Uniform seed size predicted

The seed size/number trade-off concept formalized in the Smith-Fretwell model predicts that: "if a mother plan is in a position to allocate more resources to seed output, it should produce more seeds of the same size...[assuming] some minimum size for a seed to have any chance of establishing and...diminishing returns at some stage as seed mass increases further. The curvature of the Smith-Fretwell function ensures that, if resources are reallocated such that one seed has higher seed mass than the optimum while another has less, the gain in fitness in the augmented seed is smaller than the loss in fitness in the diminished seed."

The mother plant's "...physiological machinery of seed provisioning " should have been selected to produce the maximum number of uniform, optimally sized seed, rather than simply producing larger seeds.

However, seeds vary within a species and even within an individual plant. Explanation?

"The moderate observed variation in seed mass within a species can be attributed either to the machinery of seed provisioning having limited capacity to deliver a completely standardized seem mass or to variability in the Smith-Fretwell function that seedlings are exposed to."

Source: Leishman, M.R., Wright, I.J., Moles, A.T., Westoby, M. (2000) The evolutionary ecology of seed size. In Seeds: the ecology of regeneration in plant communities.Fenner, M., ed. CABI Publishing, Wallingford: p. 31-57.