Costs and benefits of evolving under experimentally enforced polyandry or monogamy

Reproduction has classically been viewed as a predominantly cooperative process. However, over the last 20 years this concept has steadily yielded ground to one of continual conflict in which the interests of the sexes are typically discordant. Within this framework, males and females are seen to be locked into a perpetual arms race, each adaptation by one sex promoting the evolution of countermeasures in the other sex. However, under strict genetic monogamy, the interests of the sexes become congruent, and hence antagonistic coevolution does not occur. We subjected the fly Sepsis cynipsea, a species with conspicuous sexual conflict, to experimentally enforced monogamy or polyandry for 29 generations and evaluated the microevolutionary consequences. We found that there were longevity costs to females consistent with sexually antagonistic coevolution. However, our measure of female fitness, offspring emergence, did not differ between treatments, even though life-history characters such as fertility and fecundity did. Results are discussed in terms of costs and benefits of sexual selection and sexual conflict.