Re: Population genetics question regarding sexual selection

Perplexed in Peoria wrote:
A discussion in talk.origins cross-posted here to get some
comments from population genetics experts. Joe?

I know I'm not Joe, but I'll leap into his shoes anyway....

"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:HXoJg.4132$tU.2558@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Perplexed in Peoria wrote:

"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:_5nJg.21829$gY6.19405@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx


Perplexed in Peoria wrote:

[snip]

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.

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).

Bob

--
Bob O'Hara

Dept. of Mathematics and Statistics
P.O. Box 68 (Gustaf H?llstr?min katu 2b)
FIN-00014 University of Helsinki
Finland

Telephone: +358-9-191 51479
Mobile: +358 50 599 0540
Fax: +358-9-191 51400
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Journal of Negative Results - EEB: http://www.jnr-eeb.org



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Relevant Pages

  • Re: Population genetics question regarding sexual selection
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