On Mon, 31 May 2004 17:25:45 +0000 (UTC), 
Tim Tyler  wrote:

> I've written an essay about genetic drift and population size.
> 
> This essay is intended to dispell some of the myths (recently
> expressed on other threads in this forum) suggesting that the
> effects of genetic drift are independent of the population size.

The rate of substitution of alleles in a population by random genetic 
drift alone is independent of population size. The rate of substitution
of beneficial alleles alleles by natural selection depends on population 
size (N). This rate also depends on the selective advantage (s) of an 
allele and on the rate of formation (u) of beneficial alleles.

The formula is 

                 K  = 4 Nsu

> Supposedly this is common knowledge - but in fact it's nonsense.

No, it's correct common knowledge that the rate of gene substitution
by random genetic drift alone is independent of population size.

> The mix-up seems to arise from getting genetic drift muddled
> up with neutral evolution :-(

Neutral mutations are the only ones that aren't affected by natural
selection. I'm not confusing the effects of random genetic drift
with neutral theory. I'm well aware of the fact that there are alleles
affected by natural selection.

> ``Genetic drift and population size''
> 
>  - http://alife.co.uk/essays/genetic_drift_and_population_size/
> 
> This page shows the results of experimental modelling of the 
> probability of fixation of near neutral alleles by genetic
> drift in populations of various sizes.

When natural selection operates, the rate of substitution depends on
population size. This is true whether the selective advantage (s) is 
very small (i.e. near neutral) or very large.

> It illustrates pretty clearly that the rate of fixation of
> alleles due to genetic drift is usually a function of the
> population size.

Your data shows that the probability of fixation of a deleterious allele 
depends on population size. This is natural selection in action. 

> The fixation rate of slightly-deleterious alleles by genetic drift is 
> *dramatically* reduced in large populations - since there the effects 
> of genetic drift on near-neutral alleles are more powerfully and 
> effectively opposed by selective forces. 

Yes, when natural selection operates, population size is important. 
The question you didn't ask is what percentage of mutations have
selection coefficients that are large enough to ensure fixation in
reasonable-sized populations. You also need to ask whether there's 
any evidence that the nucleotide substitutions we see in most DNA 
could possibly be slightly advantageous or slightly deleterious.

> Enjoy,

I did. Thanks.




Larry Moran
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 6/1/04 6:50:24 AM