A levels-of-selection approach to evolutionary individuality

Abstract

What changes when an evolutionary transition in individuality takes place? Many different answers have been given, in respect of different cases of actual transition, but some have suggested a general answer: that a major transition is a change in the extent to which selection acts at one hierarchical level rather than another. The current paper evaluates some different ways to develop this general answer as a way to characterise the property ‘evolutionary individuality’; and offers a justification of the option taken in Clarke (J Philos 110(8):413–435, 2013)—to define evolutionary individuality in terms of an object’s capacity to undergo selection at its own level. In addition, I suggest a method by which the property can be measured and argue that a problem which is often considered to be fatal to that method—the problem of ‘cross-level by-products’—can be avoided.

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Notes

  1. 1.

    The phylogeny of multicellularity is very hard to unpick, but a popular theory is that metazoans evolved by heterochrony from an ancestor that was closely related to a sponge, descended from a choanoflagellate, around 780 million years ago (Valentine and Marshall 2015).

  2. 2.

    Note that transitions are not inevitable or unidirectional. For example, various fungal lineages are thought to have gained multicellularity and then later transitioned back to unicellularity (Sharpe et al. 2015, 9).

  3. 3.

    I will argue in section five that, thanks to the action of individuating mechanisms, much of the time we will get the same result regardless of which trait we choose.

  4. 4.

    Even those authors who dissent will concur that there is a fact of the matter about which of two hierarchical levels is dominant, in any case (Sober 2011).

  5. 5.

    ANOVA of fitness would fail in respect of cases, such as germ separated cases, in which some parts of the individual exhibit much higher fitness than others.

  6. 6.

    Another alternative would be to simply compare the levels of genetic variance at the different hierarchical levels. However, genetic variance is neither necessary nor sufficient for evolution by natural selection. It is not necessary because there can be non-genetic sources of heritable variance in fitness, such as differential vertically transmitted symbionts. It is not sufficient because genetic variants can be prevented from passing their traits onto offspring, as in the case of sterile worker insects.

  7. 7.

    Although, as Birch points out, the extent to which a population is group-structured versus network-structured may itself be continuous, so that groups may have an intermediate level of groupishness (Birch Forthcoming).

  8. 8.

    This characterisation of evolutionary individuals is far from universal. For example, when Hull discusses the individuality of species he is concerned with their particularity, rather than with whether selection acts at the level of species (Hull 1978).

  9. 9.

    It is unlikely, however that this variable can be empirically measured. Shelton and Michod introduce a notion of ‘counterfactual fitness’ in which we try to make informed judgments about how a unit would fare if it was removed from its social setting (Shelton and Michod 2014).

  10. 10.

    More precisely, facts about how a lineage of the unit in question will respond to selection in the future.

  11. 11.

    Which include but are not limited to ‘policing mechanisms’ (Reeve and Keller 1999), and ‘conflict modifiers’ (Michod and Roze 2001).’ Individuating mechanism’ forms a broader class, because it includes what I call ‘demarcation mechanisms’, which enhance focal-level selection, in addition to policing mechanisms, which suppress lower-level selection.

  12. 12.

    The full definition which is defended in Clarke (2013) says that an individuating mechanism is a mechanism that either limits an object’s capacity to undergo within-object selection, by decreasing the availability of within-object heritable variance in fitness (Policing kind), or increases its capacity to participate in a between-object selection process, by increasing the availability of object-level heritable variance in fitness (Demarcation kind).

  13. 13.

    To avoid circularity, we will need to appeal to cases in which there is actual selection at the focal level to justify consideration of a particular mechanism as an individuating mechanisms—as grounding the capacity, in other words.

  14. 14.

    Rare exceptions have been known (Rong et al 1988).

  15. 15.

    Another is to abandon the Price analysis in favour of the contextual approach. This technique of regression analysis avoids the problem of cross level by products, but it has problems of its own. In particular, it yields the counterintuitive result that group selection can occur even in the absence of variation between groups (Okasha 2006).

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Acknowledgments

With many thanks to Samuel Alizon, Pierrick Bourrat, Matthew Herron, Samir Okasha, Thomas Pradeu, Paul Ryan and two anonymous referees.

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Correspondence to Ellen Clarke.

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Clarke, E. A levels-of-selection approach to evolutionary individuality. Biol Philos 31, 893–911 (2016). https://doi.org/10.1007/s10539-016-9540-4

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Keywords

  • Levels of selection
  • Evolutionary individuality
  • Major transitions