On Fri, 28 Apr 1995, Alvaro Patricio Jaramillo wrote:
> Tweeters,
>
> I think it was David Wright who asked if a feature could be caused by
> sexual selection if it was present on both sexes, I hope I have the
> question correct as I accidentally erased the message. My personal answer

Actually, I questioned your description of *territorial defense* by both
sexes as an example of mutual sexual selection. I don't question that
mutual sexual sexual selection can happen.

> would be yes. Characters will be sexually selected if they improve the
> reproductive success of the carrier, when compared to individuals that
> don't have the character. Male and female choice for the same character
> can be operating at the same time, but likely with different strength,
> and this may reach the same equilibrium point (balancing costs and
> benefits of the character) for both sexes. So bright monomorphic plumages
> can evolve from a mix of male and female choice, that may be acting
> somewhat independently from each other. Slight dimorphism results when
> the cost/benefit tradeoff reaches a different equilibrium point than the
> males, likely because the benefits for having that plumage are not all
> that high for the females of that particular population, this may happen
> by females losing or gaining plumage brightness from the ancestral
> population. I imagine that sexual dichromatism is much more fluid in its
> evolution, since it is only the expression of the genes that has to
> change, the genes are already there since the male has the bright
> plumage. The same would not apply to a feature that like wingbars which
> both sexes either have or don't, it is rare that one sex has wing bars
> and the other does not.

But a new plumage character when it first appears is *expected* to be
expressed in both sexes (like your wingbars), because genetic correlations
for between the sexes for homologous characters tend to be high.
A new character will likley be expressed in *both* sexes even in cases of
sexual selection on only *one* sex. In models such as that of Lande 1980
(Evolution 34:292-20) "changing expression" (i.e., getting rid of it in
females) of such a character in only one sex can take millions of generations,
given high genetic correlations between the sexes for homologous characters.
The point relevant to discussions of bird plumage evolution is that we should
*expect* characters selected for in males to initially be expressed in both
sexes. The presence of bright plumage, for example, in both sexes does not
necessarily have to be adaptive in females. It can actually be maladaptive
in females and still be expressed if sexual selection for the character in
males is stronger than natural selection against the character in females.

David Wright
dwright at u.washington.edu