1. The Traditional Selected Effects Theory

The traditional selected effects theory (Neander Reference Neander1983; Millikan Reference Millikan1984; Griffiths Reference Griffiths and Griffiths1992, Reference Griffiths1993; Godfrey-Smith Reference Godfrey-Smith1993, Reference Godfrey-Smith1994) holds, roughly, that the function of a trait is whatever it was selected for—selected for recently—by natural selection or some comparable selection process. The primary virtue of the selected effects theory is that it helps us make sense of what are often called the teleological and normative dimensions of function (to be shortly described) in a naturalistically satisfying way. Another virtue is that it serves as a cornerstone for various fruitful philosophical projects such as naturalizing representation, disease (or disorder), and mechanisms.

“Teleological,” “normative,” and “proper function,” have several meanings in philosophy. Some outright reject that there is any teleology or normativity in nature. It is worth clarifying, then, exactly what I mean by each, so that the explanatory force of the selected effects theory can be properly appraised. By “teleological,” I just mean that function statements are often deployed as causal explanations. Specifically, they often purport to explain the existence, form, or distribution of a trait, by pointing to one of that trait’s useful effects. As I’ve sought to document (Garson Reference Garson2019, ch. 1), often, when biologists attribute functions to traits, such as the function of deterring biting flies to zebra stripes, they purport to explain why zebras have stripes: Zebras have stripes because stripes help deter biting flies. (The reigning theory holds that tsetse flies are averse to landing on striped surfaces, for reasons yet to be fully understood—see Caro et al. Reference Caro2014).

This explanatory feature of functions raises a serious conceptual puzzle: How can the fact that stripes deter flies explain why zebras have them? How is it possible for an effect of having stripes to somehow turn around and explain its own cause? The selected effects theorist answers that by saying: It does so by virtue of a natural process of selection. The fact that, in the past, stripes deterred flies, and the fact that that led to the differential reproduction of striped zebras over stripeless ones, allows the ability to deter biting flies to figure into a correct causal explanation of the current non-zero frequency of stripes in zebra populations. (To be clear, the fact that a trait was recently selected for performing some activity does not count as a complete causal explanation for the current nonzero frequency of that trait; it counts as part of that explanation.) By making sense of this puzzling feature of functions, the selected effects theory shows how teleological thinking in biology can be vindicated within a naturalistic and evolutionary worldview (Garson Reference Garsonforthcoming).

One might think that the explanandum of the function statement—that function statements are meant to explain the current nonzero frequency of a trait in a population (following Griffiths Reference Griffiths1993, 418)—is too tepid. In paradigm cases (“the function of zebra stripes is to deter flies”), aren’t function statements supposed to explain why all, or most, members of a population have the trait in question? Not always. In cases such as frequency dependent selection, selection can maintain a trait at a certain stable frequency less than 100 percent, such as the three different morphs of the male ruff, Philomachus pugnax, as described in Zuk and Simmons (Reference Zuk and Simmons2018, 63). Selection might even maintain a trait at a very low frequency in a population: Consider the “faeder,” or female-mimicking ruff, found in a mere 1 percent of the male ruff population. Still, its unusual features have functions. Some evolutionary theorists even conjecture that psychopathy (or some of its attributes such as lack of empathy) is such a trait, one that is selectively advantageous only when it occurs in an extremely low proportion of the population (for discussion, see Krupp et al. Reference Krupp2012).

Functions are also said to be “normative.” They do not necessarily describe what a trait does—after all, hearts do not always beat, and zebra stripes do not always deter flies—but in some vague sense of the term they seem to tell us what a trait is “supposed to do.” Although the meaning of the term “normative” is contested, I use the term in a fairly deflationary way: All I mean is that if a trait has a function, it is possible for that trait to fail to function (to malfunction, to be dysfunctional) (Schulte and Neander Reference Schulte and Neander2022). If traits could not literally malfunction or dysfunction, then much of biomedicine would be incoherent, as arguably, one of the whole points of biomedicine is to document the ways that bodily parts and processes fail to do what they’re supposed to do.

