Relatedness in trait group models of social evolution

Genetic relatedness is a central concept in the study of social evolution. Though originally defined in terms of genealogy, the modern version of rela tedness accommodates genetic similarity of any origin. This paper examines relatedness in group structured modes, in which a trait affects the fitness of all group members. Such traits can be divided into two types, based on whether their group fitness effects encompass all group members including t he actor ("whole-group traits"), or only group members other than the actor ("other-only traits"). Both trait types are common in nature as well as in theoretical models, but they have rarely been distinguished clearly. The a verage relatedness of recipients to actors differs for the two trait types within the same population and even the same individual, leading to differe nt selection pressures and evolutionary outcomes. Total relatedness in grou p-structured models can be partitioned into two components: structural rela tedness due to the size and number of groups in the population, and assorta tive relatedness due to the distribution of genotypes among groups. Each co mponent differs for whole-group vs. other-only traits, both in terms of the ir values and the factors that influence them. Some key differences include : positive relatedness requires positive assortment for other-only but not for whole-group traits; negative relatedness is possible for other-only but not whole-group traits; relatedness depends on average group size for whol e-group but not other-only traits, and non-random assortment into groups af fects relatedness more strongly for other-only than whole-group traits. Rec ognizing the distinction between these trait types resolves some apparent c ontradictions in the literature, and clarifies the limits of some previous results. (C) 2000 Academic Press.