Subject: Re: orioles and blackbirds (was wingbars)
Date: Apr 29 16:45:04 1995
From: Burton Guttman - guttmanb at elwha.evergreen.edu



I haven't had time to get involved in this thread (which I guess started
with Dennis's heart-felt reply to my skepticism about universal
adaptation), but let me help Teresa Michelsen with one small genetic
misconception. Teresa wrote, "When a new plumage character develops, how
it will be expressed should be entirely dependent on what chromosome the
mutation develops on. If it develops on the X or Y chromosome, it may
be sex-linked (i.e., only show up in the male or the female), but if it
develops on any other chromosome, it will necessarily be expressed in both
sexes, regardless of whether it only benefits the male, and regardless of
whether its desirability is related to mating."

Which chromosome the gene is on doesn't make a lot of difference here.
The relevant phenomenon here is sex-limited expression. That is, genes on
any chromosome may be limited in their expression in one sex or the other.
The classical textbook example is pattern baldness, which is only
expressed in men, but there are many other such genes. I'd better check
some of the new human genetic maps that are coming out these days (huge
foldouts in _Science_), but surely most of the genes that make us look
like men or like women are autosomal, not on the X or Y. And, after all,
we all have both X and Y chromosomes, but half of us don't express all
those funny genes on the Y chromosome. Oh, BTW, just to be sure we keep
all our facts straight, birds don't have the X-Y system. They have Z and
W chromosomes, and in birds its the male that is homogametic ZZ while the
female is heterogametic, ZW.

Teresa quotes David Wright as saying,

> 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.

I don't know the Lande paper, but think about the situation. If mama is
sitting on the nest and attracting predators because she's wearing papa's
bright feathers, there's going to be mighty strong selection for modifying
genes that will repress the genes for bright plumage in females. Ain't
going to be many kids hatched out of those eggs she's sitting on.

It occurs to me that I don't know the answer to a question that I think
is relevant: I assume there's a strong correlation between sexual
dimorphism, females being much duller, and female-only incubation of
eggs. Is there? A quick check of my references doesn't tell me. (And
forgive me if this has already come up--I have this whole thread saved in
another file.)

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 360-866-6000, x. 6755
Olympia, WA 98505 FAX: 360-866-6794