PROXIMATE AND ULTIMATE SOURCES OF WITHIN-INDIVIDUAL VARIATION IN SEED MASS IN PRUNELLA VULGARIS (LAMIACEAE)

Substantial variation in seed mass within individual seed parents is at odds with predictions of models for the evolution of optimum offspring size and with empirical observations of directional selection on seed mass. To elucidate the ultimate causes of this variation, I examined several proximal sources of within-individual variation in seed mass in the perennial herb Prunella vulgaris. Position of inflorescence, position of flower within an inflorescence, date of anthesis, and number of seeds produced per flower all explained some within-individual variation in seed mass. Hand pollination in the field failed to reveal any effect of pollen source (self pollen or outcross pollen) on seed mass. My results, in conjunction with those from studies of selection on seed mass in P. vulgaris, do not support hypotheses that within-individual variation in seed mass is favored by the pattern of natural selection on seed mass. Rather, the results suggest that seed parents are not capable of producing a uniform seed crop in the face of changes in resource availability in the course of a season. The inability to produce a uniform seed crop may persist because of selection for variability in traits correlated with seed mass or because of a true constraint on the evolution of uniform offspring size.