Koinophilia stabilizes bi-gender sexual reproduction against asex in an unchanging environment.

Koinophilia, the tendency to choose mates with predominantly common phenotypic features, may be favourable because maladapted traits tend to have low frequencies. We used computer simulations to study competition between asexual and sexual reproduction, with and without koinophilia. The mutation process created new alleles, most of which were deleterious, but some small fraction was beneficial. With random mating, genderless sex (which does not suffer the two-fold cost of producing males) had a modest advantage over asex, while bi-gender sex was rapidly replaced by asex. In contrast, koinophilia allowed sex to persist, even when males represented half the population. The advantage of koinophilia was greatest when populations were large, the incidence of mildly disadvantageous mutations was high, and beneficial mutations were rare. Under these circumstances koinophilic bi-gender sexual reproduction resisted replacement by asexuals for over 10,000 generations, even when one sexual mutated into a parthenogen every generation. Thus, koinophilia may be an important factor maintaining sexual reproduction.