MULTILEVEL SELECTION IN NATURAL-POPULATIONS OF IMPATIENS-CAPENSIS

This study partitions selection in natural populations of jewelweed, Impatiens capensis, into group- and individual-level components. Group selection has been a subject of controversy for decades, yet this is the first study to partition phenotypic selection in a natural population. Using contextual analysis combined with path analysis, we measured the correlation between fitness components (survival rate to first reproduction, chasmogamous [open-pollinated] seed production, and cleistogamous [selfed] seed production) and several group- and individual-level traits. Survival rate was studied for 2 yr, and the reproductive traits were studied for 1 yr. For survival rate and cleistogamous seed production, both group and individual selection occurred, and the two types of selection were in opposition. For chasmogamous seed production, only individual selection was detected. Group selection may be responsible for the constant yield law in plants. It may be more common than previously believed because it may be mistaken for frequency-dependent selection. Evolutionary theory suggests different components of genetic variation are available to different levels of selection. Thus, the demonstration of group-level selection in nature challenges evolutionary biologists to consider new components of variation as raw material for selection. The results are discussed with respect to the evolution of altruism and the use of multiple regression versus path analysis in studies of selection.