The first seeded plants emerged in the late

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

370 million years ago. Selection pressures shaping seed size stem from physical and biological sources including drought,

predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

, seedling-seedling

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

, optimal

dormancy Dormancy is a period in an organism's Biological life cycle, life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolism, metabolic activity and therefore helps an organism to conserv ...

depth, and dispersal. Coco de mer - BOT

History

Since the evolution of the first seeded plants ~370 million years ago, the largest change in seed size was found to be at the divergence of gymnosperms and angiosperms ~325 million years ago, but overall, the divergence of seed size appears to take place relatively consistently through evolutionary time. Seed mass has been found to be phylogenetically conservative with most differences in mean seed mass within types of seed dispersal (dispersal modes) being

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

. This type of information gives us clues about how seed size evolved. Dating fossilized seeds of various sizes and comparing them with the presence of possible animal dispersers and the environmental conditions of the time is another technique used to study the evolution of seed size. Environmental conditions appear to have had a larger influence on the evolution of seed size compared to the presence of animal dispersers. One example of seed size evolving to environmental conditions is thought to have been abundant, closed forest vegetation selecting for larger seed sizes during the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch. A general increase or decrease in seed size through time has not been found, but instead a fluctuation in seed size following the environmental conditions of the

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

, and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 Today we also see a pattern with seed size distribution and global environmental conditions where the largest mean seed size is found in tropical forests and a steep decrease in seed size takes places globally as vegetation type changes to non-forest.

Mechanism

Modern seed sizes range from 0.0001 mg in orchid seeds to in double coconuts. Larger seeds have larger quantities of metabolic reserves in their embryo and

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...

available for the seedling than smaller seeds, and often aid establishment under low resource availability. However, smaller seeds can be produced in larger quantities which has the potential to produce more offspring and have better chances of some of the seeds dispersing into suitable habitat. This seed size-number trade off has led to the evolution of a wide range in size and number of seeds in response to environmental selection pressures.

Selective pressures

No single event, such as a large divergence in the phylogeny of seeded plants, is seen as the cause of major divergences in seed size. Rather, small events are thought to occur fairly consistently through time with minor evolutionary influence.

Shade

Species growing in shaded environments tend to produce larger seeds and larger seeded species have higher seedling survivorship in low-light conditions. 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. A few large seeded trees that occur in closed canopy wooded areas such as

old-growth forest An old-growth forest or primary forest is a forest that has developed over a long period of time without disturbance. Due to this, old-growth forests exhibit unique ecological features. The Food and Agriculture Organization of the United Natio ...

s are the many

oak An oak is a hardwood tree or shrub in the genus ''Quercus'' of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisp ...

species,

hickory Hickory is a common name for trees composing the genus ''Carya'', which includes 19 species accepted by ''Plants of the World Online''. Seven species are native to southeast Asia in China, Indochina, and northeastern India (Assam), and twelve ...

pecan The pecan ( , , ; ''Carya illinoinensis'') is a species of hickory native to the Southern United States and northern Mexico in the region of the Mississippi River. The tree is cultivated for its seed primarily in the U.S. states of Georgia ( ...

, and butternut trees.

Drought

Small seeds are seen to be predominant in arid, desert environments. In some desert systems the vast majority of annual seeds weigh between zero and two milligrams. 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, deserts 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.

Predation

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, leads to higher fitness of smaller seeds (e.g. kangaroo rats in desert systems selectively forage on the larger seeds in the seed bank. Similarly, sometimes smaller seeds are selectively preyed upon such as with Australian granivorous ants which are only capable of carrying smaller seeds.

Seedling-seedling competition

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 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.

Optimal dormancy depth

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. 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).

Dispersal

The smaller the seed, the further they can disperse, which can be beneficial for avoiding competition with siblings and the parent as well as having better chances of some of the seeds dispersing into suitable habitat. Dispersal may also lead to greater fitness in future generations if further dispersed individuals are more likely to

cross pollinate Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds. Pollinating agents can be animals such as insects, for example bees, beetles or butterflies; birds ...

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. 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.

References

{{Reflist, 30em Evolution of plants Phylogenetics Tree of life (biology)