Abstract
The theories of kin selection and heterogeneous advantage have been central to studies of altruistic behaviour and the evolution of sex over the last 35 years. Yet they predict diametrically opposite effects of genetic diversity on population density. Close relatives gain inclusive fitness advantages by preferentially associating with and behaving altruistically towards one another. However, heterogeneous advantage, which predicts competition to be highest when genetic diversity is low, suggests that benefits will be greater for individuals in groups of non-kin. Here we test how these two processes balance and affect the productivity of populations of animals in natural habitats. We report from a study of juvenile Atlantic salmon in the wild that heterogeneous advantage outweighs the benefits of kin-biased behaviour, resulting in a 1.8-fold higher population biomass and significantly better condition of individual fish.