Testing theories of sex against the observation that sex is biparental

Understanding the purpose of sex remains one of the most important unresolved prob-lems in evolutionary biology. Two leading theories of sex are the mutational determin-istic hypothesis that sex reduces mutational load, and the Red Queen hypothesis that sex permits a host organism to maintain parity in a never-ending conict with parasites. These theories have been di ¢ cult to distinguish. We propose to distinguish between them by asking if either is consistent with the fact that sexually produced o¤spring have only two parents, not three or more. It is shown that the mutational deterministic hypothesis implies that a suitable triparental system dominates biparental sex, so the latter should never be observed, contrary to fact. In contrast, it is shown within a highly stylized model that the Red Queen hypothesis can potentially explain biparental sex without conferring an additional advantage to triparental sex