O. Box 4 (Yliopistonkatu 5)
FIN-00014 University of Helsinki
Finland
Telephone: +358-9-191 23743
Mobile: +358 50 599 0540
Fax:  +358-9-191 22 779
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