Subject: Re: orioles/genetics Date: May 3 22:35:38 1995 From: Burton Guttman - guttmanb at elwha.evergreen.edu
Teresa caught me saying something silly (How do you make an embarrassed smiley?) about our having X and Y chromosomes. (Very embarrassing for someone who regularly teaches genetics. Don't know what I was [non]thinking.) Leaving the blunder aside, there are some important points to be made.
First, it doesn't matter which chromosomes have the genes that produce a character, such as a plumage. What's important for sexual dimorphism is that the Z and W chromosomes have _regulatory_ genes. The factors that turn genes on and off are proteins that diffuse freely through the nucleus and bind to regulatory sites (they're called promoters and enhancers) on any of the chromosomes. Teresa's models seem quite sensible, but what will be on the Z and W chromsomes won't be genes "for" blue or green plumage but rather regulatory elements that will turn on, or off, a bunch of other genes probably scattered among the other chromosomes that actually encode proteins, such as enzymes, that produce the plumage.
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 birds, because, as I recall, birds tend to have many (rather small) chromosomes. 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.
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.
So, David, when you say, "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," my reaction is to say that there's something terribly wrong with the model.
I'm still trying to understand everything David is saying (and I apologize for not having time to give it all the attention it deserves right now), but maybe we could try to get some agreement on this point.
Burt Guttman guttmanb at elwha.evergreen.edu The Evergreen State College Voice: 360-866-6000, x. 6755 Olympia, WA 98505 FAX: 360-866-6794