Data from: 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 on water availability and conspecific density. 5. Contrary to our
predictions, increasing conspecific density relaxed directional selection
favoring larger seeds, but only in the wettest conditions and an increase
in water availability strengthened it, but only at low density. A possible
explanation of these counter‐intuitive results relies on the observed
absence of survival selection and increased plant growth rates under high
water and low density. 6. Larger seeds require more resources to construct
and when this cost is taken into account; there is no overall fitness
increase with seed size. This nicely follows the life history theory
predictions for optimal seed size. At the evolutionary equilibrium, if
seeds could be larger, per seed fitness would still increase, which is
what we observed. But cost corrected fitness should be flat. Maternal
fitness equals per seed fitness times seed number, so any increase to per
seed fitness of making a bigger seed is balanced by the resulting cost to
seed number. Our results indicate flat cost corrected fitness of seed size
as theory predicts.