The selected effects theory explains the normativity of function by turning functions into historical properties—similar to the property of being a sibling, an adaptation, an igneous rock, or a volcanic mountain. Whether a trait has a function, and which function it has, depends entirely on its history, not on its current dispositions. Whether a trait is able to perform its function depends on its current dispositions, not its history. On the selected effects theory, then, there is nothing particularly puzzling about how a trait can possess a function it is nonetheless constitutionally unable to perform.

There are three further aspects of the selected effects theory worth mentioning here: pluralism, its meta-analytic (or “metaphilosophical”) ambitions, and realism. First, the selected effects theory is compatible with a modest pluralism about functions. What I mean by “pluralism” is the rather unremarkable fact that scientists use the term “function” differently in different contexts. The literature on how, exactly, we should think about pluralism (What are those meanings? In which contexts are those meanings deployed?) has become rather expansive (see Sterner and Cusimano Reference Sterner and Cusimano2019 for a good entry point).

The version of pluralism I adopt here cleaves tightly to Neander (Reference Neander2017a). As she puts it, there are at least two pretheoretical notions of function circulating in the life sciences: the proper function sense and the minimal function sense. The proper function sense admits both a function-side effect distinction and a function-malfunction distinction. The minimal function sense does not. If I say, “the function of the heart is to pump blood,” I usually imply that many other things it does are not functions (such as making beating sounds) and among those other things, some are malfunctions (such as going into cardiac arrest). In contrast, I suspect that the minimal notion of function is at play when the distinction between form and function is invoked: here, “form” is more closely aligned with physical structure, and “function” with something like “activity enabled by that structure” (see Novick Reference Novick2023 for discussion). In regard to the human patella bone (kneecap), “form” denotes shape and connectivity; “function” an activity it permits (knee extension). Such a notion of function, I believe, is at work in Bock and von Wahlert’s (Reference Bock and von Wahlert1965) discussion of function, in which a trait’s functions include “all physical and chemical properties arising from its form.” This minimal notion of function would seem to be ahistorical through and through but also, as Neander (Reference Neander2017a) notes, incapable of supporting a robust notion of malfunction. If one’s patella were crushed so as to no longer permit knee extension, it wouldn’t be “malfunctioning” in the minimal sense of function: It simply would be altered in such a way that it no longer sustains the same profile of activities as before.

The selected effects theory is intended to provide a rigorous articulation of the former, pretheoretical, proper sense of function, rather than the minimal sense of function. Unfortunately, function theorists are not always clear which of the two pretheoretical senses, if either, they intend to explicate. As a consequence, it is often difficult to decide whether two function theorists are disagreeing with one another about what functions are, or whether they are seeking to explicate two different senses. Even biologists sometimes switch back and forth between these two senses or lump them together in ways that are confusing and counterproductive (e.g., Doolittle Reference Doolittle2013 demonstrates how this conflation occurs when we’re trying to estimate what proportion of the human genome is “functional”).

Selected effects theorists hold diverse views about its meta-analytic ambitions. The failure to recognize the diversity of those ambitions is another source of potentially fruitless debate. Is the theory meant to be a conceptual analysis of what lay folk “have in mind” when they attribute functions to traits? Or only what modern biologists “have in mind”? Or is it supposed to work as a theoretical definition—that is, is it meant to say what functions are in nature, regardless of what anyone has in mind, as in “aluminum is that element with atomic number 13?” Or is it meant to have a stipulative character, to work as a “Carnapian explication” or an exercise in conceptual engineering (for discussion, see Millikan Reference Millikan1989a; Neander Reference Neander1991; Schwartz Reference Schwartz2004)? These are four quite distinctive projects which will potentially yield four different answers. Sometimes it is not entirely clear which meta-analytic project(s) a philosopher is pursuing. For example, Wakefield has, at times, described his “harmful dysfunction” account of disorder, which prominently involves the selected effects theory, as a conceptual analysis (1992; Wakefield et al. Reference Wakefield2006) and at other times as a form of “black box essentialism” (1999) which is usually synonymous with theoretical definition (see Fagerberg Reference Fagerberg2023 for an exploration of Wakefield’s complex meta-analytic goals).

