Social heterosis and the maintenance of genetic diversity.

Genetic diversity in species is often high in spite of directional selection or strong genetic drift. One resolution to this paradox may be through fitness benefits arising from interactions of genetically diverse individuals. Advantageous phenotypes that are impossible in single individuals (e.g. being simultaneously bold and shy) can be expressed by groups composed of genetically different individuals. Genetic diversity, therefore, can produce mutualistic benefits shared by all group members. We define this effect as 'social heterosis', and mathematically demonstrate maintenance of allelic diversity when diverse groups or neighbourhoods are more reproductively successful than homogenous ones. Through social heterosis, genetic diversity persists without: frequency dependence within groups, migration, balancing selection, genetic linkages, overdominance, antagonistic pleiotropy or nonrandom allele assortment. Social heterosis may also offer an alternative evolutionary pathway to cooperation that does not require clustering of related individuals, nepotistic favouritism towards kin, or overt reciprocity.