I. Introduction

Recent years have seen a resurgence of interest in evolutionary models of culture. The picture envisaged in much recent work is something like this: the general capacity for culture presumably has a genetic basis and is an adaptation. The capacity for learning by imitation is crucial here (Tomasello 1999). But once this capacity is in place, cultural change tends to acquire its own dynamic, which has a partially Darwinian character (Richerson and Boyd 2004, Dennett 1995, Mesoudi et al. 2004). The most contentious versions of this idea posit discrete cultural “replicators”, which Dawkins (1976) called “memes”.

These ideas have been discussed at several points in the webconference, for example in Bouchard's and Bryson's papers. Here I will offer some general ideas on the relation between evolutionary theory and cultural change. I argue for a framework that recognizes three nested categories: (i) theories that apply “population thinking”, (ii) theories that apply the concept of evolution by natural selection (a Darwinian dynamic), and (iii) theories that look for replicators. In the final section I will discuss some arguments in Bouchard's contribution to the webconference, as he sets things up quite differently.

II. Population Thinking, Darwinism, and Replication

The general picture I will defend can be represented in terms of three nested categories, describing theories of change.

Figure 1: Three categories

In setting up the broadest of the categories I draw on a concept due to Ernst Mayr (1959). He argued that a subtle but important innovation that can be associated with Darwin and his time is “population thinking”. This involves approaching a domain (the living world, in Darwin's case) in a way that recognizes the reality and causal importance of variation within populations, and avoids treating such variation as imperfection in the worldly realization of ideal “types”. This is useful when thinking about the particular contrasts between Darwin and his precursors in biology, but the “populational” approach I have in mind here has some more concrete features as well. When we embark on population thinking, we think of a system as an ensemble of components that each have a degree of autonomy, a life (or something like a life) of their own, and a significant number of properties in common. Change at the level of the ensemble is a consequence of interactions within the population. I will use the term “populational” for a framework that applies ideas of that kind. So it contrasts not only with what Mayr called “typological” or Platonic approaches, but with various other explanatory strategies as well.

The second category I label “Darwinian”. Here I mean the Darwinian dynamic associated with change within a population by means of natural selection. Darwin described this process in fairly concrete terms, assuming units that were (almost always) organisms in the usual sense. Since then, there have been two ways of trying to abstract the core Darwinian idea, so it can be applied more broadly. One approach I call the “classical” tradition. The other is the “replicator” view.

The classical tradition dates at least to Weismann (1909), has perhaps its most-cited formulation in Lewontin (1970), and has also been expressed by many others[1]. The main idea is that we expect evolutionary change whenever we have a population in which there is (i) variation, (ii) which is responsible for differences between individuals in reproductive output, and (iii) which is heritable to some extent. The “population” here need not consist of ordinary biological individuals. It could consist of any entities at all for which the notion of reproduction is well-defined[2].

Heritability, in the relevant sense, is a statistical and comparative matter. We do not need copying or preservation of structure. Everyone in the population could be unique with respect to the evolving trait. Imagine an asexually reproducing population in which everyone has a different height, and these heights are evenly spaced without even a “clumping” into rough types. There is no copying of the distinctive properties of an individual in reproduction. But if parents and offspring are more similar than randomly chosen individuals, we have heritability and evolutionary change via selection is possible.

The second tradition of abstract description of Darwinism uses the idea of a replicator (Dawkins 1976, Hull 1988). Many definitions of a replicator take some notion of “copying” as a primitive. Dawkins said “[w]e may define a replicator as any entity in the universe which interacts with its world, including other replicators, in such a way that copies of itself are made” (1978:132). Hull defined a replicator as “an entity that passes on its structure largely intact in successive replications” (1988:408). This is somewhat metaphorical, but what is being described is in effect a special case of the phenomena recognized by the classical view. We have replicators when we have a population featuring high-fidelity reproduction, and a single parent for each individual. That is indeed how genes are made; in the copying of DNA there is a parent molecule and an offspring molecule, and the copying is very high-fidelity. In many organisms, including us, low-fidelity sexual reproduction at the level of organisms is made possibly by high-fidelity asexual reproduction at the level of genes. The defenders of the replicator framework often seem to suppose that something like this is essential to evolution, so all evolution by natural selection has to involve replicators somewhere. But this is not true. What is needed for evolutionary change is heritability, which is a comparative matter, need not be high-fidelity, and which can, in principle, involve contributions from many parents. Evolution involving replicators is a special case[3].

If we apply this framework to the case of cultural change, we see that there are three questions to ask. To what extent should cultural change be treated as a populational phenomenon? To what extent should it be treated as a Darwinian phenomenon? Is it possible and useful to recognize cultural replicators?