Here, I accept the Millikanian approach which sees the selected effects theory as a theoretical definition (“water is H₂O”). Like all theoretical definitions, its articulation should be guided by core features of biological usage—including the explanatory and malfunction-permitting dimensions alluded to above. My view is that selected effects are the only kinds of things in nature that can underwrite the explanatory and malfunction-permitting features of functions while hanging together reasonably well with paradigm biological judgments about what sorts of things have proper functions (hearts, wings), what functions they have, and what sorts of things do not have functions (parts of rocks, hurricanes, and candle flames) (Garson Reference Garson2019).

The selected effects theory has a final feature that will be central to my argument. This is its commitment to a realist worldview, in the sense that functions, as a rule, are natural, mind-independent features of reality. Whether something has a function, and what function it has, doesn’t depend on how anyone thinks about it or feels about it; it doesn’t depend on anyone’s values, goals, or research agendas. Whether the activity of a trait represents a function, a side-effect, or a malfunction, depends on the structure of the world, not the structure of the mind. For the selected effects theorist, functions are woven just as tightly into the fabric of nature as causation, gravity, or mass. Of course, some traits acquire a function by virtue of a selection process that involves cognition or consciousness, such as some forms of sexual selection or artifact selection. But even in those cases, the fact that a trait has an effect as its function does not constitutively depend on mental states. For example, perhaps male jealousy has the biological function of helping males to be wary of cuckoldry. But the fact, if it is a fact, that male jealousy has that effect as a function, rather than as an incidental by-product, or a dysfunction, does not constitutively depend on anyone’s current mental states (our current day interests, goals, and values) but its selection history.

Realism about functions is quite critical for the selected effects theorist, for numerous reasons. First, the theory is usually framed within the larger project of naturalism—which is about helping to reveal how puzzling phenomena such as teleology, normativity, and mental representation fit into the natural world, and appeals only to entities and properties that are recognized by contemporary natural sciences. Naturalism is often taken to forbid not only appeals to supernatural entities (intelligent design of nature, say) but also to values (e.g., “naturalism” and “normativism” in the philosophy of medicine is a debate about the exact role of values in medical judgment—see Boorse Reference Boorse1975; Bedau Reference Bedau1991; Kingma Reference Kingma2010). Admittedly, the use of “naturalism” to exclude not only supernatural agents but values is strange, as Conley and Glacklin (Reference Conley and Glacklin2021) recently emphasize. It seems to presume that if values exist, they must exist subjectively, a point that, for example, neo-Aristotelians deny. My point here is merely that if functions depend, constitutively, on goals, interests, or the subjective sort of values, then the naturalistic character of functions would be jeopardized.

This realist or mind-independent character is pivotal for several traditional applications of the functions debate to other areas of philosophy and science, and in particular, for its application to understanding disease and mental representation. Many theorists believe or suggest that diseases (disorders, pathologies) essentially involve dysfunction (or related features like failure of function or loss of function), including Boorse (Reference Boorse1975), Neander (Reference Neander1983), Wakefield (Reference Wakefield1992), Griffiths and Matthewson (Reference Griffiths and Matthewson2018), and Jefferson (Reference Jefferson2022). One attractive feature of this view is that whether or not something is a disease (disorder) doesn’t merely depend on whether society happens to think it’s bad, unusual or weird; it depends on underlying facts about the world. This reigns in the tendency to “overmedicalize” or “overpathologize” conditions that large swaths of society consider to be undesirable or inconvenient. For example, the question of whether ADHD is really a dysfunction, rather than a different mode of cognitive functioning (see Swainpoel et al. Reference Swainpoel2017 for discussion), depends, for the selected effects theorist, on facts about how it evolved, and not just what society happens to find desirable or convenient.

