EVOLUTION OF BRASSICA-RAPA L (CRUCIFERAE) POPULATIONS IN INTRASPECIFIC AND INTERSPECIFIC COMPETITION

Populations of Brassica rapa were grown for three generations in each of two environments: intraspecific competition, with four surrounding Brassica rapa neighbors per pot, and interspecific competition, with t wo Raphanus sativus neighbors per pot. In each environment, the larges t (by flower number) 10% of the plants were outcrossed and provided se eds for the next generation. As a control, a randomly chosen 10% of th e plants in each environment were outcrossed to produce seed for the n ext generation. Each of these four treatments, the selected lines in i ntra- and interspecific competition and the corresponding control line s, was maintained for three generations. After a single generation of growth in a common, no-competition environment, replicate plants from each treatment were grown with no competition and with intra- and inte rspecific competition for determination of growth responses. After two generations of selection, flower number in the intraspecific-selectio n line had increased by more than 50% over that in the control line an d by more than 19% over that under interspecific selection. After a co mmon-environment generation, plants from the intraspecific-selection l ine were shown to have significantly faster growth in height and flowe r number as seedlings. Plants in the interspecific-selection line show ed similar but nonsignificant trends. No differences in seed mass, eme rgence time, or photosynthetic rate were found between control and sel ected lines in either intra- or interspecific competition. Some differ ences between control and selected lines were noted in biomass allocat ion related to differences in phenology. The results demonstrate that performance in competitive environments can evolve through changes in plant development but that rates of evolution will differ in intra- an d interspecific competition.