Tim Tyler wrote:
> dkomo  wrote or quoted:
> 
>>Jeffrey Turner wrote:
>>
>>>dkomo wrote:
>>>
>>>>Jeffrey Turner wrote:
>>>>
>>>>>Tim Tyler wrote:
>>>>
> 
>>>>>>Drift is most effective when population sizes are
small.  Selection
>>>>>>is most effective when population sizes are large. 
I reckon this 
>>>>>>fact (in conjunction with nature's population
sizes) will often 
>>>>>>limit's drift's usefulness as an explanation for features of 
>>>>>>organisms.
>>>>>
> 
> [...]
> 
> 
>>>>1.  The jar will contain 100 red marbles.
>>>>2.  The jar will contain 100 blue marbles.
>>>>3.  The jar will contain a mix of red and blue marbles in
>>>>        approximately the same ratio as it had initially.
>>>>4.  It's not possible to predict the marble ratios.
>>>>5.  None of the above.
>>>>6.  I don't know.
>>>>7.  Who cares?
>>>>
>>>>Note that if the number of either the red or blue marbles
reaches 100,
>>>>no further change is possible because "mutations"
are disallowed
>>>>(red->blue, or blue->red during reproduction).
>>>
>>>Off-hand, I'd say 3 is probably correct.  There is a finite
>>>probability of any other distribution, however.
>>
>>The answer is (5) and is given in the thread in which the original
>>post appeared:
>>
>>http://tinyurl.com/2crm3
> 
> 
> [...]
> 
> 
>>The main thing I wanted to point out with this example is that a 
>>statistical process can drive an allele distribution in a population 
>>either to fixation, where the frequency of the allele becomes 100%, or 
>>to extinction, where it disappears from the population.  And such a
>>process is independent of any selection pressure -- that is, the
>>selection coefficient of the allele is near zero.
> 
> 
> I think you mean "*IF* the selection coefficient of the allele is near
> zero"  Large selection pressures are not irrelevant - and would
> change the answer to the problem from 5 to 1 or 2.
> 
> 
>>>Is one hundred individuals a realistic population?
>>
>>According to what people have stated here on sbe, subpopulations this
>>small or smaller are the *norm* for genetics.
> 
> 
> For the genetics of small populations on the verge of extiction?
> 
> The quantitly of relevance of the calculaton is known as the
> "effective populaton size".
> 
> An effective population size of 500 is considered to be a threshold
> for status as an endangered species.
> 
> The figures you are talking about (in isolated populations) would
> represent effective population sizes of 25-50 - putting them straight
> into the red, flashing region of the endangered species list.
> 
No, Ne would be 100 - the population described is behaving as an ideal 
population.

Bob

-- 
Bob O'Hara

Dept. of Mathematics and Statistics
P.O. Box 4 (Yliopistonkatu 5)
FIN-00014 University of Helsinki
Finland
Telephone: +358-9-191 23743
Mobile: +358 50 599 0540
Fax:  +358-9-191 22 779
WWW:  http://www.RNI.Helsinki.FI/~boh/
Journal of Negative Results - EEB: http://www.jnr-eeb.org
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