Mate choice for optimal (k)inbreeding.

Mating between related individuals results in inbreeding depression, and this has been thought to select against incestuous matings. However, theory predicts that inbreeding can also be adaptive if it increases the representation of genes identical by descent in future generations. Here, I recapitulate the theory of inclusive fitness benefits of incest, and extend the existing theory by deriving the stable level of inbreeding in populations practicing mate choice for optimal inbreeding. The parsimonious assumptions of the model are that selection maximizes inclusive fitness, and that inbreeding depression is a linear function of homozygosity of offspring. The stable level of inbreeding that maximizes inclusive fitness, and is expected to evolve by natural selection, is shown to be less than previous theory suggests. For wide range of realistic inbreeding depression strengths, mating with intermediately related individuals maximizes inclusive fitness. The predicted preference for intermediately related individuals as reproductive partners is in qualitative agreement with empirical evidence from mate choice experiments and reproductive patterns in nature.