Condition-dependent sex and the rate of adaptation.

Recent modeling has shown that condition-dependent sex can evolve much more readily than sex that occurs at a uniform rate, even in the face of substantial costs of sex. Specifically, evolution favors genes that cause organisms to allocate more resources to sexual reproduction when they are in poor condition and to asexual reproduction--including increased life span--when they are in good condition. This form of fitness-associated sex (FAS) evolves because modifier genes that promote their own escape from low-fitness genetic backgrounds and that remain longer in high-fitness genetic backgrounds rise in frequency alongside the spread of high-fitness genotypes due to selection. Importantly, FAS does not evolve because it is good for the individual or good for the species but because it is in the selfish interest of modifier genes that promote FAS to move from low- to high-fitness genetic backgrounds. Even though FAS does not evolve for the good of its descendants, we show here that FAS often hastens the rate of adaptation. Ironically, the rate of adaptation is most likely to be accelerated by FAS when sex is costly, because FAS makes it more likely that individuals in poor condition will suffer the costs of sex, improving the efficiency with which less fit alleles are eliminated.