Subject: Re: orioles and blackbirds (was wingbars) Date: May 1 16:19:12 1995 From: wrightdb at pigsty.dental.washington.edu - wrightdb at pigsty.dental.washington.edu
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.