On Wed, 3 May 1995, Burton Guttman wrote:
> That's why I'm troubled when David Wright says, "Teresa Michelson
> correctly interpeted the assumptions of the Fisher-Lande model for
> evolution of sexually dimorphic characters via sexual selection. If the
> genes responsible for the new, sexually selected character are *not* on
> one of the sex chromosomes (i.e., they are on an autosome, thus
> "autosomal," as in my previous description), then the new, sexually
> selected character will appear in both sexes (by default, in a sense)."
> Y'see, I don't believe it. (Fisher was writing long before we knew
> anything about gene regulation; I don't know about Lande.) The genes could
> be on any chromosome; it's pretty unlikely, in fact, for genes that create
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> new features to pop up suddenly on the sex chromosomes, especially in
> chromosomes.

Isn't that what I said?

> The sex chromosomes have been making significant differences
> between males and females all along, before the appearance of any new
> characteristics, and they should be able to continue making those
> differences if it's advantageous.

But the characters we are dealing with here were *not* sexually dimorphic
before the new, flashy version of the character appeared. So there would be
no reason for regulatory genes having sex-limiting effects to be involved
until *after* the new version of the character has appeared.

> And with plumage it often _is_ advantageous, isn't it? Think about a
> scenario. A complicated characteristic like plumage will surely evolve
> rather slowly, through the action of many genes. Now suppose a new gene,
> or new combination of genes, starts to produce just a little bit of bright
> plumage that is expressed equally in males and females, and suppose
> females who show it have reduced fitness. Then right from the beginning
> there's going to be strong selection (natural selection) to bring the
> relevant genes under control of the female regulatory systems and repress
> those features in females.

In this scenario, involvement of regulatory genes *follows* the
appearance of the new, sexually selected character. So far, so good.
But if sexual selection *for* the trait in males is much stronger that
natural selection *against* the trait in females, then the new character can
spread rapidly through the population. You seem to be arguing that
intense NS on females will select for regulatory genes to come into
play immediately and quickly effect sex-limitation. But if NS against the
trait in females is that intense from the outset, SS for the character will be
stymied, and we will have no character to discuss. It boils down to the
relative strengths of SS vs NS. It is this tension between SS and NS that
makes the situation interesting.

David Wright
dwright at u.washington.edu