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Evolution of seed size

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The first seeded plants emerged in the late Devonian 370 million years ago. Selection pressures shaping seed size stem from physical and biological sources including drought, predation, seedling-seedling competition, optimal dormancy depth, and dispersal.

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Mechanism

Modern seed sizes range from 0.0001 mg in orchid seeds to 42 kg in double coconuts.[1][2] Larger seeds have larger quantities of metabolic reserves in their embryo and endosperm available for the seedling[3] than smaller seeds, and often aid establishment under low resource availability.[4] 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.[5] This seed size-number trade off[6] 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.[2]

Shade

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.

Drought

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.

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,[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]

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 [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]

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.[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).

Dispersal

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]

Evolution of Seed Size

Since the evolution of the first seeded plants ~370 million years ago,[20] 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.[2] Seed mass has been found to be phylogenetically conservative[5] with most differences in mean seed mass within types of seed dispersal (dispersal modes) being phylogenetic.[21] This type of information gives us clues about how seed size evolved.[2] 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 epoch.[22] 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, Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs.[22] 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.[23]

References

  1. ^ Harper, J. L.; Lovell, P. H.; Moore, K. G. (1970). "The Shapes and Sizes of Seeds". Annual Review of Ecology and Systematics. 1: 327–356. doi:10.1146/annurev.es.01.110170.001551. JSTOR 2096777.
  2. ^ a b c d Moles, Angela T.; Ackerly, David D.; Webb, Campbell O.; Tweddle, John C.; Dickie, John B.; Westoby, Mark (2005-01-28). "A Brief History of Seed Size". Science. 307 (5709): 576–580. doi:10.1126/science.1104863. ISSN 0036-8075. PMID 15681384.
  3. ^ a b c Westoby, Mark; Jurado, Enrique; Leishman, Michelle (1992). "Comparative evolutionary ecology of seed size". Trends in Ecology & Evolution. 7 (11): 368–372. doi:10.1016/0169-5347(92)90006-w. PMID 21236070.
  4. ^ a b c d Leishman, Michelle R; Wright, Ian J; Moles, Angela T; Westoby, Mark (2000). 'The Evolutionary Ecology of Seed Size', in Seeds: The Ecology of Regeneration in Plant Communities. CABI. ISBN 9780851999470.
  5. ^ a b c d e Westoby, Mark; Leishman, Michelle; Lord, Janice (1996-09-30). "Comparative ecology of seed size and dispersal". Phil. Trans. R. Soc. Lond. B. 351 (1345): 1309–1318. doi:10.1098/rstb.1996.0114. ISSN 0962-8436.
  6. ^ Smith, Christopher C.; Fretwell, Stephen D. (1974). "The Optimal Balance between Size and Number of Offspring". The American Naturalist. 108 (962): 499–506. doi:10.1086/282929. JSTOR 2459681.
  7. ^ Saverimuttu, Tharman; Westoby, Mark (1996). "Seedling Longevity under Deep Shade in Relation to Seed Size". Journal of Ecology. 84 (5): 681–689. doi:10.2307/2261331. JSTOR 2261331.
