Species selection: the advantages of sex

The mean fitness of an asexual clone of organisms (left) adapted for a particular narrow niche may exceed that of a clade of sexual organisms in the same niche, since the optimally adaptive genotype is constantly recombined during sexual reproduction and produces a wider range of sub-optimally adapted types. However, an ecological disaster (black block in right-hand figures) that eliminates the clonal niche will not have as severe an impact on the sexual clade, because of the diversity of genotypes.