Subject: Re: orioles and blackbirds (was wingbars)
Date: Apr 29 15:18:00 1995
From: "Michelsen, Teresa" - TEMI461 at ecy.wa.gov




I wonder if I'm missing something here - When a new plumage character
developes, how it will be expressed should be entirely dependent on what
chromosome the mutation developes on. If it developes 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 developes 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. I think
this is why it's more likely for a new mutation to occur in both sexes -
there are more other types of chromosomes than there are X and Y chromosomes
for mutations to occur on (assuming it's equally likely that a mutation
could occur on any chromosome). Is this what you mean by "genetic
correlations between the sexes for homologous characters tend to be high",
or does that mean something else entirely? (I'm having a translation
problem) I can only think of two ways that a non-sex-linked trait could
become a sex-linked trait. First, there are occasional mutations where
chromosomes get crossed and trade parts. This could move a trait from a
regular chromosome to an X or Y chromosome (assuming the bird lived and
could reproduce with this type of mutation). Alternatively, a second
mutation could appear on an X or Y chromosome that would cause the
non-sex-linked trait to be expressed or not according to sex (e.g., a
hormone that inhibited or enhanced expression of the trait). Either of
these processes would be statistically highly unlikely, which is why it
would take many generations after expression of the original trait for it to
become sex-linked (if it ever did). Please let me know if the process being
discussed is something else entirely - my genetics is pretty basic and I'd
like to learn more about it.

Teresa Michelsen
temi461 at ecy.wa.gov
___________________________________
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