Re: Sexual selection of disadvantageous traits

Perplexed in Peoria wrote:

"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.

That was the entire original assumption of the handicap principle.
Perhaps you should go back and read Zahavi's original paper.

... 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.

Unfortunate. I wonder what you have noticed.

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.

Of course it will. Let's suppose we have a mixture of four types, for
simplicity: the four combinations of long and short tails, high and low
quality. Suppose that the long tail has a cost, and that low quality has
a cost too, such that the costs are equal. So after a round of survival,
the population is enriched in short-tail, high-quality, depleted in
long-tail, low-quality birds. Try any numbers you like for the cost
itself. I'll say 10% each just for fun. So if we start with a population
of 100 males of each type, we end up with this table:

T
short long
high 100 90
Q
low 90 80

But now take this 2 x 2 table and put yourself in the position of a
female trying to find a high-quality male but unable to see quality
directly. Using tails as a clue, is the female more likely to select a
high-quality mail by choosing a long-tailed or short-tailed male?
Answer: long-tailed: 9/17 (.529) vs. 10/19 (.526). Or thinking of it
another way, all surviving birds have gone through an equal test of
their survivability. The expectation is that any given bad
characteristic is compensated for by some equal combination of good
characteristics. Here we are reducing all characteristics to two: tail
length and quality. If one character gets "better", the expectation is
for the other to get "worse".

I repeat that this (and Zahavi's original theory) relies on making an
artificial distinction between the handicap and quality, which there is
no reason to believe exists in nature. What the female is really looking
at would be survivability, and tail length is no clue to that.

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 ... The surviving long-tailed birds would ... Both are impacted equally by quality, ...
    (sci.bio.evolution)
  • Re: Sexual selection of disadvantageous traits
    ... an aspect of fitness. ... All you are saying is that 'genes for long tail' have ... That is, birds with those ... The statistical expectation for all surviving males is that they ...
    (talk.origins)
  • Re: Population genetics question regarding sexual selection
    ... artificially separating sexual selection from natural selection. ... quality of some sort were originally derived based on the Darwinian ... The statistical expectation for all surviving males is that they ... The surviving long-tailed birds would ...
    (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)
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