MULTILEVEL SELECTION IN NATURAL-POPULATIONS OF IMPATIENS-CAPENSIS

This study partitions selection in natural populations of jewelweed, I mpatiens capensis, into group- and individual-level components. Group selection has been a subject of controversy for decades, yet this is t he first study to partition phenotypic selection in a natural populati on. Using contextual analysis combined with path analysis, we measured the correlation between fitness components (survival rate to first re production, chasmogamous [open-pollinated] seed production, and cleist ogamous [selfed] seed production) and several group- and individual-le vel traits. Survival rate was studied for 2 yr, and the reproductive t raits 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 productio n, only individual selection was detected. Group selection may be resp onsible for the constant yield law in plants. It may be more common th an previously believed because it may be mistaken for frequency-depend ent selection. Evolutionary theory suggests different components of ge netic variation are available to different levels of selection. Thus, the demonstration of group-level selection in nature challenges evolut ionary biologists to consider new components of variation as raw mater ial for selection. The results are discussed with respect to the evolu tion of altruism and the use of multiple regression versus path analys is in studies of selection.