Sexual selection sustains biodiversity via producing negative density-dependent population growth

Mechanisms for maintaining biodiversity are still unclear despite considerable research. The classic theory predicts that stable co-occurrence of competitive species requires niche differentiation. In fact, the co-occurring species are often differentiated from each other. However, the neutral theory assuming equivalence of the reproductive rate of all individuals regardless of the species in a biological community has successfully recreated the observed patterns of species abundance distribution. This success is based on the unrealistic assumption suggesting that some mechanisms eliminate interspecific differences in the reproductive rates. Here, I present sexual selection as a candidate of the mechanisms by constructing analytical and simulation models. Sexual selection affects the traits that increase mating success even at the expense of fecundity when the species is abundant. By contrast, when the species is at a relatively low density, this negative effect on fecundity is mitigated because less competition for mating occurs in the rare species. The analytical model of this effect on fecundity predicted that sexual organisms stop population growth before exhausting resources due to the effect. This prediction was confirmed by simulation models. The simulations also showed that hundreds of competitive species with interspecific differences in reproductive potential can coexist over 10,000 generations. Moreover, species abundance distributions obtained from the simulations were similar to the patterns observed in field data. Given the generality of sexual reproduction in nature, sexual selection is likely to play a significant role in sustaining biodiversity over a broad range of environments. Synthesis. Evolution does not always maximize population growth rate. This study shows that evolution of sexual selection controls the population growth rate according to density and stabilizes the population size. This stabilizing effect has a potential to rescue endangered species from extinction, prevent overgrowth of common species, promote coexistence of competitive species, and successfully recreate the observed patterns of species abundance distribution.