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Natural Selection and Drift as Individual-Level Causes of Evolution

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Abstract

In this paper I critically evaluate Reisman and Forber’s (Philos Sci 72(5):1113–1123, 2005) arguments that drift and natural selection are population-level causes of evolution based on what they call the manipulation condition. Although I agree that this condition is an important step for identifying causes for evolutionary change, it is insufficient. Following Woodward, I argue that the invariance of a relationship is another crucial parameter to take into consideration for causal explanations. Starting from Reisman and Forber’s example on drift and after having briefly presented the criterion of invariance, I show that once both the manipulation condition and the criterion of invariance are taken into account, drift, in this example, should better be understood as an individual-level rather than a population-level cause. Later, I concede that it is legitimate to interpret natural selection and drift as population-level causes when they rely on genuinely indeterministic events and some cases of frequency-dependent selection.

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Notes

  1. In population genetics, the term ‘force’ is used more often than the term ‘cause.’ I consider them as roughly equivalent, although I will use the term ‘cause’ throughout the paper. This is because I will make use of concepts developed in the philosophical literature on causation. For more on the difference between the use of these two terms in evolutionary theory see Hitchcock and Velasco (2014).

  2. This division into four causes is to some extent arbitrary. Some authors would for example add sex or recombination as other forces or causes.

  3. See for example Bouchard and Rosenberg (2004), Matthen and Ariew (2002, 2009), Millstein (2006), Northcott (2010), Reisman and Forber (2005), Rosenberg and Bouchard (2005), Shapiro and Sober (2007), Stephens (2004), Walsh (2000, 2007, 2010), Walsh et al. (2002), Ariew et al. (2015), Walsh et al. (2017), and Otsuka (2016).

  4. To be sure, most causal explanations involve the use of statistics in evolutionary biology. The statisticalists have no objection to that, but they claim that referring to causes is in principle dispensable for evolutionary explanations.

  5. Otsuka et al. (2011, see also Northcott 2010), argue that context independence is far from being a necessary condition for a relationship to be considered as causal. Context dependence is in fact a feature of causal modeling.

  6. In this paper, “individual” should be understood here as “any entity below the level of the population.” As pointed out by Charles Pence (personal communication), this notion of “individual” might refer to something different from what the individual causalists mean by this term, since for them “individuals” often refer to “individual organisms.” If that is the case, I accept this departure from that of the individual causalists.

  7. Stephens’ (2004) position seems close to that of Reisman and Forber. He assumes that drift and NS are population-level causes: “the point is that the effect of drift is only properly understood at the population level. It is a population level cause. One sees the differential causal impact of drift only by comparing populations of different sizes” (p. 556, see also pp. 563–564).

  8. Reisman and Forber’s exact formulation is as follows: “[Premise] 1. The manipulation condition. [Premise] 2. Manipulating the character of selection and drift can result in systematic changes to population-level dynamics. [Conclusion] Selection and drift are causes of population-level dynamics.” (2005, 1114) Reisman and Forber explicitly refer to population-level variables as causes when they claim: “we can manipulate the strength of drift in a population by manipulating the size of the population” (p. 1115). When they refer to the authors of the experiment they discuss, they write “[t]hey manipulate a population-level parameter to test how selection and drift interact to produce evolutionary change” (p. 1116).

  9. Note that there exists some heterogeneity in the genetic background of fruit flies. Thus, the environment is not homogenous from the point of view of the alleles of the population. This difference is crucial for the rest of the argument.

  10. In the example proposed by Dobzhansky and Pavlovsky, the measures of frequency are made on alleles, not individual organisms. Thus, an “individual” refers here to a token allele.

  11. Note, that using the binomial distribution implies the assumption of a founding population of unlimited size (equivalent to a drawing without replacement). If we were to release this assumption and have a source population with a finite size (equivalent to a drawing with replacement), the appropriate distribution would be a hypergeometric one. It can be shown that as the population size increases, the hypergeometric distribution tends toward a binomial one.

  12. This assumes that the variability in the genetic background is not neutral. Of course, such a manipulation would have consequence of a higher magnitude in a small than large population since the frequency of one allele is a small population is different from that of a large one. But that is beside the point.

  13. For more on the subtleties of the notion of invariance see Pocheville et al (2017).

  14. This point might look, at first glance, similar to the point made by the statisticalists and discussed in the introduction, that natural selection and drift are not causes of evolution because they are context dependent. The point here however is different. All causal relationships are to some extent context dependent, and following the criterion of invariance, the less context dependent relationship leads to better causal explanation. Thus, the point is not that context dependent relationships are not causal explanations as argued by the statisticalists.

  15. For a discussion on the notion of screening-off see Brandon (1990).

  16. We could, for instance, imagine that some quantum processes percolate up in a biological process and have consequences on reproductive outputs, as done by Glymour (2001).

  17. Needless to say, both deterministic and indeterministic natural selection/drift can be at play in a single population.

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Acknowledgements

I am thankful to Paul Griffiths, Frans Jacobs, Charles Pence, Arnaud Pocheville, Joeri Witteveen and three anonymous reviewers for their comments on earlier versions of the manuscript. This research was supported under Australian Research Council’s Discovery Projects funding scheme (project number DP150102875) and a Macquarie University Research Fellowship.

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  1. Department of Philosophy, Macquarie University, North Ryde, NSW, 2109, Australia

    Pierrick Bourrat

  2. Department of Philosophy, School of History and Philosophy of Science and Charles Perkins Centre, The University of Sydney, Sydney, NSW, 2006, Australia

    Pierrick Bourrat

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Bourrat, P. Natural Selection and Drift as Individual-Level Causes of Evolution. Acta Biotheor 66, 159–176 (2018). https://doi.org/10.1007/s10441-018-9331-1

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