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Because of the production of males, sexual populations are expected to incur a 50% cost in potential growth rate. However, theory predicts that sexual competition between males can compensate for this cost by decreasing the mutation load of sexual populations. To test this hypothesis, I induced mutations in male bulb mites with ionizing radiation and subjected their progeny (F1) to two selective regimes differing in opportunity for sexual selection. Mutations which were not removed by selection acting on the F1 decreased embryonic viability in the F2. Viability was significantly higher in the treatment in which there was an opportunity for sexual selection than in the treatment in which sexual selection was experimentally eliminated. The results indicate that sexual selection can increase population fitness and, at least partly, compensate for the cost of sex. [ABSTRACT FROM AUTHOR]

Abstract The number of de novo mutations in the germline can be expected to increase with age in males, therefore females might decrease mutation load in their progeny by avoiding mating with older males. Here, I propose that female polyandry can be more effective in decreasing the risk of genetic disorders in progeny than pre-copulatory mate choice, particularly if sperm competitiveness declines more steeply with age than other traits affecting chances of males to mate. If faster ageing of spermatogenic tissue causes older males to transfer inadequate numbers of functional sperm, polyandry would also benefit females directly. [ABSTRACT FROM AUTHOR]