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 infl uencing evolutionary processes, surprisingly few studies have document ed the existence or nature of density-dependent selection. We quantifi ed the effects of density on directional selection on seed mass (the m ass of a sown seed) and emergence time in the greenhouse and field for the annual plant Plantago wrightiana. In the greenhouse, we quantifie d selection on seed mass and emergence time at each of five planting d ensities (1 m(-2) to 10,000 m(-2)) using the relationship between fina l plant mass and each trait at each density. We observed no significan t selection on either seed mass or emergence time when plants were gro wn alone. At all higher densities, there was significant selection fav oring early emergence and large seed mass, but there were no significa nt differences among the selection gradients determined at densities g reater than individually grown plants. In the field, we detected no re lationship between the magnitude of selection for early emergence and density. Our results suggest that selection on seed mass and time of e mergence is density dependent, but the relationship between density an d the magnitude of directional selection on these traits is not contin uously 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.