Fitness consequences in germination success and reproductive success in relation to seed size were investigated for two consecutive years for a population of a desert annual plant, Dithyrea californica. Seed size in this system can be readily measured in its natural setting even after the seed has germinated, making our system ideal for heritability estimates and the direct evaluation of the fitness consequences and hence natural selection on seed size in the wild.
Germination success as a function of seed size was evaluated by measuring seed sizes for 150 germinated plants and from the remaining dormant seeds in the seed bank.
Reproductive success was determined by measuring seed sizes of 200 plants and comparing them to adult reproductive success (size and number of offspring produced).
2008 and 2009 differed in the amount and timing of precipitation, the primary environmental driver in deserts, giving us the chance to observe the consequences of seed size on fitness in good and bad years. Our results suggest that temporal environmental variation results in variable selection on seed size.
Germination in both 2008 and 2009 was not significantly correlated with seed size.
Total reproductive allocation (seeds produced times mean offspring seed size) showed no significant correlation with parental seed size in 2008, the year with good environmental conditions (R2 = 0.015; p = 0.08). However, drier conditions during 2009 lead to lower seedling establishment and highly significant correlations of seed size with seedling survival (R2 = 0.35; p = 0.0001), showing differential size dependent mortality.
These results suggest that making larger seeds may be favored by natural selection in years when dry conditions result in seedling mortality, since seed size appears to provide a survival advantage in the face of drought. However, in years like 2008, when water is more readily available, seed size may be neutral on a per seed basis, leading to selection for greater numbers of seeds, hence smaller seeds.