Under stressful circumstances, seed size has important consequences for germination, survival, and reproductive success; all of these are important components of plant fitness. This study investigates the relationship between seed size and fitness in the Sonoran Desert winter annual Dithyrea californica. This species represents a unique opportunity to study natural selection on seed size in the wild due to a serendipitous detail of its life history: the seed coat remains attached and unchanged to the root throughout its life. It is thus possible to excavate the root and measure the seed size that originated each plant. We measured the relationship between seed size and germination by comparing seed sizes of germinated and dormant seeds in the field over four consecutive years. We also measured the effect of seed size on survival and reproductive success using data from censuses of plant mortality and fecundity of survivors, relating survival and fecundity to the size of their initial seed size, and the number of conspecific neighbors. Larger seeds had a higher probability of germination than smaller seeds. Plants originating from larger seeds had higher survival rates and higher fecundity than plants originating from smaller seeds. The amount of precipitation had a beneficial effect on plant fecundity and influenced seed-size survival selection. Plant competition decreased plant fecundity but not survival, creating a detrimental environment for plants only to grow and reproduce. This is the first study to show empirical evidence of seed-size selection throughout the whole life cycle in a natural setting. Further, maternal provisioning has benefits that persist into adulthood, and environmental interactions are important in determining survival and fecundity.