Effect of density on magnitude of directional selection on seed mass and emergence time in Plantago wrightiana Dcne. (Plantaginaceae).

Although population density is believed to be an important factor influencing evolutionary processes, surprisingly few studies have documented the existence or nature of density-dependent selection. We quantified the effects of density on directional selection on seed mass (the mass of a sown seed) and emergence time in the greenhouse and field for the annual plant Plantago wrightiana. In the greenhouse, we quantified selection on seed mass and emergence time at each of five planting densities (1 m-2 to 10,000 m-2) using the relationship between final plant mass and each trait at each density. We observed no significant selection on either seed mass or emergence time when plants were grown alone. At all higher densities, there was significant selection favoring early emergence and large seed mass, but there were no significant differences among the selection gradients determined at densities greater than individually grown plants. In the field, we detected no relationship between the magnitude of selection for early emergence and density. Our results suggest that selection on seed mass and time of emergence is density dependent, but the relationship between density and the magnitude of directional selection on these traits is not continuously increasing. Over broad ranges of density in the greenhouse and in the field, there was no detectable relationship between density and the magnitude of directional selection.