Darwin first raised the question of sex ratio evolution, and saw it as both important and enigmatic. He was, however, unable to make much headway with the problem and declared it a puzzle for the future. This particular future arrived about 60 years later, when R. A. Fisher (1930) pointed out that under autosomal inheritance half of the genes passed to zygotes in any generation come from males and half come from females. Fisher noted that this one-mother/one-father symmetry generates frequency- dependent natural selection on sex ratio, resulting in an evolutionary equi- librium in which half...