Chapter Four
1. Is the Levels of Selection Debate Relevant to Medicine?
In Giroux’s critique of Canguilhem, she claims that he overemphasizes the role of the individual organism for understanding evolutionary processes. If natural selection can be understood as operating on levels other than the organism, then Canguilhem’s supposed appeal to evolution to support his organismic account of health is troubled. While one could initially object by saying that it is unclear whether Canguilhem upheld the empirical claim that natural selection only acts on organisms, it will still be instructive to explore what is at stake in this debate.
The main idea in the debate concerning units of selection82 has been to clarify what is selected by natural selection. While traditional Darwinism typically focuses on the individual organism’s survival and reproduction as what drives evolution, this was challenged throughout the 20th century. During the 1960s and 70s, for example, some argued that the true unit of selection was not the individual organism, but the genes, since after all genes are what are transmitted to the next generation, they are the ‘replicators’ (e.g. Williams, Dawkins). In reaction to this, there were many biologists in the 1980s who argued that such ‘gene-centrism’
fails to account for phenomena such as altruism in which selection seems to occur on levels higher than the gene and organism, such as the group (e.g. Wilson, Sober, Lewontin, etc.). As might be expected, there were many who argued for a more pluralistic approach, such that natural selection operates on different levels of this hierarchy simultaneously (e.g. Gould, Maynard Smith, Michod, etc.). Ultimately, the general consensus seems to be that natural selection can occur on different levels (i.e. there are various beneficiaries). However, it remains hotly debated how important selection at higher levels (e.g. groups, species) is (Okasha 2006b).
While this debate has become more complex and continues to this day, what is of interest to
82 I will be using the phrases ‘levels of selection’ and ‘units of selection’ more or less interchangeably in this chapter. There are a series of related concepts and issues that I will not be able to fully address in this debate, which pertain to what it is that is replicated in each generation, e.g. genes, developmental processes, etc.
(replicators), whether organisms are merely vehicles, what is causally interacting in an environment (interactors), what interactions are relevant for changes in gene frequency, and who/what it is that benefits from these changes (the beneficiaries) (cf. Okasha 2006a). In what follows I will try to specify which of these issues is at stake (replicators, vehicles, interactors, and beneficiaries) while remaining largely agnostic regarding their utility or validity.
Giroux is that the individual organism seems to have been displaced from the center of evolutionary theory.
Before going into some further complexities of this debate, I would like to take a different approach and begin by asking what relevance this debate has for the philosophy of medicine. Philosophers on both sides of the naturalism-normativism divide have used the issue of units of selection to problematize or clarify the role of evolution (e.g. Engelhardt 1978;
Boorse 1997). The main reason for this seems to be that if some trait is beneficial for the transmission of genes or for group survival, but in either case is harmful for the individual organism, then this seems to make it difficult to discern whether the trait is normal or pathological. As Nesse asks: ‘at which level of selection should we define abnormality’ (2001, p.
43)? If natural selection acts on (or benefits) genes or groups, then maybe the concepts of health and disease need to be reworked so as to account for such occurrences.
If we look at some examples, we can see how these claims could be substantiated. On the level of genes, it has been suggested that health judgments should consider the phenomenon of pleiotropy, i.e. the various phenotypic effects of a given gene. Nesse (2001, p. 43) provides the hypothetical example of a gene that codes for improved sperm motility but which causes some serious disease later in life. As it is initially advantageous, this gene would likely be selected for despite its later deleterious effects. A non-hypothetical example of pleiotropy would be that of stem cells which are initially adaptive by aiding tissue maintenance and repair, but which increase the risk of neoplasia later in life (Gluckman, Beedle & Hanson 2009, p. 263). For an individual organism, these traits would be pathological, yet from an evolutionary perspective they seem healthy. Moving to a higher level, it has been argued that gestational diabetes might have some adaptive benefits for the fetus, even though it harms the mother, and thus traits allowing for this diabetes could be favored (Haig 1993). Whereas it seems pathological for the mother as an individual, an evolutionary perspective could suggest that it is a ‘healthy condition’ (Ananth 2008, p. 211), possibly because, like morning sickness in general, it has adaptive benefits for both the mother and her embryos (Sherman & Flaxman 2002).
While these examples raise some interesting questions, I want to suggest that the relevance of this debate has been overstated. First, it has been pointed out by many researchers working within the new field of ‘Darwinian Medicine’ that we should expect there to be a discrepancy between what is selected for in a given population and the resulting effects on individual health precisely because natural selection shapes organisms for the maximization of
reproductive success rather than health (Nesse & Williams 1994; Gluckman, Beedle & Hanson 2009). Processes at the genetic or group level can give rise to various forms of adaptations that need not result in the health of the organism but merely ensure that its capacities for survival and reproduction are sufficient to allow for the maintenance of this adaptation. Consequently, if natural selection does not promote health, then it is not clear why Giroux would focus on this debate to question Canguilhem’s approach. Second, without clarifying the different concerns of biology and medicine, it seems possible to conflate descriptions of natural variation with their medically relevant effects in a given individual83. In other words, while the pleiotropic effects of genes are ‘normal and natural’, it is a different question as to whether they constitute a disease in the organism ‘as a whole’ (Gammelgaard 2000, p. 114). Rather than asking whether a condition such as gestational diabetes is a disease, it seems more helpful to ask under what conditions it could become pathological84. Even if mild or ‘normal’ levels of it are adaptive, the mother’s potential suffering cannot be so easily bypassed. Third, such issues have led some to argue that basing medical judgments on evolutionary considerations is problematic since in the end physiological explanations and medical experience are the most useful (Cournoyea 2013).
Whether or not something is beneficial for genes or groups remains secondary to whether it affects a given organism’s ability to function in its environment. Thus, if natural selection is not concerned with the health of individual organisms, if there is an important difference between biological and medical goals85, and if physiology coupled with medical experience ultimately guides medical judgments, then it is not clear why multi-level selection would challenge a focus on organisms when it comes to conceptualizing health and disease86. As I showed in the previous chapter, even if there are multiple causal forces involved in disease etiology, this need not be conflated with disease ontology.
Despite these reservations, if we decide to take Giroux seriously that the levels of selection debate implies a rethinking of health and disease, then it is reasonable to ask whether her presentation of the debate is accurate. While accepting the possibility of multiple levels of
83 ‘The idea that some variation is “normal” and some “abnormal” has no place in evolutionary theory’ (Stearns et al. 2008).
84 It is interesting to point out that while exploring the potentially adaptive benefits of various traits, e.g. morning sickness, it is still only ‘normal’ levels of them which are adaptive (Sherman & Flaxman 2002).
85 Gammelgaard concludes that in medicine ‘all goal-states are determined in relation to the individual, while in evolutionary biology, goal-states are determined to the unit of selection and the unit of selection need not be the individual’ (2000, p. 115). Consequently, one cannot deduce a concept of disease from evolutionary theory.
86 This issue of how evolutionary considerations contribute to our understanding of health and disease is more complex than being argued here. I will return to it in detail in the next chapter.
selection, is there a reason to uphold the individual organism as at least the primary unit87 and is the ‘organism’ still a useful explanatory concept in biology?