Field tests of density- and frequency-dependent selection in Erigeron annuus (Compositae)

Selection may maintain genetic diversity in natural populations if the physical or biotic environment is variable over spate and/or time. Because density and genotype frequencies can be heterogeneous. and because genotypes may differ in competitive ability, both density- and frequency-dependent selection have been considered to be potentially important evolutionary processes. To address the possibility that intraspecific interactions among plants are a sourer of fitness variation in Erigeron annuus, we conducted field experiments over 2 yr that were designed to examine the potential of population density, genotype frequency, and their interaction to act as selective agents. In both experiments, apomictic genotypes of Erigeron were paired. Seedlings were planted into plots that differed in density and the identity of minority and majority genotype. There was evidence for a differential effect of density among genotypes for only one year's experiment, suggesting that density-dependent selection is either weak or temporally variable. Genotype frequency had no effect on fitness in either year, and thus there was no evidence for frequency-dependent selection. In addition, the lack of a frequency x density interaction demonstrates that resource partitioning, one mechanism for frequency dependence, is not strong among Erigeron genotypes. If frequency-dependent selection does occur in this species. it is either too weak to detect even in large field experiments, or occurs only in the presence of a selective agent (e.g., pathogens) that was lacking in our experiments.