Cryptic Kin Selection: Kin Structure in Vertebrate Populations and Opportunities for Kin-Directed Cooperation

Animal societies of varying complexity have been the favoured testing ground for inclusive fitness theory, and there is now abundant evidence that kin selection has played a critical role in the evolution of cooperative behaviour. One of the key theoretical and empirical findings underlying this conclusion is that cooperative systems have a degree of kin structure, often the product of delayed dispersal, that facilitates interactions with relatives. However, recent population genetic studies have revealed that many non-cooperative animals also have kin-structured populations, providing more cryptic opportunities for kin selection to operate. In this article, I first review the evidence that kin structure is widespread among non-cooperative vertebrates, and then consider the various contexts in which kin selection may occur in such taxa, including: leks, brood parasitism, creches, breeding associations, territoriality and population dynamics, foraging and predator deterrence. I describe the evidence that kin-selected benefits arise from interacting with kin in each of these contexts, notwithstanding the potential costs of kin competition and inbreeding. I conclude that as the tools required to determine population genetic structure are readily available, measurement of kin structure and the potential for kin selection on a routine basis is likely to reveal that this process has been an important driver of evolutionary adaptation in many non-cooperative as well as cooperative species.