The realist, mind-independent character of functions is also, quite obviously, pivotal for the project known as teleosemantics, which attempts to illuminate mental representation in terms of function and thereby anchor representation in the natural world. Teleosemanticists hold that what makes a bit of brain activity a representation, and what it is a representation of, depends on functions, which, in turn, depend on nonrepresentational features of the world (e.g., Papineau Reference Papineau1984; Millikan Reference Millikan1989b; Dretske Reference Dretske and Bogdan1986; Schulte Reference Schulte2012; Neander Reference Neander2017b; Garson Reference Garson2022a, ch. 8). If functions depend, constitutively, on values, goals, and interests, then they could not play this role. (Note that I am not making the erroneous claim that the selected effects theory is the only plausible theoretical foundation for teleosemantics: see, e.g., Price Reference Price2001; Schroeder Reference Schroeder2001; Nanay Reference Nanay2014; see Hundertmark Reference Hundertmark2024 for discussion. I am making the trivial claim that whichever theory of function one adopts as the intellectual backbone of teleosemantics, it cannot without circularity be one that depends crucially on intentional mental states.) Moreover, as Neander (Reference Neander2017b, 89–90) points out, all plausible naturalistic approaches to representation, such as Dretske’s (Reference Dretske1981) informational account, Fodor’s (Reference Fodor1987) asymmetrical dependence account, and Cummins’s (Reference Cummins1996) second-order similarity account, seem to lean, at some point or another, on pretheoretical notions like “proper,” “normal,” or “optimal” functioning to ground representation. So, it’s likely that anyone committed to naturalizing representation is also committed to the claim that such functions have a mind-independent existence.

2. The generalized selected effects theory

I’ve argued that the traditional selected effects theory, despite its numerous virtues, is plagued by an arbitrary restriction, one which demands a significant amendment (e.g., Garson Reference Garson2011, Reference Garson2016, Reference Garson2017, Reference Garson2019). It restricts functions to things that undergo reproduction or “copying” (e.g., Millikan Reference Millikan1984, Reference Millikan1989b, 1993; Neander Reference Neander1991; Godfrey-Smith Reference Godfrey-Smith1993; Neander and Rosenberg Reference Neander and Rosenberg2012). The basic picture of reality that informs the traditional theory is one in which biological entities form lineages bound by parent-offspring relations. The members of certain lineages, by virtue of their characteristic features, tend to increase in frequency in a population over others, or at least maintain their frequency (as in polymorphisms). Many selected effects theorists have been quite explicit in this core commitment (Millikan Reference Millikan1993, 34; Godfrey-Smith Reference Godfrey-Smith1993, 198–99; Neander and Rosenberg Reference Neander and Rosenberg2012, 618; see Kingsbury Reference Kingsbury2008, 496 for discussion).

I think there are interesting cases in nature of entities that acquire proper functions through a selection process, but one that only involves a history of “differential persistence:” one thing persisting in a population better (i.e., longer) than another. Consider a group of neurons in the CA1 area of one’s hippocampus. Neurons don’t reproduce—they do not form parent-offspring lineages—and so they cannot undergo differential reproduction. But they do persist for varying lengths of time, and some persist longer than others on account of their size, shape, or connectivity. I hold that the fact that a token neuron or neural structure (axon, neural group) “out-persists” another by virtue of some characteristic activity is sufficient to give it a new function. I also think trial-and-error learning—say, an infant learning that a particularly piercing cry is more likely to attract the attention of its father than a less piercing one, and that behavioral disposition being differentially reinforced over others on that account—is sufficient to create new functions, too. Nothing like reproduction (copying, lineages of individuals bound by parent-offspring relations, etc.) is needed (see Garson and Papineau Reference Garson and Papineau2019 for elaboration). Plausibly, some ecosystem elements (Bouchard Reference Bouchard and Huneman2013; Doolittle Reference Doolittle2014), and even some social institutions (Hindriks and Guala Reference Hindriks and Guala2019), can undergo differential persistence—one “outlasting” another because of some characteristic activity—and hence could come to acquire functions on this liberal view.

Put concisely, the generalized selected effects theory of function (GSE) holds that a function of a trait is an activity that caused its differential reproduction, or its differential retention, in a population (Garson Reference Garson2017, 524; Reference Garson2019, 93). By “differential reproduction,” I mean to include all of the entities that the traditional selected effects theory includes. In other words, if the traditional theory gives a function to a trait, GSE gives it the very same function. By “differential retention,” I mean to include entities like neurons (or other neural structures), psychological dispositions, and perhaps some ecosystem elements and social institutions, that don’t reproduce but that undergo differential persistence. By “in a population,” I mean to rule out certain problematic cases, as will be discussed at length in the following text.

