Abstract
While sex can be advantageous for a lineage in the long term, we still lack an explanation for its maintenance with the twofold cost per generation. Here we model an infinite diploid population where two autosomal loci determine, respectively, the reproductive mode, sexual versus asexual and the mating system, polygynous (costly sex) versus monogamous (assuming equal contribution of parents to offspring, i.e. non-costly sex). We show that alleles for costly sex can spread when non-costly sexual modes buffer the interaction between asexual and costly sexual strategies, even without twofold benefit of recombination with respect to asexuality. The three interacting strategies have intransitive fitness relationships leading to a rock–paper–scissors dynamics, so that alleles for costly sex cannot be eliminated by asexuals in most situations throughout the parameter space. Our results indicate that sexual lineages with variable mating systems can resist the invasion of asexuals and allow for long-term effects to accumulate, thus providing a solution to the persisting theoretical question of why sex was not displaced by asexuality along evolution.
Highlights
The evolutionary maintenance of sexual reproduction has been considered the main unsolved problem in evolutionary biology [1,2,3,4] and it remains challenging after more than 40 years of research
The major issue in understanding the evolutionary maintenance of sex is how to balance the disadvantages of sexuality in terms of the cost of producing males who provide no resources to offspring, the so-called ‘twofold cost of sex’, which may reduce the reproductive potential of a lineage to one-half per generation ([1,2,3,4,11,12,13,14,15])
Our model can provide an answer to the old, persistent question of why sex is not displaced by asexuality in the short term: the diversity of sexual breeding systems that can occur during the evolution of a lineage, with variable degrees in the twofold cost-of-sex and in the opportunity for sexual selection, prevent the invasion and fixation of asexuality
Summary
The evolutionary maintenance of sexual reproduction has been considered the main unsolved problem in evolutionary biology [1,2,3,4] and it remains challenging after more than 40 years of research. Modifier alleles that produce changes from asexuality to small increases in the frequency of sex may be shown to spread within a small, finite population if genetic drift produces negative linkage disequilibrium, even the range of population sizes that favours sex are rather small when the twofold cost of sex is included [14,53,54] It seems that in many cases the twofold cost may not being compensated in the short term. The full twofold cost only occurs when sexual females invest 50% of their resources [11] to produce males that do not contribute with resources to the production of offspring, normally because they invest their entire reproductive budget in competition with other males to secure mates. We use game theory in a sexual selection genetic model that investigates the evolutionary dynamics of asexual and sexual individuals that use different reproductive strategies in an infinite population
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