Re: Culture is not consciously developed? Q for Wilkins

[Reposting]
"Larry Moran" <lamoran@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:ej06ks$30oi$1@xxxxxxxxxxxxxxxxxxxxxx
On Wed, 8 Nov 2006 01:25:19 -0500 (EST),
John Wilkins <j.wilkins1@xxxxxxxxx> wrote:

[snip]

*Thank* you! Another way to say this is that in the theorem of
selection, as _s_ goes to zero, you are increasingly left with drift as
the default state. Drift is the absence of selection, selection is a
bias that overwhelms drift.

For a given value of s, the probability of fixation is approximately 2s.
In other words, if a new allele confers a selective advantage of 10%
(0.1s - a pretty high value) then the probability that the allele will
be fixed in a population is about 20%. Another way of saying this is that
the allele will be lost 80% of the time even though it confers a significant
selective advantage. The other allele, which is disadvantageous, wins the
fight.

Of course, this analysis assumes that the other allele, the disadvantageous
one, has an (2N-1):1 advantage in starting population.

To me, this doesn't look like selection overwhelming random genetic drift.

It doesn't if you just look at one side, and ignore the possibility of
recurrent mutations. But to look at both sides, we should also think
about the probability that a slightly disadvantageous allele can drift
to fixation replacing a slightly advantageous one. With your suggested
(admittedly large) selection coefficient of 0.1, I would guess that the
probability of replacement is miniscule in any reasonably sized population.

Anyone know the formula for probability of fixation of deleterious alleles?

It's probably not a good idea to think of random genetic drift as the
absence of selection. Instead, think of drift as the default mechanism
of evolution that occurs in all populations with all alleles, even those
with significant selection coefficients. In such cases selection and drift
are both working and drift usually wins. That's why most of evolution occurs
by accident.

In order for a new allele to be fixed by natural selection it has to confer
a significant advantage (>0.1s) *and* be lucky.

No, the advantage doesn't have to be that significant, though admittedly
if it is not then it takes more luck. The advantage should be greater
than 1/N though, if you want to have a good chance of staying fixed.

That's where most just-so
stories go wrong. Their authors think that all they have to do is imagine
*any* selective advantage, no matter how small, and that will explain why an
allele is fixed. That's not good enough. In order to have a valid explanation
you have to propose that the selective advantage is large, and that chance
played a role.

Or that the mutation is recurrent, so that you get lots of chances.



.

Relevant Pages

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