There are numerous benefits to GSE. It applies to a large gamut of entities we’d like a theory of function to apply to. It integrates evolutionary and ontogenetic selection processes into a single, streamlined account. It solves the problem of novel functions—how new functions arise over the course of one’s lifetime—and, as applied to teleosemantics, the problem of novel representations. At root, GSE is meant to point to a basic, rather abstract, process, one that is ubiquitous in the natural (and social) world, and to present functions as depending on this process. In that respect, it shares an intellectual affinity with the project sometimes described as “universal selection theory” or “universal Darwinism” while departing from it on key points (e.g., Darden and Cain Reference Darden and Cain1989; Cziko Reference Cziko1995; Dennett Reference Dennett1995; Hull et al. Reference Hull, Langman and Glenn2001; see Garson Reference Garson2019, 67–69 for discussion).

The main justification for GSE, however, is not that it has those benefits. After all, a false or groundless theory could have various benefits. At root, my core justification is a parity of reasoning argument (Garson Reference Garson2017, 533; Reference Garson2019, 94). The reason for accepting the traditional selected effects theory is that it makes sense of the explanatory and malfunction-permitting features of function in a naturalistically satisfying way, while hanging together well with ordinary biological usage. But GSE does the same, minus an arbitrary restriction (“copying”). Hence, it’s the better theory.

3. GSE and the liberality problem

It’s natural to wonder whether GSE errs on the side of encompassing too much. This is precisely the point at issue in several of the criticisms lodged against it. Here is an example of “differential retention” that does not create new functions (see Kingsbury Reference Kingsbury2008, 496). Consider a bunch of rocks scattered on a beach. Some are harder, some are softer, and the softer ones erode more quickly than the harder ones. Eventually, only the harder ones remain. Some rocks, one might say, are “differentially retained” on account of their hardness. But the hardness of a rock doesn’t have a proper function: One wouldn’t say that the purpose of the rock’s hardness is to withstand erosion, or that, if the rock erodes anyway, there must have been a malfunction.

The fact that GSE appears to give rise to trivializing counterexamples seems to be a nonarbitrary reason to insist that, after all, functions do require something like “copying” or reproduction. As noted in the preceding text, a germ of this argument is found in Millikan (Reference Millikan1993, 34), Godfrey-Smith (Reference Godfrey-Smith1993, 198–99), and Kingsbury (Reference Kingsbury2008, 496). I think we can avoid the counterexamples without rejecting the idea that differential persistence gives rise to new functions. But I’ve come to realize that the way that I articulated GSE in the past created several new problems and puzzles—problems and puzzles I could have avoided had I elucidated the theory more carefully. This is precisely what I seek to do here.

Initially, I thought that the solution to the puzzle of the rocks, and similar counterexamples, was implicit in the very idea of a selection process (Garson Reference Garson2017, 536; Reference Garson2019, 104). Selection, we’re often reminded, takes place in a population (Godfrey-Smith Reference Godfrey-Smith2009; Millstein Reference Millstein2009, Reference Millstein, Bell, Futuyma, Eanes and Levinton2010; Matthewson Reference Matthewson2015). So, what kind of thing is a population? Following recent work in philosophy of biology, I endorsed the following quite minimal criterion: For a collection of entities to form a population, those entities must impact one another’s absolute fitness (survival or reproduction chances). Having sex, fighting over food, and hunting a wild boar together, are examples of interactions by which we impact each other’s fitness. Millstein (Reference Millstein, Bell, Futuyma, Eanes and Levinton2010; also see her Reference Millstein2009, 271) calls such groupings “causal-interactionist populations” and I will follow suit. There is much more to be said by way of fleshing this idea out. The idea of a collection of entities that impact one another’s absolute fitness is only meant to be a minimal criterion for populationhood. It is not a complete account. A complete account of populationhood would also have to make reference to features such as the degree of “connectivity” the members of a population must have. It would also have to specify that the members of a population are all of the same “type,” rather than different types (e.g., that they are all of the same species, rather than different species).

Equipped with this minimal criterion of populationhood, I was able to quickly dispatch problematic cases like rocks on a beach. It’s easy to see that a bunch of rocks on a beach don’t satisfy this minimal criterion. That is because the rate at which one rock erodes has no impact on the rate at which other rocks erode. Whether a rock erodes quickly or slowly does not affect the erosion rate of others. Of course, this solution forces us to expand what populations are to include things that merely impact one another’s persistence prospects, rather than their survival or reproductive prospects, but that struck me as a modest expansion of the idea.

