Haldaneʹs rule and its legacy: Why are there so many sterile males?

A general pattern of animal hybridization, known as Haldaneʹs rule, is that the XY (ZW) sex is more severely affected in its viability or fertility than the XX (ZZ) sex. Recent evidence suggests that three different forces have shaped this pattern: (1) the X chromosome and autosomes are in greater disharmony in the XY sex; (2) evolution of hybrid male sterility is greatly accelerated, at least in species with XY males; and (3) maternal-zygotic incompatibility preferentially affects the viability of the XX sex. In species with XY males, the rapid evolution toward hybrid male sterility may be responsible for the bulk of observations pertaining to Haldaneʹs rule. One interesting and testable hypothesis is that sexual selection drives this rapid evolution.