Re: Population genetics question regarding sexual selection

Jim,

I'm glad to see that you are interacting with John on talk.origins. He used
to be a frequent contributor to sbe and he is a good evolutionary biologist.

My take on your debate is that you are both right. We can blame Darwin for
artificially separating sexual selection from natural selection. I
understand why he did this, and I think his strategy was successful in its
realized heuristic value. However, it has become clear over the past 30
years or so that this is a false distinction. Sexual selection is just a
form of natural selection that operates within a population without the
source of selective pressure emanating from the external environment.

Models of the evolution of epigamic traits that honestly signal overall
quality of some sort were originally derived based on the Darwinian
tradition. Therefore, the logic of the models started with the assumption
that epigamic traits are ornaments that evolved through a process that was
different than, and independent of, natural selection. This meant that the
fitness effects of epigamic traits were treated as additive to fitness
effects of traits evolved in response to natural selection. These models
then predicted that an epigamic trait will be favored if it yields a net
increase in fitness (typically approximated as lifetime reproductive
success) by increasing reproductive output sufficiently to offset the cost
in viability.

Of course, these are just models that make simplifying assumptions, like
additivity. Your point about non-linearity is right on target, but the
validity of the models does not rest on the accuracy of their assumptions;
rather it depends on how robust the predictions of the models are to
relaxation of those assumptions. I think John is trying to explain the
logic of the models, while you are pointing out an important point about
reality.

Cheers,

Guy


in article ed5v0r$63c$1@xxxxxxxxxxxxxxxxxxx, Perplexed in Peoria at
jimmenegay@xxxxxxxxxxxxx wrote on 8/30/06 11:19 PM:

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

"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) then selection will not create a
correlation between these traits. Your argument to the contrary
is just wrong, but I don't see any quick-and-easy argument to
convince you of this. One or the other of us needs to do some
rethinking. I will leave it to any lurkers to suggest which of
us ought to do this rethinking.

I'm going to cross-post this to sci.bio.evolution to see if
we can get Joe Felsenstein to comment.

Snip remainder, as we are mostly disagreeing on terminology or
on how to partition up the causality.




.

Relevant Pages

  • Re: Population genetics question regarding sexual selection
    ... The statistical expectation for all surviving males is that they ... Both are impacted equally by quality, ... If selection on the tail handicap and selection on ... correlation between the two traits, and selection on both of them, then ...
    (sci.bio.evolution)
  • Re: Population genetics question regarding sexual selection
    ... The statistical expectation for all surviving males is that they ... The surviving long-tailed birds would ... Both are impacted equally by quality, ...
    (sci.bio.evolution)
  • Re: Sexual selection of disadvantageous traits
    ... The statistical expectation for all surviving males is that they ... The surviving long-tailed birds would ... Both are impacted equally by quality, ... That was the entire original assumption of the handicap principle. ...
    (talk.origins)
  • Re: Sexual selection of disadvantageous traits
    ... The statistical expectation for all surviving males is that they ... The surviving long-tailed birds would ... Both are impacted equally by quality, ... That was the entire original assumption of the handicap principle. ...
    (talk.origins)
  • Re: Sexual selection of disadvantageous traits
    ... The statistical expectation for all surviving males is that they ... The surviving long-tailed birds would ... Both are impacted equally by quality, ... ignore the cost of the long tail. ...
    (talk.origins)