r norman  wrote in
news:c9nhg4$ikb$1{at}darwin.ediacara.org: 

> On Wed, 2 Jun 2004 20:39:54 +0000 (UTC), Tim Tyler 
> wrote:
> 
>>Larry Moran  wrote or quoted:
>>
>>> For beneficial alleles that arise in a population the probability of
>>> fixation can be approximated by 
>>> 
>>>                      P = 2s
>>> 
>>> where s is the selective advantage. This equation holds for small
>>> values of s (s < 0.05) and populations greater than about 100
>>> individuals. It is pretty much independant of population size for
>>> realistic populations. 

>>AIUI, population sizes in the real world range from one individual up
>>to billions of them.  Probability of fixation for a particular,
>>non-neutral allele can range from around 0.5 in small populations to 
>>0.0001 in large ones.

>>So - it seems rather misleasing to assert that the probability of 
>>fixation in "realistic populations" is "pretty much
independent of 
>>population size".

(snip)

> What is your favorite endangered species?   Snow Leopard, Siberian
> Tiger, Whooping Crane, Cheetah, California Condor?  These all number
> in excess of 100.  How long do you think a population can remain less
> than 100, especially down in the low dozens or single digits where the
> population size really does make a difference for fixation?  What is
> the probability of extinction for such populations? 
 
> Real life science uses weasel words like "realistic population size"
> and "pretty much independent" to indicate what is useful to real field
> biologists doing real observational studies of real populations living
> in the real world.  And fewer-than-100 doesn't "really" occur. Larry
> Moran's claim is quite clear, specific, and correct.
 

I have recently been reading Wilson's "Sociobiology", in which he 
discusses effective population size. I quote: "the effective population 
numbers of the few real populations measured so far have generally turned 
out to be low", often in the range of 50-100. Of course, this small 
effective size implies that sampling error will be quite significant - so 
I appear to be disagreeing with you and Larry about the probability of 
fixation due to selection  being independent of population size but at 
the cost of greatly increasing the probability of drift causing fixation 
of even non-neutral mutations. To paraphrase the Wicked Witch - You 
cursed brat! Look what you've done!I'm drifting! Drifting! :-)


Yours,

Bill Morse
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