Selection for seed size: the unexpected effects of water availability and density

1. Seed size is a functional trait with important fitness consequences that potentially extend throughout the life cycle of plants. Dithyrea californica experiences selection for larger seeds in post‐germination stages but it is still uncertain how environmental factors mediate the strength and the direction of natural selection on seed size. 2. D. californica represents a unique opportunity to investigate selection on seed size in natural conditions due to a persistent seed ring that stays attached to the root throughout the plant's life. This makes it possible to unearth plants at any stage and measure the size of the seed from which they originated. 3. We conducted a factorial experiment manipulating water availability and intraspecific competition using plants that naturally germinated in the wild. 4. Selection on seed size via survivorship was nil because all individuals survived to reproduce. The strength and the direction of selection on seed size via fecundity depended ...