Why do zebras have black and white stripes? No, it’s not to amuse the fans of Juventus, an (in)famous Italian football club with a black-and-white-striped home kit. It’s probably because black and white stripes have a function in the life of zebras. This function, available evidence suggests, is to deter disease-carrying flies from biting the zebra.
A question such as ‘Why do zebras have a striped coat?’ is interesting in its own right. But it also motivates a number of more general questions, which lie at the foundations of the life sciences. For example, what are biological functions? What sorts of explanations are biologists offering when they say that a certain trait has a certain function? And what are concrete philosophical and scientific consequences of a theory of biological functions?
Justin Garson’s What Biological Functions Are and Why They Matter answers these questions, defending a novel theory of biological functions, namely, the generalized selected effects (GSE) theory. According to this theory, ‘the function of a trait is whatever it did, in the past, that contributed to the trait’s differential reproduction or differential retention within a population’ (p. 3).
GSE combines three ideas. First, it retains the core idea of selected effects theories of function (for example, Millikan ; Neander ): the biological function of a trait is what it was selected for, either by natural selection or some other historical process that caused individuals with this trait to leave behind more offspring than individuals without it.
Second, GSE goes beyond existing selected effects theories by saying that selection does not require reproduction or copying: differential retention suffices. For some biological traits, like synapses in the brain, the trait’s biological function would be whatever caused it to persist longer than some other trait within a population. Although the causal effects of synapses do not contribute to their own reproduction, they can contribute to their differential strengthening and weakening, and so to their differential retention compared to other synapses in the same ‘population’ (cf. Innocenti and Price ; Riccomagno and Kolodkin ). Therefore, selective processes need not operate only over entities that can reproduce or make copies of themselves.
Third, GSE emphasizes that selection processes operate over entities in a population, which is, roughly, a grouping of individuals whose behaviours causally influence each other so as to make a difference to the probabilities of reproduction or persistence of those individuals. Because rocks scattered on a beach do not constitute a population, they cannot undergo processes of selection, even though some of those rocks will quickly erode, while harder rocks on that beach will persist for longer. The features of a rock, unlike zebra stripes, do not have functions.
Building on these three ideas, GSE sets out to make better sense of three central features of biologists’ practice: First, GSE would make sense of the distinction between biological function and ‘lucky accident’, clarifying why a trait’s biological function is not simply anything the trait does. For example, zebra stripes amuse some football fans, but also ward off biting flies. Why is the latter but not the former a function of zebra stripes? Because warding off biting flies is one of the selected effects of having stripes; amusing football fans is not.
Second, GSE would also provide us with a good account of when and why biological functions have ‘explanatory depth’. Scientists often ascribe certain functions to certain traits because by making these ascriptions, they purport to explain why individuals within a population have these traits instead of others. Warding off biting flies, for example, would causally explain why zebras have black and white stripes instead of a coat with pink polka dots. The causal effects of some trait in the past can explain the very existence of the trait now, because of historical selection processes. What we want to explain is why a trait is present—why, for example, zebras or some particular zebra have a certain physical feature. And we appeal to processes of selection over an evolutionary or ontogenetic timescale to explain this fact; we appeal, for example, to the idea that over the evolutionary history of the zebra, black and white stripes have helped zebras survive by warding off biting flies.
Third, GSE would help us understand when and in what sense traits malfunction. That is, it helps us make sense of the ‘normativity’ of biological functions. If the stripes of the zebra systematically fail to deter biting flies, then they malfunction: they fail to do what they should do. Because the function of a trait depends on its selective history rather than on its present-day capacities, it’s easy to understand why a trait may possess a function it systematically fails to perform now. Malfunctioning traits fail to bring about the effects for which they were selected.
Garson fleshes out these three claims and develops a sustained argument in support of GSE over the twelve chapters of What Biological Functions Are and Why They Matter. The argument has the form of ‘a parity of reasoning’ argument: If traditional selected effects theories make good sense of three central features of biologists’ practices—namely, the explanatory and normative features of functions and the function/accident distinction—then we have reason to accept them. Like traditional selected effects theories, GSE makes good sense of those three features. But unlike selected effects theories, GSE accounts for all these features without ad hoc extensions or arbitrary restrictions, such as that all processes of selection must consist in processes of natural selection or must operate on entities that reproduce. So, we have greater reason to accept GSE than selected effect theories.
