Natural Selection on Two Seed-size Traits in the Common Morning Glory Ipomoea Purpurea (Convolvulaceae): Patterns and Evolutionary Consequences

Plants may express two separate seed-size characters during their lifetimes: the size of the seeds from which they germinate (initial seed size) and the mean size of seed they produce as adults (maternal seed size). Many empirical studies indicate that selection often favors a larger initial seed size. In contrast, patterns of natural selection on maternal seed size have not been measured, although theory often predicts stabilizing selection. Here, I report on a field study of the common morning glory Ipomoea purpurea, which provided measurements of natural selection on both initial and maternal seed size. For initial seed size, selection favored larger seeds, but a greenhouse study indicated no genetic variation for this trait. For maternal seed size, there was no evidence of either directional or stabilizing selection, but there was significant additive genetic variation. The genetic correlation between the number and size of seeds was not significant, indicating no trade-off between these traits, but a negative genetic correlation was found between maternal seed size and the probability of surviving to reproduce. The absence of the predicted pattern of stabilizing selection on maternal seed size in the study population highlights the need for more empirical work on the evolution of seed size.