It might initially seem obvious that cultural change is a populational phenomenon, but in the present sense this claim is far from trivial. Although cultural phenomena of all kinds clearly depend on the activities of individuals who make up populations, it is possible for a population to generate products that are not best treated in populational terms. Many cultural phenomena are like this. Persisting community-level artifacts like buildings and roads are the consequences of activities of a population, but once they exist their ongoing role is not populational in character. In fact, structures like these may affect behavior in ways that reduce the populational character of social life. Culture is populational to the extent that it can be modeled informatively as interaction between partially autonomous individuals who share a significant number of properties. A highly structured network with heterogeneous and non-interchangeable parts is a different thing from a population. This is part of what Fracchia and Lewontin are pointing at in their vigorous rejection of evolutionary models of culture (1999), though they express the point in different terms. But these anti-populational observations should not be made in a wholly general way. Simpler forms of culture may have a more populational character than more complex and refined forms. An initial populational mode of interaction may give rise to something else.

There is also a related point that is simpler. Societies with top-down control are less amenable to a populational treatment. The ideas that proliferate are those that come from a certain location in the society, regardless of the content and their local consequences. In many cultures, personal fashion choice is a fairly populational phenomenon, but in a sufficiently autocratic society, it will not be. (Gold teeth are presently banned in Tajikstan, solely because of the whim of an autocrat.) Ken Reisman argued in his PhD dissertation (2005) that Darwinian models of culture become inapplicable to the extent that power relations are asymmetric. I am broadening that claim to one about populational models in general.

But suppose that some cultural phenomena are populational. This might include changes in patterns of individual everyday behavior (eating, cooperating, communicating). Are such processes also Darwinian? There are two ways that such processes might be treated in a Darwinian manner.

First, we might treat the individuals in the population as ordinary biological individuals, and treat cultural properties as aspects of phenotype. This will work easily to the extent that cultural transmission is “vertical”, from parent to offspring. The “reproduction” relation remains an ordinary biological parenting relation. It will often be that offspring culturally resemble their parents more than they resemble randomly chosen members of the population, and differential reproduction can then yield evolutionary change. Things get much complicated to the extent that cultural transmission is not vertical, but I won't worry about those problems here.

The second option is to treat the instances of cultural traits as making up their own Darwinian population. The “reproduction” relation is now between the instances of a cultural trait; your best friend's Catholicism might be said to be the parent of your Catholicism. Does it make sense to use the concept of reproduction in this way? I think it makes some sense in some cases. In general, the notion of reproduction is a vague and gradient one. Paradigm Darwinian processes involve populations with clear reproduction relations between individuals, but there are also processes that have some Darwinian characteristics because the population exhibits something like a reproduction relation. That applies to both biological and cultural cases; the lines in Figure 1 should be understood as somewhat blurred, rather than sharp. Reproduction involves the generation of a new entity of the same kind as the parent(s), with a certain kind of causal responsibility from parent to offspring. Some ways in which a cultural variant can be “passed on” or reappear in a new individual are reproduction-like, but this depends on the cognitive processes involved. The simplest kinds of imitation have some of this character, though there are interesting disanalogies even then[4]. For example, in imitation learning, it is the recipient's dispositions that are causally responsible for there being a similarity between a “parental” and an “offspring” instance of a trait, though the particular features of the offspring are then a function of the parent. And as Sperber (1996) and others have argued, most social learning is not passive in the ways that I here associate with a reproduction relation. But some simple forms of social learning might have enough of a reproduction-like relation between instances of a trait for a Darwinian framework to be applied. As emphasized earlier, the claim that there are cultural replicators is a stronger one again. 

In assessing these ideas it is useful to focus on recent formal models of behavioral change via social learning (Skyrms 2003, Nowak 2006). These are models of behavioral change in which individuals interact locally, receive payoffs, and update their behaviors as a function of their experience. These models make use of a number of different update rules and dynamics. They are all rules in which an individual derives its phenotype from local influences, via a function of some kind. But having one's behavior be a function of the attributes of one's neighbors(s) is not the same thing as having one's behavior be a copy of some neighbor. The latter is a special case of the former.

We might represent some of this structure as follows. Suppose that via observation an individual is to set the value of some behavioral characteristic Z for the next time step, Z(t+1). Z is a continuous variable (though it might be the probability of making some binary choice, such as cooperation as opposed to defection). Assume the individual has n neighbors, where neighbor i's behavior at t is represented as X_i(t). Z(t+i) will be a function of the phenotypes of the neighbors, their payoffs (W_i) at t, and the individual's previous state Z(t) and payoff V(t). So in general, Z(t+1) is some function of the following variables: (X₁(t), X₂(t),... X_n(t), W₁(t), W₂(t),... W_n(t), Z(t), V(t)). There are many possible rules, but some of them can be represented like this:

(1)       Z(t+1) = a(Z(t)) + b(X*) + (1- a - b)(S_i(X_i))/n

Here X* is the behavior of the neighbor with the highest payoff at t, or the behavior of the focal individual at t if its payoff was higher than any neighbor's. The “weights” a and b sum to no more than one. The idea is that any individual's new behavioral choice can be sensitive to (i) what it did last time, (ii) the recent success of behaviors exhibited by neighbors, and (iii) the local prevalence of those behaviors. An individual can give some role to inertia, some to tracking what has recently worked, and some to doing what is common. The a and b parameters reflect how much weight is given to each factor. So when b=1 we have an “imitate your best neighbor” dynamic; when a=1 the individual never changes.

