Plant individuality: a solution to the demographer’s dilemma

Abstract

The problem of plant individuality is something which has vexed botanists throughout the ages, with fashion swinging back and forth from treating plants as communities of individuals (Darwin 1800; Braun and Stone 1853; Münch 1938) to treating them as organisms in their own right, and although the latter view has dominated mainstream thought most recently (Harper 1977; Cook 1985; Ariew and Lewontin 2004), a lively debate conducted mostly in Scandinavian journals proves that the issues are far from being resolved (Tuomi and Vuorisalo 1989b; Fagerström 1992; Pan and Price 2001). In this paper I settle the matter once and for all, by showing which elements of each side are correct.

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Fig. 1
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Notes

  1. 1.

    Apomictic reproduction occurs when a new plant develops from a single celled zygote but without sexual fertilisation.

  2. 2.

    In all cases ‘modular’ will be used here to refer to structural, rather than developmental or evolutionary modularity, and ‘clonal’ to refer to vegetative, rather than parthenogenetic growth. See Clarke (2011) for more on these distinctions.

  3. 3.

    Estimates of the mutation rate per gene per individual generation fall between 10−7 and 10−4 (Otto and Hastings 1998, p. 510).

  4. 4.

    At least this is the view of the modern synthesis. See Jablonka and Lamb (2005) for ways in which somatic variation may affect the germ line after all.

  5. 5.

    Buss describes twenty seven out of fifty living taxa as showing some somatic embryogenesis (Buss 1987, p. 21) but he also shows that germ line sequestering and embryogenesis are not discrete alternatives. Rather, all living things fall somewhere on a spectrum where at one extreme the unitary organisms sequester the germ line early and preformistically and at the other no sequestering occurs at all.

  6. 6.

    This is growth that occurs outside of the ‘normal’ developmental program, if there is one, and which originates from non-meristematic tissue. For example in the practice of coppicing, a tree is cut down above the ground, and adventitious shoots grow from around the trunk.

  7. 7.

    Although there is substantial evidence for evolution in clonal lines in aphids, mites, and bacteria, which are relevantly similar (Weeks and Hoffman 1998; Wilson et al. 2003).

  8. 8.

    As opposed to ‘semelparous’ plants, in which module development is coordinated so that all the parts flower at once, after which the whole plant or group of modules dies.

  9. 9.

    I’ll talk about seeds for simplicity, but this should be taken as standing in for any sexually produced plant propagule.

  10. 10.

    In fact genet growth can be ascertained by counting any genet part—ramets, branches, buds, leaves, tillers, flowers—any countable unit will do. The main criteria used when choosing a focal unit are practical—is it easily accessible? Are the numbers tractable? (Wikberg 1995).

  11. 11.

    Usually the contribution of each type of offspring is weighted by relatedness (see Fagerström 1992) to account for the difference in heritability between sexual and clonal reproduction.

  12. 12.

    For simplicity I’ll use ‘ramet view’ from now on to include all lower level views, in opposition to ‘genet view’.

  13. 13.

    Wikberg argues for what she calls pluralism with respect to unit choice in plants (Wikberg 1995). However, on closer inspection her account is firmly in the genet camp—she advocates a pragmatic sort of pluralism with respect to which unit is chosen as the proxy for fitness.

  14. 14.

    Though see, Gorelick and Heng (2011) who are motivated instead by the significance of epigenetic reset.

  15. 15.

    For example Michod says germ separation prevents somatic cells from having fitness at all, whereas Godfrey-Smith says it merely decouples their fitness from their intrinsic character. See Clarke (2010) for more details.

  16. 16.

    Unless immune-suppressant drugs are administered to prevent the graft from being rejected.

  17. 17.

    This is framed negatively, for simplicity. We might also find mechanisms whose effect is to increase the extent to which these questions gain positive answers. Those qualify as individuating mechanism also, but for a different hierarchical level. Those mechanisms give us reason to call the collection of parts which possess them a collection of organisms, rather than an individual in its own right, and to count at the lower level. Both kinds of mechanism might interact to determine the level at which evolution by natural selection occurs.

  18. 18.

    Although this is not true of all advocates of the lower level view, many of whom insist on an integrated fitness measure that incorporates genetic information with the count of clonal offspring (Pan and Price 2002).

  19. 19.

    Setting non-genetic sources aside for a moment.

  20. 20.

    He also describes how conflicts can be expected to have certain paradoxical consequences. For example, “harmful somatic mutations concerning the roots are expected to accumulate in the branches (and therefore the seeds) as the tree grows older”. The roots themselves would eliminate these mutants by somatic selection, so that “the roots of vegetative offspring would be more resistant to root parasites, for example, than the roots of a young tree germinating from seed.” (Hadany 2000, 519).

  21. 21.

    I am using Okasha’s derivations of these equations (Okasha 2006), p. 22.

  22. 22.

    The assumption is that the covariance relation is locally transitive, so that covariance between an organism’s traits and its fitness, as well as covariance between an organism’s traits and its offspring’s traits, guarantees covariance between an organisms’ fitness and its offspring’s traits.

  23. 23.

    Or the ‘Berkeley Admissions Paradox’. The philosophical mistake given rise to has been termed ‘The Averaging Fallacy’ (Sober and Wilson 1999).

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Acknowledgments

I would like to thank Samir Okasha, Peter Godfrey-Smith, Torbjörn Fagerström, Thomas Pradeu, Henry J Folse III and two anonymous reviewers, as well as the University of Bristol, the Arts and Humanities Research Council and the Konrad Lorenz Institute for Evolution and Cognition Research.

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Clarke, E. Plant individuality: a solution to the demographer’s dilemma. Biol Philos 27, 321–361 (2012). https://doi.org/10.1007/s10539-012-9309-3

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Keywords

  • Biological individuality
  • Plants
  • Modular
  • Genet
  • Ramet
  • Selection