The influence of neighbor relatedness on multilevel selection in the Great Lakes sea rocket

Natural selection can operate at the individual and group level in natural populations. This study investigates the ecological factors that determine the relative importance of individual versus group selection. In particular, it determines how the relatedness of interacting neighbors influences multilevel natural selection in a population of the Great Lakes sea rocket. Focal plants were grown in groups of siblings, groups of plants that were themselves siblings but unrelated to the focal plants, and groups of plants with mixed genotypes. Significant group selection on plant size was observed only when the neighbors were siblings but not when they were unrelated. In sibling groups, individuals with heavier stems had higher fitness, and individuals growing with heavier but shorter neighbors also had higher fitness. Thus, individual and group selection on stem weight operated in the same direction. The detection of group selection in sibling groups can be attributed in part to an increased opportunity for group selection in these groups since sibling groups differed more from one another than the other group types. In addition, the quality of the selective environment in sibling groups may have differed from that for the other group types. Group selection was therefore more prevalent in the most genetically structured sample, in which responses to group selection are also most likely to occur.