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".
>
>I can only think that you are using one of the terms "realistic" or 
>"independent" in an unorthodox manner.

Populations in an abstract, conceptual world range from one individual
on to up.  You can create artificial examples of populations in the
"real" world smaller than 100 individuals and you can find instances
in the very short time span just before a population went extinct
where it passed through a range of sizes varying from one hundred down
through one to zero.

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.
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