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March 03, 2005

Slow Dimorphism

Even when favored by natural selection, sexual dimorphism evolves far more slowly than monomorphic traits: alleles tend to have the same effect in in both sexes. . I just talked to Alan Roger about a paper of his (with Arindam Mukherjee) that did a quantitative analysis of just how much more slowly: for stature: about 65 times more slowly . There is reason to think that is a general result, that for most traits, dimorphism evolves tens of times more slowly than monomorphic traits, given equal force of selection.

Every now and then we hear someone say that given the recent common origin of the human race, there just hasn't been time for geographically isolated populations to evolve significant differences. That's wrong: 150,000 years is a reasonable consensus value for most recent human common ancestry, and that's enough time for a lot of change in the population mean of just about any trait, given plausible values of the force of selection. However, it is not a long time in term of selecting for changes in sexual dimorphism: more like 2300 years. The species is young enough that you expect sex differences to be roughly constant, even when means vary widely. Pygmy men are shorter than European women, but the difference between Pygmy men and women is about the same as the difference between European men and women. I would guess that this is true in general: true for psychometric traits as well.

I bet Larry Summers would like to talk about this.

Posted by greg at 05:13 PM