Re: Sexual selection of disadvantageous traits


"r norman" <r_s_norman@xxxxxxxxxxxx> wrote in message news:uftaf2huf5jod112fpj5id57ooucf5pj2k@xxxxxxxxxx
On Wed, 30 Aug 2006 02:32:14 GMT, "Perplexed in Peoria"
<jimmenegay@xxxxxxxxxxxxx> wrote:


"Ken Shackleton" <ken.shackleton@xxxxxxx> wrote in message news:1156285214.109830.149520@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Perplexed in Peoria wrote:
"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:IsJGg.16819$gY6.2584@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Perplexed in Peoria wrote:

"Vend" <vend82@xxxxxxxxxxx> wrote in message news:1156261171.092378.199460@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Can somebody please tell me what is the most likely explanation (or
explanations) for the presence of disadvantageous traits often found in
birds. I've seen many times an explanation involving sexual selection
but it doesn't make me any sense

[snip]
Several people have given good answers. I would recommend the
book "The Red Queen" by Ridley for a readable survey of the
variety of ideas that have been advanced on this subject.

I will observe that one of the reasons that the explanation you
originally quoted doesn't make sense to you is that you are
working under the assumption that natural selection always
optimizes fitness. In fact, it doesn't, as can be shown
(using too much math to fit into the margins of this posting).

Sure it does, within the scope of the genetic variation that presents
itself to the population. Attractiveness to potential mates is of course
an aspect of fitness.

No it doesn't. I refer you to any reasonably advanced text on
population genetics for the refutation. Of course, your point
could be narrowly construed as a claim that fitness _is_ being
optimized in the usual explanations for sexual selection of
traits that reduce male survival. True enough, if you redefine
fitness in the right way to include the fitness of offspring.

The thing that is slightly odd about sexual selection is that
the argument for 'why' the female should prefer the handicapped,
but sexy mate is that she wants to have sexy sons.

Excuse the interruption in the conversation, but is not the question of
*why* the female selects a certain trait or not rather irrelevant. It
seems to me that the fact that the trait is selected strongly enough to
overcome the risks of predation would be enough. In the human example,
I prefer brunettes to blondes...I have no idea *why* I do...I just
do.....they why is irrelevant.

Perhaps I could have been clearer. The 'why' here means why
the female's preferential mating with long tailed males
is advantageous to the female's genes. It doesn't deal with
her 'conscious' goals and motivations. And the thing that
is slightly odd about it is that to see that there is an
advantage to the female's genes, you may need to count grandchildren
rather than simply counting children. Her sons, having inherited
the father's tail, have a reduced chance of surviving to
maturity, but an increased chance of siring children (her
grandchildren) if they do survive.

I don't understand why you should think this is a problem or in any
way unusual. All you are saying is that 'genes for long tail' have
higher fitness than 'genes for short tail'. That is, birds with those
alleles have a higher probability of contributing those genes to
future generations. The immediate survival of the individual carrying
the genes is only one aspect of fitness.

I realize that both survival and fecundity are involved in
fitness. And I understand that we are assuming that the long
tail trait is correlated with fitness in males. But the subject
under discussion is whether the trait of prefering to mate with
long tailed males is correlated with fitness in females.

Now, I will agree that if you start with the axiom that female
fitness is enhanced by 'choosing' to mate with high-fitness
males, then there is no problem. I might even agree that this
'axiom' can be proved as a theorem of population genetics, if
you get the definitions right. But if you are a bit careless
with the definitions, the theorem doesn't work and the sexual
selection argument doesn't work. And the usual definitions
of fitness are 'careless' in this sense, since they don't usually
address the "probability of contributing those genes to
future generations". Instead, to avoid double counting, they
are constructed to only count the "probability of contributing
genes to the next generation".

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, due to the commonness of a female
preference for long tails, it increases the reproductive
success of males that survive to sexual maturity. When
you combine the three effects together, the net effect is
(by assumption) positive.

Now look at it from the mature female's viewpoint. She 'wants'
to maximize the count of her own offspring that reach sexual
maturity. So, in choosing a mate, she notes the handicap
aspect of a tail and counts that as a negative. But she notes
the correlation aspect and counts that as a positive. Both
of these will have an effect on whether her offspring reach
sexual maturity.

But how is she to assess the sexiness of the tail? It seems
absurd for her to consider the sexiness of her mate as something
objectively positive. It doesn't help her. What about the
sexiness of her sons. According to the usual accounting rules,
she can't count that either, since it doesn't have any impact
on her sons until *after* they have reached sexual maturity.
By the usual accounting rules, it can't be taken into account
as contributing in any way to *her* fitness.

This, then, is a case where the usual accounting rules are
wrong. And that strikes me as a little bit odd, and hence
worth a mention. That's all.

M

.