Re: The Cost of Substitution [possible REPOST]

Guy A Hoelzer wrote:
seemysig@xxxxxxxxxxxxxx wrote on 12/19/06 11:07 PM:
Guy A Hoelzer wrote:

I didn't say that fixation happened faster under drift than under
directional selection. I just said that your point about fixation
taking longer in larger populations under drift also applies to
selection. Your response supports this point. I will add
that drift happens universally and no matter what the
population size the combination of drift and directional
selection leads to a faster time to fixation than does selection
alone.

This seems to depend somewhat on your point of view:

Drift can lead to the allele sometimes not reaching fixation at
all. I'd expect the average time to fixation to depend on what
sort of penalty you assigned to that case. If you treat it as
time_to_fixation=infinity, the average time goes up. If you
delete it from your calculations, the average time goes down.

If you simply simulate these conditions using something
like Joe's PopG program, you can run a selection model with
and without drift. You will find that the fixation of
alleles is almost always (not always) faster when drift is
involved. Of course, this does not account at all for the
set of alleles that drift to extinction while they are
rare. This is typically the majority of alleles, even the
advantageous ones. I was focusing on those advantageous
alleles that do ultimately go to fixation.

Right. So you are considering random fluctuations
when alleles are common (where they help fixation
happen faster) and are ignoring them when alleles
are rare (where they prevent fixation happening
at all). That explains your perspective.

Furthermore, the expected temporal path to fixation under selection
is very sensitive to dominance. In fact, selection alone
is incapable of fixing a dominant advantageous allele,
although this model approaches fixation much more quickly
than a model of a recessive (or additive) advantageous allele.

What - if you ascribe all effects due to a finite population
size as being due to drift?

When comparing the behavior of models assuming finite vs.
infinite population sizes, all else being equal, I ascribe
the differences in model behaviors to drift. The models
are designed so that the influence of selection is the same
in both cases.

In an infinite spatially distributed population, a mutation at
one location will take forever to reach fixation - whereas it
may rapidly reach fixation in a finite population.

I claim that difference has nothing to do with drift - and
that describing it as being an effect of drift would
simply be confusing.

That does not seem helpful to me. Advantageous alleles are
quite capable of going to fixation without any randomness or
sampling error being involved.

I disagree, although there is a nagging semantic issue here
due to dominance.

Yes: my statement made no mention of dominance.

An "advantageous" dominant mutation may be propelled by
selection to a very high frequency. However, before it
achieves a frequency of 100% the population becomes devoid
of homozygous recessive genotypes. All of the recessive
alleles become wrapped up in heterozygous genotypes. At
this point, the dominant allele no longer confers any
fitness advantage to any individual.

Assuming that a single dose of the allele is
precisely as good as a double dose. As far
as I know, that is not implicit in describing
an allele as being a 'dominant advantageous allele'.

I can at least now see what you meant.
--
__________
|im |yler http://timtyler.org/ tim@xxxxxxxxxxx Remove lock to reply.


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