While many chapters of What Biological Functions Are and Why They Matter draw from Garson’s previously published work, this existing material is developed in new ways and organized into three parts: Background (Chapters 1–3), Theory (Chapters 4–8), and Applications (Chapters 9–12).
Chapter 1 lays out three desiderata for a theory of biological function. A good theory should illuminate the ‘explanatory depth’ of biological functions, distinguish statements involving genuine functions from statements involving ‘lucky accidents’, and make sense of the ‘normativity’ of functions. And it should do so in a way that comports with ordinary biological usage of the term ‘function’ and with relevant biological practices aimed at explanation, discovery, testing, and causal intervention.
Chapter 2 reviews the main ideas of traditional selected effects theory and addresses some common objections against it, such as the objection that natural selection cannot explain features of specific individuals. The upshot of this chapter is that if functions are selected effects, then they possess explanatory depth.
Chapter 3 argues that if functions possess explanatory depth, then they must be selected effects. This chapter first dismisses any ‘forward-looking’ theory that does not appeal to historical processes. It then compares selected effects theories with two alternative classes of theories that appeal to the causal history of organisms but are not committed to the idea that the explanatory depth of functions should be explained in terms of the selected effects of a trait. Theories in the first class, the ‘organizational approach to function’, say that a trait’s function is whatever it does that causally contributes to its persistence. The problem with this approach is that it ascribes functionality to too many traits, even to lucky accidents or traits like panic attacks, which scientists ordinarily consider dysfunctional. Theories in the second class, ‘weak etiological theories of function’, say that a trait’s function is whatever it does to contribute to an increase in its bearer’s expected number of offspring. The problem here is that these theories cannot explain why an individual has certain traits; they can only explain why the individual exists.
After this background, Garson proceeds to unpack his own theory, GSE. Chapter 4 aims to show that natural selection is not the only type of selection process that can bestow functions upon traits. For example, trial-and-error learning and antibody selection in the immune system can bestow new functions on traits, independently of how the traits help organisms survive and reproduce.
Chapter 5 moves beyond traditional selected effects theories, arguing that things that do not reproduce can possess functions. Using the case of ‘synapse selection’, this chapter provides readers with one detailed example of processes of differential persistence.
Chapter 6 states the commitments of GSE, clarifies some of its similarities with existing attempts to extend the selected effects theory, and addresses six objections. Three objections target the idea that processes of synapse selection are genuine function-bestowing processes. The other three objections target the idea that GSE is the best theory of biological function. In particular, one objection is that the traditional selected effect theories, and not GSE, is the best fit for how biologists talk about function; another objection concerns the rocks example I mentioned above, and allows Garson to articulate what populations are.
Chapter 7 explains how mechanistic considerations can help GSE to determine which activity in a causal sequence is the genuine function of a trait. Chapter 8 also draws on mechanistic considerations to argue that a trait malfunctions just when, due to changes in its constitution, it cannot perform its most proximal function in its normal environment.
With these ideas in hand, Garson finally applies GSE to various philosophical and scientific problems. Chapter 9 focuses on the diversity of usage of the term ‘function’, and argues that GSE acknowledges and can illuminate this diversity. Chapter 10 turns to mechanisms, fleshing out the ‘functional sense of mechanism’. Chapter 11—which together with Chapter 5 I found the most interesting—suggests that mental disorders need not always depend on biological malfunctions, and that some mental disorders can be considered as functional traits possibly mismatched to the current environment. Chapter 12 argues that GSE can supplement teleosemantic accounts of mental representation by showing how synaptic selection in the brain can create novel mental representations.
I have three general remarks, concerning a salient omission, the ‘rules of the game’ laid out in Section 1.5, and the rocks example. Omitted in this book is an engagement with the epistemology of functions. How do biologists acquire knowledge about biological functions? How do they know what the normal environment for a trait is, or what the most recent selected effect of a trait is? How should they test hypotheses about functions? For example, biting flies in the zebra environment, where it’s warm and humid, tend to be abundant. How should scientists tease those two factors apart? Addressing any of these questions would have increased the importance and originality of What Biological Functions Are and Why They Matter.
Concerning the ‘rules of the game’ for a theory of function, Garson helpfully clarifies that his sources of evidence for GSE include ordinary biological usage of the term ‘function’, practices in biomedicine and biological psychiatry aimed at intervening on (dys)functions, and intuition. But what’s ordinary biological usage? When should biological usage be taken literally? And how should these sources be weighed if they give us inconsistent answers? While Garson acknowledges that biologists use ‘function’ in various ways, he doesn’t explain when this talk should be taken literally and when it should give way to intuition and stipulation. At times, Garson chalks off literal usage in favour of what scientists should ‘implicitly’ mean with ‘function’. At other times, he appeals to what scientists literally say to fend off objections that GSE does not fit explicit usage of ‘(dys)function’.
