Kin selection and local competition in a heterogeneous community.

When should an individual be willing to pay a cost in order to help or harm another individual in its community? Kin selection suggests that relatives should help each other, while competition for limited resources may select for harming behaviour against neighbours. This study considers social interactions between two individuals. For actions influencing non-dispersing reproduction, a condition is derived for selection to favour helping or harming, as a function of the actor's relationship to the rest of its community and to the recipient. Where two individuals differ in their relatedness to the community, the individual which is less related to the community will be the more helpful. Two individuals with a given relatedness to each other will be more helpful to each other as they jointly become less related to their community. The implications of these results are explored through an example involving a community derived from two ancestral groups. The directional selective effects will depend on the extent to which social interactions are assortative rather than random, and the distribution of opportunities for helping and harming.