Sex versus non-sex versus cheaters

Cheating is known to be a potential threat to cooperation. We examine the fitness of a cheater allele in an asexual-haploid versus a sexual-diploid population using three different models. The first model depicts small cooperative groups in which the altruist and cheater are distributed randomly, the second simulates large cooperative groups with varying degrees of spatial aggregation and the third simulates a hymenopteran society with cheater workers. In all the models sex helps stabilize cooperation. The results are not affected by dominance or recessiveness of the cheater allele. In either case, sexual diploidy increases the variance across groups and thus enhances group selection. As a result sex and cooperation can coevolve under conditions when cooperation offers substantial fitness gains and cheaters threaten cooperation. The. model predicts that cooperation should be more common in sexually reproducing organisms than asexuals. Among organisms where diploid as well as haploid life-cycle stages are present, cooperation should be seen more commonly in the diploid stage.