Just one example (other salient examples concern ‘dysfunction’ and mental disorder): In Chapter 6.2, he considers the objection that traditional selected effects theory better matches actual biological usage of ‘function’, because by saying that a trait has a certain function, scientists would often explicitly mean just that the trait is an adaptation that evolved by natural selection. Garson’s reply is twofold: first, some philosophers don’t think selected effects theories reflect explicit biological usage; second, sometimes scientists talk explicitly about biological functions in a way that respects their explanatory depth, their normativity, and the function/accident distinction. Because GSE better accounts for these three features, scientists are implicitly committed to GSE, even though they may explicitly say functions are just evolved adaptations. The difficulty with these replies is that the meaning of ‘function’ differs in the linguistic practices of the various scientific communities. If Garson’s goal is to offer a theory that accurately reflects existing usage, and existing usage includes the fact that by the expression ‘biological function’ many scientists just mean ‘evolved by natural selection’ or that they believe synaptic pruning increases the efficiency of neuronal transmissions without creating new functions, then we should have some transparent criterion to separate explicit usage that should be taken seriously from explicit usage that should be chalked off as sloppy or metaphorical.
One last critical remark concerns a version of the rocks example due to Karen Neander (Chapter 6.4). Rocks suitably arranged on a beach can display differential retention and also ‘persistence relevant interactions’ between them. The objection is that ‘it runs against both intuition and ordinary biological usage to give functions to rocks’ (p. 106). Garson’s reply appeals to a notion of population according to which a group of individuals is a population only if the individuals in that group display fitness-relevant interactions between them and, on average, each individual has fitness-relevant interactions with many other individuals in the group (Matthewson ). There are two problems with this: first, neurons would not count as populations since they do not have fitness-relevant interactions, only persistence-relevant ones; second, even if we replace the former with the latter, this notion of a population would not rule out versions of the rocks example where each rock is causally related to many others. Without a more stringent notion of a population, GSE actually bestows functions upon rocks.
All in all, Garson’s What Biological Functions Are and Why They Matter is a book well worth reading. As this outline suggests, it is broad in its scope, contains many interesting examples, and provides readers with a concise overview of some important portions of the philosophical literature on functions. The book is also clearly written, jargon-free, and neatly organized, which makes it accessible to readers with diverse backgrounds. Those of us lucky enough to have access to a well-stocked library or who can afford the exorbitant price of this book (even the e-book format is well over £50!) will find What Biological Functions Are and Why They Matter to be a welcome addition to current literature in philosophy of biology.
I am grateful to Dimitri Coelho Mollo for helpful comments on previous versions of this review, and to the Alexander von Humboldt Foundation for financial support.
Caro, T., Arguela, Y., Briolat, E. S., Bruggink, J., Kasprowsky, M., Lake, J., Mitchell, M., Richardson, S. and How, M. : ‘Benefits of Zebra Stripes: Behaviour of Tabanid Flies around Zebras and Horses’, PLoS One, 14, p. e0210831.
Cobb, A., and Cobb, S. : ‘Do Zebra Stripes Influence Thermoregulation?’, Journal of Natural History, 53, pp. 863–79.
Innocenti, G. M. and Price, D. J. : ‘Exuberance in the Development of Cortical Networks’, Nature Reviews Neuroscience, 6, pp. 955–65.
Matthewson, J. : ‘Defining Paradigm Darwinian Populations’, Philosophy of Science, 82, pp. 178–97.
Millikan, R. G. : Language, Thought, and Other Biological Categories, Cambridge, MA: MIT Press.
Neander, K. : ‘Functions as Selected Effects: The Conceptual Analyst’s Defense’, Philosophy of Science, 58, pp. 168–84.
Riccomagno, M. M. and Kolodkin, A. L. : ‘Sculpting Neural Circuits by Axon and Dendrite Pruning’, Annual Review of Cell and Developmental Biology, 31, pp. 779–805.
 Caro et al. () recently provided support for this hypothesis with an ingenious experiment involving cleverly disguised horses. For a different take on the existing evidence, see Cobb and Cobb (), who argue for a thermoregulation hypothesis, according to which the biological function of the stripes is to cool the zebra.