Imagine a cross between a male and female from two closely related species, which produces viable offspring. But now look more carefully at these offspring: they are all male, or all female; or if both sexes are present, then one of the two sexes is sterile or otherwise maimed. Haldane’s rule says that this sex will be the male in mammals and the female in birds. The catalogue of examples that has been pieced together since 1922 is impressive: hundreds of cases conform to the rule, across many phyla, with surprisingly few exceptions for a subject as confounded by exceptions as biology.
