Multilevel selection in groups of groups

Natural selection occurs at many levels. We evaluated selection acting on collectives at a level of multilevel selection analysis not yet quantified: within and between clusters of groups. We did so by monitoring the performance of natural colonies of social spiders with contrasting foraging aggressiveness in clusters of various sizes. Within-clusters, growth rates were suppressed when colonies were surrounded by more rival groups, conveying that competition is greater. When colonies were surrounded by few rivals, the more aggressive colonies in a cluster were more successful. In contrast, relatively non-aggressive colonies performed better when surrounded by many rivals. Patterns of selection between-clusters depended on the performance metric considered, but cluster-wide aggressiveness was always favored in small clusters. Together, selection both within-and between natural clusters of colonies was detectable, but highly contingent on the number of competing colonies.