Re: Selection Pressure
- From: "Walter ReMine" <science@xxxxxxxx>
- Date: Fri, 20 May 2005 18:46:24 -0400 (EDT)
Anon. (Bob O'Hara) wrote:
> Walter ReMine wrote:
>> Perplexed in Peoria wrote:
>>>.... The answers of Tim and Bob, especially, indicated
>>>to me that the term was meant to be used in a directional
>>>sense. There is selective pressure in a direction; high
>>>pressure means large selection coefficients for or against
>>>some trait, and hence rapid response to selection.
>>
>> That is one logical interpretation of "selection pressure".
>> Though there is another, and it is exemplified in Haldane's
>> use of the term "intensity of selection". That is, it can
>> also mean selection for MANY independently substituting
>> alleles simultaneously each with a LOW selection coefficient,
>> (which is how Haldane used the term in his 1957 paper, and
>> equates to a high cost of substitution per generation).
>>
> I don't think these have to be contradictory interpretations.
I agree, they do not have contradictory interpretations. However, they
have alternative interpretations, and Haldane's usage in his 1957 paper
was limited to one of those interpretations. That is, Haldane used the
term "intensity of selection" to mean selection for MANY independently
substituting alleles SIMULTANEOUSLY, where each one has a very LOW
selection coefficient.
Many people make the mistake of thinking Haldane 1957 was referring to
a small number of alleles with a HIGH selection coefficient. That would
be a valid use of the term "intensity of selection", however that was
not how Haldane used it in that paper.
Note: When a scenario invokes high selection coefficients, the total
cost of substitution increases, and (other things being equal), this
will REDUCE the substitution rate over the long-run. That is, the
individual substitutions will fixate faster, but the overall
substitution rate, which is the focus of concern in Haldane's argument,
will be lower. This is the mystery that got people on this thread
wondering about "intensity of selection" in the first place.
-- Walter ReMine
Haldane's Dilemma
http://www1.minn.net/~science/Haldane.htm
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