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
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.

Of course some of these patterns can be caused by genetic drift, but
that's no fun ;)

Al "foaming at the mouth adaptationist" Jaramillo
Vancouver, B.C.
jaramill at sfu.ca