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The size of a seed is known to affect the fitness of the plant growing from it; larger seeds often have the higher fitnesses. Based on the assumption that most seed size variation will be found among genotypes, reductions in the variance of seed size, due to selection towards seeds of similar and larger sizes have been predicted, but unfortunately, not observed. Many plant species produce seeds which vary in size due to their position in the inflorescence or fruit. I thus wanted to see if the variation in seed size within a plant created variation in fitness and if so, whether this might explain why the expected reduction in variance is not observed. Pairs of seeds from the self-fertilizing .Hordeum vulgare ssp. spontaneum, sharing maternal and paternal genotype, maternal environment and growth environment but differing in the amount of endosperm available to the embryo, were used. Initial seed size affected time of germination, seedling growth rate and the number of seeds produced. There were, however, no differences in the means of the sizes of seed produced by genetically identical individuals, which can be explained if the position effects are regenerated by seeds of different sizes. It is proposed that it is not a particular seed size which characterises a genotype but a particular distribution of sizes. Position effects are the agents creating the size distributions which ensure that seed size and fitness variation is produced every generation. Evidence is presented which suggests that variation in the expression of position effects exists and may be available to selection. The evolutionary role of this variation is considered in light of current models.

The effects of variation in seed size on growth and reproduction in the wild barley Hordeum vulgare ssp. spontaneum