The evolution of group size and kin structure in complex societies

Abstract

Molecular and pedigree data reveal substantial variation in genetic relatedness within social groups across animals. Yet kin selection theory typically treats relatedness as a fixed parameter rather than an emergent property of group dynamics. We develop a demographically explicit model in which group size fluctuates through births, dispersal, mortality and floater immigration. Residents and potential joiners resolve conflict over membership, with joining probabilities set by inclusive-fitness effects at demographic equilibrium. Membership control jointly shapes group size distributions, stability and relatedness. Under insider control, unrelated outsiders are admitted primarily to small, low-productivity groups, so mean relatedness rises with group size and stays high in larger groups. Groups are most stable near the per capita productivity optimum, with equilibrium sizes slightly above it. Low fecundity relative to resident mortality increases floater reliance, promoting mixed-kin groups and reducing relatedness. Under outsider control, groups grow larger by admitting unrelated floaters, and relatedness remains low. By treating kin structure as an emergent outcome of demographic processes and membership decisions rather than a fixed parameter, our framework links relatedness to the processes that form and regulate social groups, offering new insight into the evolution of sociality.