From: pad@galaxy.nsc.com (Paul Denny x8349)
Newsgroups: sci.bio
Subject: Re: Evolution
Keywords: Sexual Selection
Date: 18 Aug 92 21:49:26 GMT
Organization: National Semiconductor, Santa Clara

In article <1992Aug18.005140.5137@u.washington.edu>, toby@milton.u.washington.edu (Toby Bradshaw) writes:
> In article <1992Aug17.230830.23118@voder.nsc.com> pad@galaxy.nsc.com (Paul Denny x8349) writes:
> >I read Richard Dawkins' book The Blind Watchmaker several years ago
> >and it argued that the reason for the evolution of such things as 
> >Peacock tails (that do not seem to very suited to survival) was due to
> >a genetic preference by females for the trait. This leads to an increased
> >likelyhood of reproduction for males with longer tails with offspring that
> >have longer and longer tails (males) and more and more preference for long
> >tails (female).
> >
> >In isolation this argument seems to work but it seems to me (not a biologist)
> >that it excludes the possibility of females with a preference for tails that
> >are good for surviving with - these females would mate with "sensible" males
> >and their offspring would do better than the females with a "silly" preference
> >and hence win the battle to survive.
> >
> >Where am I going wrong?
> 
> If the long tail *is* a positive indicator of male health, for example,
> females preferring long-tailed males are making a good decision if the
> health character is under genetic control.

I suppose if you argue that having long tails makes survival more probable
then there is nothing to be explained and no need for a theory that includes
sexual selection, however I am uncomfortable with this as a satisfactory 
explanation.

  Suppose males with longer
> and longer tails [or gaudier and gaudier plumage, or bigger and bigger
> antlers, or faster and faster cars :) ] survive in spite of their
> "handicap"; perhaps they "deserve" to reproduce more than their
> more circumspect competitors.  Of course, in the afterglow they
> may wish to molt into eclipse plumage, shed their ponderous headgear,
> and buy a Yugo.
> 
If males survive despite the handicap then where is the selection pressure
upon which evolution theory is founded? (p.s. a Yugo may be a good descision
as any crashes will happen at low velocity 8-) ).

> If the majority of females in the population prefer large ornaments,
> the sons of small-ornament-preferring females will be unlikely
> to have high fitness, especially in a lek or harem breeding system.

This is pretty much the argument as I remember it from Dawkins book where
I assume that by fitness you mean likelyhood of *mating*. I think that 
I understand the argument but also think that it only works when isolated
from survival fitness. For example in the population above there will be
a range of ornament sizes and prefences for them centred around a large
ornament size which for the sake of this example we say *is* a handicap
to survival. In this population those males with the mean preferred ornament
size will have the greatest probability of mating (and will produce 
female offspring with a preference for large ornaments) those males with 
smaller ornaments will find it harder to find an interested partner and 
hence the probability of mating will decrease with ornament size below the 
mean preferred size. However the probability of survival to maturity will
increase with decreasing ornament size. The maximum probability of survival
will be that value of ornament size where probability of mating multiplied
by the probability of survival to maturity is a maximum. In the above case
this will be a smaller ornament size than the mean preferred value. This will
mean that male offspring will have slightly smaller ornaments (obvious) and
female offspring will slightly prefer smaller ornaments (not so obvious but
it follows from the sexual selection argument) hence the population should
evolve to a sensible ornament size.

> 
> There are studies showing that some bird plumage characteristics
> vary with parasite load, and that females of some species
> prefer not only large male decorations, but also those that are
> symmetrical.  Symmetry is supposed to be evidence of "normal development".
> 
This seems to say that a handicap isn't, if this is so why the sexual selection
theory?

> >Thanks - Paul    
> 
> You're welcome.
> 
> Toby Bradshaw
> Biochem and Forest Resources
> University of Washington