  8. ^ Salisbury, Edward J (1975). "Seed size and mass in relation to environment". Proc. R. Soc. Lond. B. 186 (1083): 83–88. doi:10.1098/rspb.1974.0039. ISSN 0080-4649.
  9. ^ Gross, Katherine L. (1984). "Effects of Seed Size and Growth Form on Seedling Establishment of Six Monocarpic Perennial Plants". Journal of Ecology. 72 (2): 369–387. doi:10.2307/2260053. JSTOR 2260053.
  10. ^ Salisbury, Edward James (1942). The Reproductive Capacity of Plants. G. Bell And Sons.
  11. ^ Moles, Angela T.; Westoby, Mark (2004). "What Do Seedlings Die from and What Are the Implications for Evolution of Seed Size?". Oikos. 106 (1): 193–199. doi:10.1111/j.0030-1299.2004.13101.x. JSTOR 3548409.
  12. ^ Mazer, Susan J. (1989-02-01). "Ecological, Taxonomic, and Life History Correlates of Seed Mass Among Indiana Dune Angiosperms". Ecological Monographs. 59 (2): 153–175. doi:10.2307/2937284. ISSN 1557-7015. JSTOR 2937284.
  13. ^ a b Chen, Ting C.; Valone, Thomas J. (2017-07-01). "Rodent granivory strengthens relationships between seed size and plant abundance in a desert annual community". Journal of Vegetation Science. 28 (4): 808–814. doi:10.1111/jvs.12529. ISSN 1654-1103.
  14. ^ Chen, Ting C.; Valone, Thomas J. (2017-07-01). "Rodent granivory strengthens relationships between seed size and plant abundance in a desert annual community". Journal of Vegetation Science. 28 (4): 808–814. doi:10.1111/jvs.12529. ISSN 1654-1103.
  15. ^ Hughes, Lesley; Westoby, Mark (1990-02-01). "Removal Rates of Seeds Adapted for Dispersal by Ants". Ecology. 71 (1): 138–148. doi:10.2307/1940254. ISSN 1939-9170. JSTOR 1940254.
  16. ^ Bond, W. J.; Honig, M.; Maze, K. E. (1999). "Seed Size and Seedling Emergence: An Allometric Relationship and Some Ecological Implications". Oecologia. 120 (1): 132–136. doi:10.1007/s004420050841. JSTOR 4222367. PMID 28308044.
  17. ^ Venable, D. Lawrence (1992). "Size-Number Trade-Offs and the Variation of Seed Size with Plant Resource Status". The American Naturalist. 140 (2): 287–304. doi:10.1086/285413. JSTOR 2462610.
  18. ^ Leishman, Michelle R.; Westoby, Mark; Jurado, Enrique (1995). "Correlates of Seed Size Variation: A Comparison Among Five Temperate Floras". Journal of Ecology. 83 (3): 517–529. doi:10.2307/2261604. JSTOR 2261604.
  19. ^ Hughes, Lesley; Dunlop, Michael; French, Kristine; Leishman, Michelle R.; Rice, Barbara; Rodgerson, Louise; Westoby, Mark (1994). "Predicting Dispersal Spectra: A Minimal Set of Hypotheses Based on Plant Attributes". Journal of Ecology. 82 (4): 933–950. doi:10.2307/2261456. JSTOR 2261456.
  20. ^ Linkies, Ada; Graeber, Kai; Knight, Charles; Leubner-Metzger, Gerhard (2010-06-01). "The evolution of seeds". New Phytologist. 186 (4): 817–831. doi:10.1111/j.1469-8137.2010.03249.x. ISSN 1469-8137.
  21. ^ Lord, Janice; Westoby, Mark; Leishman, Michelle (1995). "Seed Size and Phylogeny in Six Temperate Floras: Constraints, Niche Conservatism, and Adaptation". The American Naturalist. 146 (3): 349–364. doi:10.1086/285804. JSTOR 2463112.
  22. ^ a b Eriksson, Ove; Friis, Else Marie; Löfgren, Per; Schemske, Associate Editor: Douglas W. (2000). "Seed Size, Fruit Size, and Dispersal Systems in Angiosperms from the Early Cretaceous to the Late Tertiary". The American Naturalist. 156 (1): 47–58. doi:10.1086/303367. JSTOR 10.1086/303367. PMID 10824020. {{cite journal}}: |first4= has generic name (help)
  23. ^ Moles, Angela T.; Ackerly, David D.; Tweddle, John C.; Dickie, John B.; Smith, Roger; Leishman, Michelle R.; Mayfield, Margaret M.; Pitman, Andy; Wood, Jeff T. (2006). "Global patterns in seed size". Global Ecology and Biogeography. 0: 061120101210018––. doi:10.1111/j.1466-822x.2006.00259.x.
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