Evolution of seed size

Species growing in shaded environments tend to produce larger seeds and larger seeded species have higher seedling survivorship in low-light conditions.[7][8][4][3][9][5][10][11] The increased metabolic reserves of larger seeds allows the first shoots to grow taller and leaves to grow broader more quickly in order to compete for what little sunlight is available.[4] A few large seeded trees that occur in closed canopy wooded areas such as old-growth forests are the many oak species, hickory, pecan, and butternut trees.

Small seeds are seen to be predominant in arid, desert environments.[12] In some desert systems the vast majority of annual seeds weigh between zero and two milligrams.[13] small seed size may be a favorable adaptation in desert plants for a couple reasons. Small seeds have been found to have the ability to store in dry environments for several years without desiccating. Also, in many cases, desserts have rainy seasons that provide opportunity for small seeds to germinate under conditions with ample external resources available. Due to the great importance that seeds germinate when water is available, seeds often sense the presence of water and use it as a cue to germinate. Also, many desert plants have evolved the ability to produce a fraction of their seeds to not germinate at the same time as the rest of the plant's seeds as a safe guard known as bet hedging in which if the majority of a plant's seeds germinate at one time and then die due to rain followed by drought, the potential for the plant to have successful offspring is not completely lost.

Granivors (those that feed on seeds and grains) can selectively eat either smaller or larger seeds, favoring seeds on the opposite side of the spectrum. Commonly, granivorous predation by rodents, which selectively feed on larger seeds,[14] leads to higher fitness of smaller seeds (e.g. kangaroo rats in desert systems selectively forage on the larger seeds in the seed bank.[13] Similarly, sometimes smaller seeds are selectively preyed upon such as with Australian granivorous ants which are only capable of carrying smaller seeds.[15]

Competition between seedlings for limited resources can result selective pressures on seed size. In dense mats of competing seedlings, those from larger seeds have higher survivorship [3] due to their ability to more quickly grow taller shoots, broader leaves, and thus out-compete smaller seeded seedlings for resources. Germinated seedlings from larger seeds could also possibly outlive the smaller seeded seedlings which cannot live as long off their stored energy reserves.[4]

If there is a selective pressure favoring the survival of seeds buried deeper in the soil, larger seed size may evolve because of their larger reserves of energy required to emerge from further depths.[16] One such pressure causing this type of selection is the recurrence of fires (e.g. in prairies the heat from a fire can damage or kill seeds near the surface of the soil but leave seeds buried deeper unharmed).

The smaller the seed, the further they can disperse, which can be beneficial for avoiding competition with siblings and the parent[17] as well as having better chances of some of the seeds dispersing into suitable habitat.[5] Dispersal may also lead to greater fitness in future generations if further dispersed individuals are more likely to cross pollinate with an unrelated individuals, leading to greater genetic variation. The type of seed dispersal evolved has been highly correlated to seed size in floras across the world.[18] In general, seeds smaller than 0.1 mg are often unassisted (wind dispersed), seeds larger than 100 mg are often dispersed by vertebrates or by water, and seeds between 0.1 and 100 mg are dispersed by a large variety of dispersal modes including dispersal by a great variety of animals.[5][19]