The key virtue of this solution, it seemed to me, was that it solved the problem of liberality without introducing any arbitrary or ad hoc devices that stood in need of further justification. To say that a selection process takes place within a population, that is, a group of individuals that impact each other’s fitness, struck me as bordering on tautology: “[R]estricting functions to the members of populations is not an ad hoc device, but it emerges by reflecting on what natural selection is” (Garson Reference Garson2019, 104). Therefore, by restricting functions to individuals within populations, I merely purported to clarify something that was already latent in the very idea of a selection process, rather than adding something brand new. (By analogy, to say that a bachelor must not merely be an unmarried male, but an unmarried male that is eligible to be married—not an infant, a priest, or a person with advanced dementia—seems to be simply clarifying the standardly accepted understanding of “bachelor” rather than stipulating some new rules for its application.)

Still, I realized that more needed to be said about the liberality problem. As Neander pointed out to me (cited in Garson Reference Garson2017, 537), one could modify the rocks example slightly to make the rocks satisfy this minimal criterion. This time, imagine a pile of rocks by the shore. As the waves come crashing in, they rub against each other, and the harder ones contribute to the erosion of the softer. Now we have a collection of entities that impact one another’s persistence chances, and hence that satisfies our minimal criterion for populations—so GSE must give functions to them.

To avoid this modified rocks problem, I took the following route. Suppose we analyze, more carefully, the ordinary biological notion of a population. Perhaps we will find other features that populations have that such collections of rocks do not have. In other words, maybe we will find an additional, but still rather minimal, feature of a population that excludes such cases. Then we will not have to resort to any arbitrary or ad hoc restrictions. The solution to the rocks on a beach problem would still, as it were, flow from the very nature of populationhood.

One feature of populations is what we might call their “strong interconnectivity.” The idea is that for a collection of individuals to form a population, each individual must, on average, have fitness-relevant interactions with many other individuals in the group, not just a few others. Matthewson (Reference Matthewson2015) has shown how, barring such a condition, we could take two random populations on opposite sides of the world and call the collection consisting of both a “population,” too. So, it’s plausibly a rather minimal criterion for a population. If we go back to our pile of rocks, it doesn’t exhibit strong interconnectivity. Each rock only has persistence-relevant relations with its immediate neighbors. A nice feature of this notion is that it can be measured quite precisely. An intriguing consequence of this for the functions debate is that, if we accept that functions depend on populations, then functions come in degrees, because populationhood comes in degrees (Matthewson Reference Matthewson2020).

Unfortunately, critics were quick to point out that one could further modify the rocks example to satisfy this “strong interconnectivity” criterion (Conley Reference Conley2020; Columbo Reference Columbo2020; Bourrat Reference Bourrat2021; Schulte Reference Schulte2021; Dussault Reference Gayon, de Ricqlès and Dussault2023; see Matthewson Reference Matthewson2020 for discussion). Schulte offers a rockfall example: A bunch of rocks are rolling down a mountain, and smashing into each another as they roll. They could affect each other’s persistence chances and exhibit strong interconnectivity in the process. For that matter, one could put a bunch of rocks in a rock tumbler and leave them for a long time (Conley Reference Conley2020). Such a group would exhibit strong interconnectivity.

It’s worth emphasizing why it’s important to exclude rocks from having proper functions. Why all this fuss over a handful of counterexamples? The issue is not, as one might think, that giving functions to rocks violates my “intuitions.” Many feel, rightly or wrongly, that relying heavily on intuitions to win a philosophical argument is a distasteful activity, and at any rate one that is inappropriate for a theoretical definition, rather than a conceptual analysis. Rather, my view that rocks don’t have proper functions is based on a general methodological principle: A good theoretical definition of function—that is, a definition that seeks to identify a kind of process or pattern underlying ordinary biological usage—should be guided by broad features of that very usage of “function,” in the same way that a good theoretical definition of water should be guided by broad features of ordinary scientific usage of “water” (Garson Reference Garson2022b). One of these features includes the fact that there are certain sorts of things in the world that biologists do attribute proper functions to, and other sorts of things they do not. They attribute functions to things like bodily parts and processes (the heart), mental states (male jealousy), and group behaviors (V-formation in birds). They do not generally attribute proper functions to inanimate objects, such as rocks, clouds, hurricanes, clay crystals, and so on—with the exception of artifacts. If my theoretical definition of “function” yields function ascriptions that are at significant odds with the usage that inspires it, that would be evidence that my definition picks out the wrong kind of thing in the world.

