The contagion mechanism for the origin of sex.

The origin of sex is one of the least understood aspects of the evolution of sex. Recently it has been proposed that sex arose as a result of a contagious genetic element which acted so as to foster its own transmission. Two continuous-time deterministic models of this mechanism are analysed. The first model lacks population regulation. With this assumption, sex is always established in the population, predominating if the sexual form is viable. The second model includes population regulation, giving rise to evolutionary dynamics in which sex need not be established in the population. In addition, with the second model, when the sexual form is viable, it need not fix. Thus, the contagion mechanism can give rise to sex, but need not do so.