"Jim McGinn"  wrote in message
>
> What causal forces--if any at all--are associated with
> an allele becoming fixed?  (Think carefully before you
> answer.)
>
Is this a trick question? Say we have a population of mice. One mouse has
the only allele in the population for a furry tail. It is recessive so it
isn't expressed in his phenotype.
One day he and another mouse emerge from a burrow. A hawk swoops. Our mouse
runs for the bushes at the right, the other mouse runs for the long grass at
the left. The hawk swerves right - end of the furry tail allele.
The hawk's decision to swerve right has caused the elimination of the furry
tail allele, and the hawk's decision was itself caused by who knows what.
However from our perspective we can ignore the myriad causes of alleles
being eliminated or passed on independently of their selective power. We
just say "random chance has eliminated the furry tail allele".
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