"Tim Tyler"  wrote in message
news:c9b74j$12f$1{at}darwin.ediacara.org...
> Perplexed in Peoria  wrote or quoted:
> > "Tim Tyler"  wrote in message
> > > Larry Moran  wrote or quoted:
> > > > The overall rate and extent of evolution by random
genetic drift is
> > > > independent of population size. [...]
> > >
> > > You're the expert on this, Larry - but I don't know how you can
> > > possibly expect to get away with making assertions like that ;-)
> >
> > What Larry says is true, if by "the rate of evolution" you
> > mean the number of base-pair changes that become fixed
> > in a given period of time.  That is the natural definition
> > of "rate" to use if you are into molecular taxonomy.  This
> > theorem - that rate is independent of population size - was
> > proved long ago by Kimura and independently by King and Jukes.
>
> Surely there's a significant misunderstanding here:
>
> Kimura's theory dealt with neutral alleles.
>
> The term "random genetic drift" refers to changes in the frequency of
> alleles which are caused by chance.
>
> Those alleles do not *have* to be neutral to be subject to drift -
> just close enough to neutral for randomness to stand a chance
> of influencing them.
>
> When the alleles in question are *not* neutral - but instead are
> nearly neutral - the chance of them being eliminated (or fixed)
> in the population depends on the balance between statistical
> fluctuations (i.e. drift) and selection.
>
> In large populations, that gives selection an advantage,
> while in small populations, small selective advantages
> are easily swamped by chance.
>
> In other words, in a large population, an allele is much
> less likely to be effectively "neutral" in the first place -
> and so Kimura's theory is likely to be relevant at a much
> smaller number of loci.
>
> That's basically why you get founder effects in small populations
> and stability in larger ones - there are fewer effectively-neutral
> alleles - and thus reduced possibilities for genetic drift - in
> larger populations.
>
> Kimura's result would still apply to any exactly-neutral alleles.
>
> However, the effects of genetic drift are *not* confined to
> *exactly*-neutral alleles - and on *near*-neutral alleles, the
> result about independence from population size no longer applies.
[Snip the rest.  I think I understand your point]

You raise an interesting point.  How neutral is "neutral enough"?

Kimura has a formula for this too.  That is, the theory doesn't
just apply to exactly neutral alleles (if such things even exist).
I don't have the formula in front of me, but I believe that you
are correct that the threshold (in "fitness" or in a selection
coefficient) for "neutral enough" depends upon the population size.
Perhaps Larry can provide it.

However, I suspect that a confirmed neutralist would reply
that your point doesn't make any difference, because even in very
large populations, the vast majority of non-deleterious
mutations are "neutral enough", so the fact that the majority
is "even vaster" in a small population isn't noticible in
the proportions.

That is, the neutralist would disagree with your claim that
  in a large population, an allele is much less likely to be
  effectively "neutral" in the first place - and so Kimura's
  theory is ... relevant at a much smaller number of loci.

The neutralist would say that it is only a little less
likely to be effectively neutral.  However, he might agree
with you if you said that in a large population, an allele is
much more likely to be non-neutral.  A 5% chance is much bigger
than a 1% chance, even though 95% is only a little less than 99%.

But I am pulling these numbers out of the air.  We are
talking about an empirical question now, not a theorem
of population genetics.  I don't know what the true numbers
are.  Again, perhaps Larry would care to provide his
estimates.
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