The evolution of obligate sex: the roles of sexual selection and recombination

Abstract

The evolution of sex is one of the greatest mysteries in evolutionary biology. An even greater mystery is the evolution of obligate sex, particularly when competing with facultative sex and not with complete asexuality. Here, we develop a stochastic simulation of an obligate allele invading a facultative population, where males are subject to sexual selection. We identify a range of parameters where sexual selection can contribute to the evolution of obligate sex: Especially when the cost of sex is low, mutation rate is high, and the facultative individuals do not reproduce sexually very often. The advantage of obligate sex becomes larger in the absence of recombination. Surprisingly, obligate sex can take over even when the population has a lower mean fitness as a result. We show that this is due to the high success of obligate males that can compensate the cost of sex.

Condition-dependent sex: who does it, when and why?

“…Sexual selection makes sexual reproduction more advantageous specifically for individuals in good condition and can lead to obligatory sex [72,73]. A model considering both environmental changes and sexual selection can further extend the parameter range allowing the evolution of obligate sex [72,73].…”

“…Sexual selection makes sexual reproduction more advantageous specifically for individuals in good condition and can lead to obligatory sex [72,73]. A model considering both environmental changes and sexual selection can further extend the parameter range allowing the evolution of obligate sex [72,73]. When poor condition is induced by pathogens, sexual selection also applies: sexual selection can increase the mating success of hosts that are less infected [74], due to preference for either resistance genes or pathogen avoidance [75].…”

The adaptive function of sexual reproduction: resampling the genotype pool

“…This iterative process can explain why sexual reproduction needs to occur continuously. Peck and Waxman (2000) wondered why intermittent sexual reproduction is not more common in nature and a number of other authors have sought explanations for obligate sex (Kleiman andHadany 2015, Crouch 2017).…”

Increased adaptability to rapid environmental change can more than make up for the two-fold cost of males

“…Second, instead of only considering competition between fully isogamous and fully anisogamous populations, anisogamy should be treated as a quantitative trait, capable of evolving by degrees. A model of competition between alleles that leads to only slightly different degrees of anisogamy could lead to qualitatively different conditions, as carriers of the different alleles would likely interbreed, potentially breaking down the associations between the anisogamy locus and the directly-selected loci [32].…”

“…By contrast, under anisogamy, if the males favored by sexual selection are the ones carrying "good genes" that increase other fitness components, this form of sexual selection can greatly enhance natural selection without a reduction in the reproductive output of the population [3,22,23], although it is unclear how often it actually does so in nature (see, e.g., [24]). "Good genes" sexual selection allows anisogamous populations to greatly reduce their mutational load and the probability that deleterious mutations fix, potentially overcoming the two-fold cost if deleterious mutation rates are large [25,26,27,28,29,30,31,32]. In other words, males can act as dead ends, where deleterious mutations go to die.…”