Re: The Cost of Substitution [possible REPOST]
- From: Guy A Hoelzer <hoelzer@xxxxxxx>
- Date: Wed, 3 Jan 2007 13:47:00 -0500 (EST)
in article emejef$2tm6$1@xxxxxxxxxxxxxxxxxxx, seemysig@xxxxxxxxxxxxxx at
seemysig@xxxxxxxxxxxxxx wrote on 12/21/06 10:23 AM:
Guy A Hoelzer wrote:
Tim Tyler at seemysig@xxxxxxxxxxxxxx wrote on 12/18/06 10:00 PM:
One way of measuring fitness is as a count of immediate offspring.
This is a pretty good proxy for the notion of fitness.
For a given species, that seems to be about as material as
count of dollars to me.
Absolutely. I am just trying to make more concrete the claims being made in
the debate about a cost imposed by the fixation of alleles in populations.
Are you saying the cost comes in the form of immediate offspring?
The lack of them, relative to some other members of the population.
In other words, when a fixation happens individuals are somehow compelled to
produce more offspring as a sort of compensation?
I do not see how that is 'in other words'.
The scenario where the 'cost' is being paid (in reduced fertility of
some
members of the population) is *before* fixation happens - and some
members of the population have the advantageous gene while others do
not.
First, I see no reasonable basis to ascribe the deaths or poor reproductive
performance of some individuals to the possession of an "advantageous"
allele by others. I don't think we have any idea as to how often there is
any sort of direct or indirect cause and effect in those dynamics.
I can also imagine a scenario where selection rapidly pushes an allele to
fixation without ANY demographic cost at the population level. Imagine a
population with some variance in reproductive success among individuals
prior to the origin of an advantageous mutation. Some individuals have lots
of offspring and others have none in this population with a stable size.
Now the advantageous mutation occurs. It could be driven rapidly to
fixation by biasing the reproductive success of the individuals carrying the
mutation to perform at the high end of the reproductive success spectrum,
while other individuals compensate by shifting to other positions in the
spectrum without changing the shape of the spectrum at all. This
advantageous allele will go quickly to fixation without any change in the
distribution of reproductive success for the population. In other words,
there is no cost at all to the population in this scenario.
Cheers,
Guy
.
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