Re: Population genetics question regarding sexual selection
- From: "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx>
- Date: Fri, 1 Sep 2006 01:20:27 -0400 (EDT)
"Anon." <bob.ohara@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:ed7oit$11ls$1@xxxxxxxxxxxxxxxxxxxxxx
Perplexed in Peoria wrote:
A discussion in talk.origins cross-posted here to get someI know I'm not Joe, but I'll leap into his shoes anyway....
comments from population genetics experts. Joe?
"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:HXoJg.4132$tU.2558@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Perplexed in Peoria wrote:[snip]
"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:_5nJg.21829$gY6.19405@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Perplexed in Peoria wrote:
There are three ways a tail affects male fitness. First, it
really is (by assumption) a handicap - it decreases survival
chances over the period from birth to sexual maturity. Second,
due to the non-linear feature that Harshman insists on,
it is correlated (post-survival-selection) with other positive
genetic traits. [Third way - sexiness to females - snipped]
Actually, a non-linear relationship is not necessary for this to be
true. The statistical expectation for all surviving males is that they
will have some mean "survivability". Since the long tail is costly in
terms of survival, the expectation is that long-tailed birds will have
compensating average increased levels of "quality".
I think you are wrong here. Without non-linearity, there would be
no compensating correlation. The surviving long-tailed birds would
not be particularly high in quality - at least no more so than the
surviving short-tail birds. Both are impacted equally by quality,
if there is no non-linearity.
You have to remember that "quality" is specifically defined so as to
ignore the cost of the long tail. So if the expected survivability of
all surviving birds is S, the cost of long tails is C, and the quality
is Q, then for short-tailed birds S = Q, while for long-tailed birds S =
Q - C. But since we agreed that the expected S is the same for long- and
short-tailed birds, ...
I agreed to no such thing.
... Q(short-tailed) = Q(long-tailed) - C. And thus the
long-tailed birds have higher expected quality. What you have noticed is
that expected S is the same for all birds, long-tailed and short-tailed.
I notice no such thing.
But Q is indeed higher for the long-tailed birds.
I'm really quite surprised that you are making such an elementary
mistake. If selection on the tail handicap and selection on
'quality' are independent (which is another way of saying that
no non-linearity exists)
Ah, this is your mistake: you've mis-understood linearity...
In fact, it is the linear response that's important: if there is a
correlation between the two traits, and selection on both of them, then
the response depends on both traits.
The theory was outlined in this paper:
lande, R. & Arnold, S.J. (1983) the measurement of selection on
correlatedcharacters. Evolution 37: 1210-1226.
You have confused me. I haven't looked at the cited paper yet, but ...
The question (as I understood it) was whether simultaneous
selection on two traits uncorrelated in the pre-selection
population can create a correlation post-selection. You seem
to be discussing the question of whether partial fitnesses
can be simply multiplied if there is already a correlation before
selection.
Well, OK. I can see that an assumption of pre-selection
non-correlation might be unjustified in an argument about
sexual selection theory.
then selection will not create a
correlation between these traits.
This is true, but not the whole story: selecion could be independent,
but the traits themselves could be controlled by common genes (i.e.
pleiotropy).
Hmmm. I've really got to take the time to dig in and learn
quantitative genetics. But in my current unenlightened state,
it seems to me that pleiotropy is just an extreme case of close
genetic linkage. Hence, it affects only inheritance, not selection.
Well, I guess that becomes relevant in that it means that a
post-selection correlation in one generation will still be there
as a pre-selection correlation in the next generation.
While I thank you for your response, I would like to ask that
you expand on the "it is the linear response that's important"
above. A linear response is important to whom or for what?
I had been focused on whether the female is justified in taking
a long tail as evidence of 'quality'. But linearity may be
important at some other stage of the whole sexual selection
argument - a stage I was not focused on.
.
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