Competition, extinction, and the sexuality of species

Considering the well-known two-fold cost of males associated with sexual re production, the maintenance of sex despite natural selection remains an eni gma for population biologists. The prevalence of sex among eukaryotes is mo st commonly explained by hypotheses associated with either the purging of d eleterious mutations, the generation of favorable gone combinations, the fi xation of beneficial mutations, or, less frequently, ecological theories de aling with the coexistence of competing populations. Almost all these hypot heses ignore the fact that in stochastic environments, asexual populations exhibit higher rates of extinction than sexual populations because the latt er generally exploit a wider spectrum of resources than their asexual count erparts. Here we develop a model to demonstrate, in populations where mutat ions from sexual to asexual reproduction are possible, that three reproduct ive phases - sexual, mixed, and asexual - naturally arise among competing s exual and asexual lines. The particular phase observed is related to the le vel of stochasticity in the environment experienced by the population compl ex in question (e.g. a partially competing group of congeneric species) and is a manifestation of the tension that exists between the reproductive sup eriority of asexual populations and their higher rates of extinction. I ter m this explanation the demographic balance hypothesis and suggest the endeo stigmatid mites provide a suitable taxon for testing this hypothesis.