Seed size and emergence time within a stand of wild radish (Raphanus raphanistrum L.): the establishment of a fitness hierarchy.

To determine the role of seed size in creating adult plants of different reproductive success, individual seedlings were marked and periodically censused in a natural stand of wild radish (Raphanus raphanistrum L.) in Hamden, Connecticut. Maximum cotyledon witdth is a useful field estimator of seed weight in this species, although dramatic variation in the availability of water can modify this relationship. Using maximum cotyledon width as a linear estimate of seed weight showed that an increase in seed weight gives rise to a disproportionately large increase in an individual's total reproductive output. Analysis of covariance demonstrated that reproductive output is influenced more heavily by seed weight (maximum cotyledon width) than by emergence time. Genetic variation for seed size has been maintained in this population, suggesting that selection favoring large seeds during the seedling stage may be counteracted by selection for large numbers of seeds.