Experimental evolution in Heterandria formosa, a livebearing fish: group selection on population size

Group selection has historically been an important and controversial subjec t in evolutionary biology. There is now a compelling body of evidence, both theoretical and experimental, that group selection not only can be effecti ve, but can be effective in situations when individual selection is not. Ho wever, experiments in which true population-level traits have been shown to evolve in response to group selection are currently limited to two species of flour beetle in the genus Tribolium and RNA viruses. Here we report the results of an experiment wherein we imposed group selection via differenti al extinction for increased and decreased population size at 6-week interva ls, a true population-level trait, in the poeciliid fish Heterandria formos a. In contrast to most other group selection experiments, we observed no ev olutionary response after six rounds of group selection in either the up- o r down-selected lines. Populational heritability for population size was lo w, if not actually negative. Our results suggest that group selection via d ifferential extinction may be effective only if population sizes are very s mall and/or migration rates are low.