Evolutionary biology: Survival of the fittest group.

Abstract

Experiments with social spiders find that colony size and composition affect colony survival in a site-specific manner, indicating that natural selection on group-level traits contributes to local adaptation. See Letter p.359 Evolutionary theory predicts that group selection could drive the evolution of traits in individuals that promote the success of their group which, in turn, will promote the long-term fitness interests of group constituents. However, there is scant experimental evidence in support of group selection. This study uses colonies of the social spider Anelosimus studiosus to investigate the evolution of a group-level trait, the ratio of docile to aggressive individuals in a colony. Wild colonies display characteristic, site-specific docile-to-aggressive ratios. Jonathan Pruitt and Charles Goodnight generated experimental colonies with varying docile-to-aggressive ratios, and established them in the wild. Colonies with ratios closer to the native ratio were more successful, and experimental colonies adjusted their ratio over two generations to what would have been favoured at their native site. This experimental evidence suggests that group-level selection is acting in the wild.