A physiological barrier for the maintenance of anisogamous sex.

There are at least four conceivable factors which could be responsible for the maintenance of anisogamous sex: (1) long-term group selection, (2) short-term ecological, natural selection, (3) trade-offs between female sexual fertility and parthenogenetic fertility and (4) female dependence on a male gametic contribution. There is evidence for the action of each of the first three factors, while the fourth seems to be evolutionarily impossible, at first sight. But selection on male and female gamete production physiology could, in principle, produce female germ lines which depend on male gametes for their perpetuation. There is circumstantial evidence that this has in fact occurred in the Mammalia, and critical experiments to test this hypothesis are proposed.