Rocks might have “functions” in some other sense of the term, in something like Neander’s “minimal” sense. Rocks have various benefits, such as creating soil or containing valuable minerals, and these can correctly be called “functions” of rocks, in the same way that climate change is a “function” of deforestation. But this isn’t the proper function sense. If a rock does not contain valuable minerals, it might be disappointing to a prospector, but the rock isn’t malfunctioning or dysfunctional. Nothing objectively went wrong with the rock. There seems to be a more-or-less tacit consensus among biologists that rocks, like clouds or hurricanes, aren’t the sorts of things that can fail to work the way they’re supposed to.

In short, while strong interconnectivity is an important feature of populations, and while pointing to strong interconnectivity probably avoids Neander’s specific counterexample, it doesn’t do what I meant for it to do—namely, to let us ascribe functions to things (or withhold such ascriptions) in ways that hang together well with the actual biological usage that is intended to guide it.

Before moving on to criticisms pertaining to the rocks example, I should note that some have rejected GSE in favor of the traditional selected effects theory for quite different reasons: not on the grounds that it is overly liberal, but on the alleged ground that it yields conflicting functional norms (Fagerberg Reference Fagerberg2022). GSE allows for the possibility that one and the same trait can have the function F by virtue of one selection process and the function G (where G is contrary to F) by virtue of another. An antibody might have the evolved function of attacking foreign particles, and the ontogenetically selected function of attacking healthy tissue. The brain’s reward system might have the evolved function of pursuing fitness-relevant goals (like food and sex) and the ontogenetically derived function of pursuing dangerous substances (like cocaine) to the exclusion of food and sex (see Garson and Papineau Reference Garson and Papineau2019 for discussion).

My response to Fagerberg’s objection is twofold. First, I accept that GSE allows for conflicts between functions—but that strikes me as an interesting and fruitful implication of it, not an embarrassment (Garson Reference Garson2019, 73). By the same token, suppose one had a theory of belief formation according to which it is possible for a person to hold two conflicting beliefs. That fact would not be an objection against one’s theory of belief formation but an interesting and potentially theoretically fruitful consequence.

Second, even if one thinks that the possibility of function conflict is a serious problem, that problem is inherent in the traditional selected effects theory as well. That is because the traditional theory allows for multiple levels of selection (such as group level, organism level, and chromosome level), and therefore the possibility that a trait can be an adaptation for doing F by virtue of one level but an adaptation for doing G (which is contrary to F) by virtue of another. For example, D. S. Wilson (Reference Wilson2015, ch. 2) describes how sexual aggression in water striders may be selected for at the individual level and selected against at the group level. Therefore, conflicting functional norms would not supply a reason for preferring the traditional theory to the generalized theory, which was the point of the parity of reasoning argument.

Intriguingly, Millikan (Reference Millikan2005, 99–100) has explored the issue of how her version of the selected effects theory allows for such conflicts. One of her examples comes from language. A phrase might have the “direct” proper function of inducing one set of actions in its hearer (e.g., “pass the salt” might have the proper function of inducing its hearer to pass the salt) and a “derived” proper function that is contrary to the first (e.g., if the speaker uses the term in a metaphorical, sarcastic, or idiosyncratic way). The fact that there are occasional function conflicts should not, alone, count against any particular theory of function.

4. Three solutions

I turn now to three critics, Conley, Bourrat, and Schulte (as Dussault Reference Gayon, de Ricqlès and Dussault2023 largely reviews these criticisms I will not address that paper here). Each of them takes, as a starting point, the observation that GSE would imply that rocks in a pile can have functions, and each offers a different diagnosis of how GSE errs. Although I think the first two criticisms can be largely handled within the existing framework of GSE, responding adequately to the third demands a substantive revision.