Eliminating the cost of sex with sexual selection

Abstract

Why should males be so common when, in many species, they contribute few resources to the production and rearing of offspring? Indeed, if males do not provision offspring, a sexual species that divides resources evenly between daughters and sons would halve its intrinsic growth rate relative to an ecologically equivalent asexual species that invests only in daughters. What advantage might pay for this twofold cost of sex? Two recent papers 1xSexual selection and the maintenance of sexual reproduction. Agrawal, A.F. Nature. 2001; 411: 692–695Crossref | PubMed | Scopus (153)See all References, 2xSexual selection and the maintenance of sex. Siller, S. Nature. 2001; 411: 689–692Crossref | PubMed | Scopus (133)See all References suggest that sexual reproduction might allow the more efficient purging of deleterious mutations from the genome as a result of sexual selection. If males that succeed in siring offspring carry fewer deleterious mutations than does the average male within a population, deleterious mutations are eliminated without reducing either the fitness of females or the growth rate of a population.The idea that females might choose mates that carry ‘good genes’ is an old one. What is novel in these papers is that the authors determine the extent to which sexual selection reduces the load of deleterious mutations and whether this could offset the cost of sex. With strong enough sexual selection and a high enough mutation rate, they find that the cost of sex can be completely repaid. For example, Siller 2xSexual selection and the maintenance of sex. Siller, S. Nature. 2001; 411: 689–692Crossref | PubMed | Scopus (133)See all References2 found that, with a mutation rate of U = 1 per diploid genome per generation, sexual selection can balance the twofold cost of sex as long as males that reproduce bear, on average, 0.25 standard deviations fewer deleterious mutations than does the average male. This result assumes that each mutation is weakly selected (causing a 1% fitness reduction, hs, in heterozygotes), that the number of mutations (m) is normally distributed, and that females choose among the males that they encounter on the basis of a male trait correlated with m.Agrawal 1xSexual selection and the maintenance of sexual reproduction. Agrawal, A.F. Nature. 2001; 411: 692–695Crossref | PubMed | Scopus (153)See all References1 considered how much stronger the combination of natural and sexual selection against deleterious mutations in males would have to be than selection in females for the twofold cost of sex to be removed. With U = 1 and hs = 0.01, selection that is 5.6 times stronger in males than in females suffices to make sexual populations fitter, on average, than asexual populations. Even in species with a low mutation rate, sexual selection could increase the chance that fathers are better matched to the current environmental conditions (including the biotic environment posed by parasites, competitors, and predators) than the average male, which could also reduce or eliminate the cost of sex.The beauty of these ideas is that they rely on assumptions that can often be met: (a) mate choice or male–male competition occurs; and (b) compared with males that remain unmated, sexually selected males pass on genes that, on average, increase the fitness of their offspring. Although these theoretical models might not explain the maintenance of sex in every species, they do suggest that males, when properly chosen, might not be so bad after all.