Direct selection under resource allocation trade-offs enables the evolution of obligate and facultative sexuality

Sexual reproduction is considered to incur multiple costs relative to asexual reproduction. Although previous research has identified indirect selective advantages that help explain the widespread occurrence of sex, the emergence and maintenance of high rates of sexual reproduction remain a central puzzle in evolutionary biology, because indirect selection favoring sex becomes weak when the sexual rate is high. Using a modifier framework that allows the simultaneous evolution of sexual rate and sex allocation, I investigate the evolution of sex via direct selection by incorporating resource allocation trade-offs between sexual and asexual reproduction. The results show that such trade-offs can substantially facilitate the invasion of sex. Crucially, the evolutionarily stable sexual rate depends on the return exponents of female fertility and asexual fertility with respect to resource investment, as well as on the initial allocation to female function within sexual reproduction. Obligate sexuality is evolutionarily stable when asexual fertility exhibits linear or accelerating returns on investment and when both the initial sexual rate and female function allocation within sexual reproduction exceed certain levels. In contrast, facultative sexuality will be evolutionarily stable when female fertility within sexual reproduction and/or asexual fertility exhibit diminishing returns. Contrary to previous theoretical predictions, self-fertilization often inhibits the evolution of sex or reduces the evolutionarily stable sexual rate. This study provides insights into the prevalence of high sexual rates, as well as the continuous spectrum of sexual rates in some groups, highlighting the importance of key parameters in reproductive ecology in shaping the evolution of sex. Significance statementWhy sexual reproduction is so common despite its substantial costs remains a longstanding puzzle in evolutionary biology. Most previous explanations focus on indirect genetic benefits of sex, which generally become weaker as the rate of sexual reproduction increases, and therefore cannot explain the prevalence of intermediate to high sexual rates in nature. This study shows that with resource allocation trade-offs between sexual and asexual reproduction, obligate or facultative sexuality can evolve and be stably maintained via direct selection alone, depending on the marginal returns of asexual and female fertility, as well as the initial allocation to female function. These results highlight the underappreciated role of reproductive ecology in shaping the evolution of sex in nature.