Earlier I pointed to the possibility of biophysical constraints and parameters in terms of inheritance shaping the local trajectory of evolution. Today Olivia Judson has a nice post [link fixed] on how the existence of two sexes in many species results in a strange metastable tug-of-war in terms of phenotypic evolution:

In sum, the traits that make a “good” male are often different from those that make a “good” female. (Note: I’m only talking about “good” in evolutionary terms. That means a trait that improves your chance of having surviving offspring.) Since many of these traits have a genetic underpinning, male and female genes are thus being sculpted by different forces.

But — and this is the source of the tension I mentioned — males and females are formed from the same underlying set of genes. After all, in humans, whether you’re a boy or a girl comes down to whether you have a Y chromosome or not: boys do, girls don’t. The rest of the genes occur in both sexes.

The X choromosome in mammals spends about 2/3 of its time in females and 1/3 in males.* And obviously the Y is found only in males. But the rest of the genome is found in both males and females. Judson notes that traits which may be attractive in males may not in females, and which may be attractive in females may not in males. There’s a fair amount of evolutionary psychological work in humans in this vein in regards to the heritability of testosterone and estrogen levels in females and males and how it effects the same and opposite sex (in short, there is suggestive data that “sexy” individuals of one sex, those who exhibit strong secondary sexual characteristics, may be prone to having less sexy offspring of the opposite sex).

Of course you can overcome the balancing tug of war; that’s why you have sexual dimorphism in things like size or facial proportion. But these sorts of traits emerge very slowly because of the equilibrium described above, modifier genes and sex-specific gene expression have to slowly engineer around the overwhelming problem that males and females are genetically no different on a sequence level aside from the Y chromosome. Some estimates put the rate of evolutionary change of sexual dimorphism, that is, trait differences between sexes, between 1 and 2 orders of magnitude slower than conventional population level evolution. Ergo, one would expect that sexual dimorphism differences varying across populations have great time depth, and are probably more interspecific than intraspecific (for example, gorillas vs. humans).

There is naturally a whole field devoted to the study of the origin of sex. But whatever its ultimate rationale and utility an evolutionary context, its existence as a background condition in many taxa may result in a constraint of the exploration of phenotype space, as species divided into two sexes characterized by strong phenotypic differences dance between two sex-specific phenotypic optima.

* Sex determination varies by taxon.