Harriet Whitehead wondered about males and females in sexually monomorphic
species getting confused. Well, they must be using cues other than
plumage to identify sex (they can simply continue to use whatever cues
they were using before the new, sexually selected plumage character
appeared).


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).


------------------------
In Lande's model (a major extension of Fisher's earlier work) the
evolution of sexually dimorphic characters is a two-step process:


1) sexual selection *for* a character in males; if the character is
autosomal (the usual case), it will be expressed in both sexes

2) if there is natural selection against expression of the character in
females, this can act to reduce or prevent expression of character in
females (sex-limitation).


This *second* step is where Burt Guttman's scenario of brightly colored
females sitting on a nest comes into play. For a new character to appear
and be sex-limited from the start would require simultaneously changing
the character *and* sex-limiting its expression. This apparently happens
only rarely (Lande's assessment is based on evidence from artificial
selection experiments, including experiments where selection is only on
one sex). But if natural selection against the trait in females is strong
(e.g., nest predation, as in Burt's example), the accumulation of genes
that modify expression of the character can reduce or prevent its
expression in females (i.e., "sex-limitation," which is thus the result of
the second step). For a new, sexually selected character to spread
through a population, sexual selection for the character in males has to
overcome any natural selection against the character in females -- there
is a sort of tug-of-war going on between SS on males and NS on females.
One of the major conclusions of Lande's analysis is that SS can proceed
*even if* it results in maladapation of females (e.g., being conspicuous
on the nest).


An implication of all this is that we should pay more attention to species
that have monomorphic expression of traits such as plumage brightness if
we want to better understand how SS works. To the extent that behavioral
characters are genetically determined, this two-step model should apply to
sexually "dimorphic" behaviors as well. And if mechanisms of
sex-limitation are the result of selection *against* a character, it will
be misleading to dissect these mechanisms in detail in an attempt to shed
light on the *origin* of characters via SS.


Lande's analysis shows that sexual selection under the above conditions
works on paper. There are only a few case histories that document how
sexually dimorphic characters actually evolved in the real world, but
these cases do fit the two-step model. In peccaries, when a new character
(zygomatic process similar to a warthog's headgear) first appeared, it was
large in both sexes, but later became sexually dimorphic (DW 1993;
Paleobiology 19:52-70). The two-step pattern is also supported by
phylogenetic analysis of bright color in orioles, as I already mentioned.


David Wright
dwright at u.washington.edu