Sources and consequences of seed size variation in Lupinus perennis (Fabaceae): adaptive and non-adaptive hypotheses.

Seed size variation within species and individuals is common. This variation may be adaptive in heterogeneous landscapes if the fitness consequences of seed size differ among environments or through time. Variation may also arise from constraints that limit control of seed size. I manipulated resource availability in both maternal and offspring environments to test conditions underlying these explanations for seed size variation in the herbaceous perennial Lupinus perennis. A fivefold variation in seed size arose primarily from differences among individuals and within-plant variability rather than from environmental conditions manipulated in the experiment. Environmental conditions had little effect on mean seed size; in contrast, within-plant variation in seed size increased with reduced resources. Fitness benefits from large seed size were similar across offspring environments, suggesting that environmental heterogeneity alone may not maintain seed size variation in this species. Surprisingly, seed size affected long-term fitness measures, including a plant's size and probability of flowering through its second year. These results are consistent with non-adaptive but not adaptive explanations for seed size variation. They also suggest that offspring size variation per se may contribute to variation in maternal fitness.