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Why do big plants make big seeds?

Authors

  • MARK REES,

    Corresponding author
    1. Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK, and
      *Author to whom correspondence should be addressed: Mark Rees. E-mail: m.rees@sheffield.ac.uk.
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  • D. LAWRENCE VENABLE

    1. Department of Ecology and Evolutionary Biology, University of Arizowna, Tucson, AZ 85721, USA
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*Author to whom correspondence should be addressed: Mark Rees. E-mail: m.rees@sheffield.ac.uk.

Summary

  • 1The conventional explanations for large plant species producing larger seeds on average than small plant species have recently been challenged, and it has been suggested that the pattern is better explained by the theory developed by Charnov (1993). Here we use simple life-history theory to explore the logic underlying Charnov's models and show that under most reasonable conditions they predict no relationship between seed mass and size at maturity.
  • 2Using a simple general model incorporating size-specific growth and survival, we explore the joint evolution of seed mass and size at maturity, and argue that seed mass will be correlated with adult traits, such as the timing of reproduction and size at reproduction, only if seedling and adult growth and mortality rates are correlated. Evidence for such correlations is briefly explored.
  • 3It has also been suggested that the standard model for seed mass evolution (Smith & Fretwell 1974) has been misinterpreted, and that application of the model requires measurement of survivorship to reproductive maturity. Using a simple model incorporating size-specific growth and survival we show that this criticism is unfounded.
  • 4Our results differ from those of Moles and colleagues because they look at the effect of long juvenile period on survival to maturity, but do not recognize that this may be compensated by covarying life-history traits, such as plant size and reproductive lifespan. Also, they seem to argue that life-history evolution is constrained by cross-angiosperm correlations, such as that between seed mass and longevity, while the life-history models presented here seek selective causes of such correlations, rather than regarding them as constraints.
  • 5Models similar to those of Charnov (1993) only predict a positive relationship between seed size and plant size if unrealistic assumptions are made about the effects of seed mass on survival, such as the effect of seed mass on instantaneous survival persisting to adulthood.

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