Density-dependent multilevel selection in the great lakes sea rocket

Multilevel natural selection has been demonstrated in natural plant populations, but the ecological conditions that influence the strength of different levels of selection are poorly known empirically. One of the factors most likely to influence the relative strength of individual and group selection is density. If density determines the intensity of interactions among neighbors, one expects that individual selection may be strong at low density but that group selection would be strongest at high density. This study characterized how density influences multilevel selection on size in the Great Lakes sea rocket (Cakile edentula). Plants were grown at three different densities in the field, and multilevel natural selection on plant size was estimated. As predicted, only individual selection was detected at the lowest density, but group selection was detected at higher densities. Surprisingly, group selection was strongest not at the highest density, but at intermediate density, where it opposed individual selection; shorter, heavier individuals growing in groups of tall, light plants had the highest fitness. At the highest density, the effect of neighbor phenotype on reproduction differed between central and peripheral group members, leading to no overall group selection detected at high density. The observation that group selection was more prevalent at higher density than at the lowest density supports the general prediction that more intense interactions among group members can lead to stronger group selection. However, the strength and direction of group selection changed unpredictably with increasing density and spatial placement. This result emphasizes the need for spatially explicit investigations of how interactions among individuals influence patterns of multilevel selection in plant populations.