Haldane's Rule

Haldane's rule—the preferential sterility or inviability of hybrids of the heterogametic (XY) sex—characterizes speciation in all known animals. Over the past decade, an enormous amount of experimental and theoretical work has been devoted to explaining this pattern. This work has falsified several once-popular theories and, more important, has produced a strong consensus on the likely causes of Haldane's rule. Experiments show that the dominance theory, which posits that “speciation genes” act as partial recessives in hybrids, can explain Haldane's rule for hybrid inviability. Dominance likely also contributes to Haldane's rule for sterility. Recent experiments further show that faster evolution of hybrid male steriles plays an important role. Faster evolution of X-linked loci may also contribute, though the evidence here is weaker. Evolutionary geneticists now largely agree that the simultaneous action of these forces explains Haldane's rule.