What's wrong with a little sex?

In many species, most (or all) offspring are produced by sexual means. Howe ver, theory suggests that selection should often favour the evolution of sp ecies in which a small fraction of offspring are produced sexually, and the rest are produced asexually. Here, we present the analysis of a model that may help to resolve this paradox. We show that, when heterozygote advantag e is in force, members of species in which sex is rare will tend to produce poorly adapted offspring when they mate. This problem should be less sever e in species where most offspring are produced by sexual means. As a conseq uence, once the rate of sexual reproduction becomes sufficiently rare, the benefits of sex may vanish, leading to the evolution of obligate asexuality . Substantial benefits of sexual reproduction may tend to accrue only if a large proportion of offspring are produced sexually. We suggest that simila r findings are likely in the case of epistatic interactions between loci.