The relationship between sexual dimorphism and intersex correlation: do models support intuition?

The evolution of sexual dimorphism (SD) (the difference in average trait values between females and males) is often thought to be constrained by shared genetic architecture between the sexes. Indeed, it is commonly expected that SD should negatively correlate with the intersex correlation (the genetic correlation between effects of segregating variants in females and males, rfm), either because (1) traits with ancestrally low rfm are less constrained in their ability to respond to sex-specific selection and thus evolve to be more dimorphic, or because (2) sex-specific selection, driving sexual dimorphism evolution, also acts to reduce rfm. Despite the intuitive appeal and prominence of these ideas, their generality and the conditions in which they hold remain unclear. Here, we develop models incorporating sex-specific stabilizing selection, mutation, and genetic drift to examine the relationship between rfm and SD. We show that the two commonly-discussed mechanisms with the potential to generate a negative correlation between SD and rfm could just as easily generate a positive association, since the standard line of reasoning hinges on a hidden assumption that sex-specific adaptation more frequently favors increased dimorphism than reduced dimorphism. Our results provide, to our knowledge, the first mechanistic framework for understanding the conditions under which a correlation between rfm and SD may arise and offer a compelling explanation for inconsistent empirical evidence. We also make the intriguing observation that-even when selection between the two sexes is identical-drift generates nonzero SD. We quantify this effect and discuss its significance.