Assessing the alignment of sexual and natural selection using radio-mutagenized seed beetles

A major unsolved question in evolutionary biology concerns the relationship between natural and sexual selection. Sexual selection might augment natural selection, e.g. if mutations that harm female fecundity also reduce male mating success. Conversely, sexual selection might favor traits that impair naturally-selected fitness components. We induced detrimental mutations in Callosobruchus maculatus beetles using X-ray irradiation, and then experimentally measured the effect of pre-copulatory sexual selection on offspring number and survival rate. Sexual selection treatment had a negative effect on egg-to-adult survivorship, though the number of progeny reaching adulthood was unaffected, perhaps because eggs and juveniles that failed to develop lessened competition on the survivors. We hypothesize that the negative effect of sexual selection arose because sexually competitive males transmitted a smaller nuptial gift, or carried alleles that conferred reduced survival. Although we found no evidence that sexual selection on males can purge alleles that are detrimental to naturally-selected fitness components, such benefits might exist in other environmental or genetic contexts.