Ecological and Evolutionary Consequences of Seed Size in Prunella Vulgaris

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Abstract
Seed size has significant demographic consequences in Prunella vulgaris. Measurements of the effects of seed size were obtained by sowing seeds of known size at four field sites and recording seedling emergence, survival, and size. The intensity of selection on seed size was calculated from these data. Large seeds had a significantly greater probability of emergence at most study sites in each of 2 yr. The effects of seed size were expressed most strongly during the early part of the life cycle, between sowing and emergence, and much less so in later phases of the life cycle. In contrast to a prediction based on the greater relative frequency of large—seeded species in later successional habitats, the effect of seed size on percent seedling emergence did not differ significantly between an old—field and a woodland habitat. However, it is likely that the intensity of natural selection on seed size is greater in an old—field than in a woodland population because the natural distributions of seed size in old—field populations include more small seeds than do those in woodland populations. An analysis of the costs and benefits of producing large and small seeds revealed that in addition to selection favoring large seeds, there was selection favoring individuals that produced large seedlings at three of the four study sites. At the fourth site, there was no selection favoring larger seeds or parents that produced larger seeds. Substantial capacity for phenotypic plasticity in seed size suggests that there may be little opportunity for an evolutionary response despite strong selection favoring large seeds.