Site-specific group selection drives locally adapted group compositions

Group selection may be defined as selection caused by the differential extinction or proliferation of groups. The socially polymorphic spider Anelosimus studiosus exhibits a behavioral polymorphism where females exhibit either a “docile” or “aggressive” behavioral phenotype. Natural colonies are composed of a mixture of related docile and aggressive individuals, and populations differ in colonies’ characteristic docile:aggressive ratios. Using experimentally-constructed colonies of known composition, we demonstrate that population-level divergence in docile:aggressive ratios is driven by site-specific selection at the group level: certain ratios yield high survivorship at some sites but not others. Our data also suggest that colonies responded to the risk of extinction: perturbed colonies tended to adjust their composition over 2 generations to match the ratio characteristic of their native site, thus promoting their long-term survival in their natal habitat. However, colonies of displa...