It would be interesting to see what the consequences are of various intermediate values of a and b, in different contexts. Equation (1) also omits another way for success to figure in behavioral updating, which is via a “best response” rule. An individual can produce on the next time step the behavior that would have been the best overall response to the behaviors produced by its neighbors last time. This is another way in which behavior can be a function of what was done earlier, but it is certainly not intrinsically Darwinian. In some cases the best response to X is X (coordination games) but in others the best response to X is Y (eg., the hawk-dove game).

This process of behavioral change by individuals could operate “on top” of an ordinary Darwinian process involving biological reproduction. An individual will then be born with an initial behavior, and will update it according to some specific rule. The learning rules (eg., a and b) could then evolve. When does an individual do best to stick with the behavior it inherited from its (evidently) successful parent, and when does it do best to adjust in the light of experience? If so, how quickly should it adjust? And should it adjust by simple imitation, by success-modulated imitation (“imitate your best neighbor”), by an even smarter “best response” rule, or by a non-social rule such as trial-and-error?[5] This gives the model a link to the larger literature on the evolution of learning (Stephens 1991, Godfrey-Smith 1996, Kerr forthcoming), and the evolution of imitation learning as opposed to other kinds (Richerson and Boyd 2004). In this scenario, Darwinian processes give rise to a rule for social learning, and this rule may or may not have a partially Darwinian character itself. The rules that produce a partially Darwinian dynamic will be, as argued above, the simple forms of imitation.[6] Darwinian processes may produce non-Darwinian forms of social learning.

III. Origin Explanations and Persistence

In this final section I will contrast part of the discussion above with some claims made in an interesting earlier paper from this webconference, by Frédéric Bouchard.

Bouchard claims that it is mistake to see all Darwinian processes as involving reproduction or replication. Differential persistence is sufficient for Darwinian change, and once the notion of persistence is suitably broadened, it may be the key to understanding all cases of change by the Darwinian mechanism. Bouchard then notes that this move may help us understand cultural change, because many cultural cases are like the biological ones in which reproduction is problematic but persistence is not. To assess reproductive output, we must be able to count offspring, which is hard in some biological cases and in many cultural ones. It is more straightforward to assess differential persistence.

In the framework outlined above, pure cases of differential persistence count as populational phenomena, but not Darwinian ones. This is because I required reproduction for Darwinian change. (Note that when Bouchard claims that some of his phenomena do not involve “populations”, this is because he is assuming that all populations include reproduction. So his “population” concept is narrower than mine.)

I think that Bouchard is right to press the importance of cases (both in biology and culture) in which reproduction is a difficult concept to apply. So I will say something about why I set things up differently from him.

If we have a set of objects which do not reproduce in any sense, and some process then acts as a “filter” that eliminates some and retains others, this is certainly a Darwin-like process. Standard definitions of evolution by natural selection handle the case awkwardly. Some (Lewontin 1980) explicitly require reproduction. Some (Lewontin 1970) are ambiguous. And others, such as a definition in terms of “change in gene frequencies” would include such cases. As a matter of terminology, both broader and narrower uses of the key terms could be defended. On the narrower use, differential persistence is only part of a genuine Darwinian process, not sufficient alone.

But the matter is not merely terminological. Here is one argument for resisting Bouchard's proposal to make persistence primary, and treat reproduction as optional or as a special case of persistence.

Let us distinguish two kinds of explanations in which natural selection can figure, Distribution explanations and Origin explanations. (These terms are modified from Karen Neander (1995), who distinguished “Creation” and “Persistence” explanations in a debate with Elliott Sober. I also broaden both her categories here.) In a Distribution explanation, we assume the presence of some range of variants, and explain how they came to be distributed as they are – why some are common, others rare, why one has gone to fixation, or why an equilibrium is being maintained. In an Origin explanation, we explain how some particular variant came to exist in the population at all, regardless of its frequency and regardless of which individuals bear the relevant trait. It is obvious that natural selection figures in Distribution explanations, less obvious but very important that it can (in some circumstances) figure in Origin explanations. Suppose we want to explain how some novel adaptive characteristic came to appear. Novelty arises proximally, in an evolutionary context, via mutation and recombination. But natural selection can reshape a population in a way that makes a given combination of characteristics much more likely to be produced via mutation and recombination, than it would otherwise be. It does this by making intermediate stages common rather than rare, thus increasing the number of ways in which a given mutational event (or similar) will suffice to produce the combination in question. (See Forber 2005 for a discussion that clarifies and isolates this role particularly well.)

We can then note that if we have no reproduction in a population, then although there can be the kind of filtering or culling that has a partially Darwinian character, we cannot have the kind of natural selection that is involved in Origin explanations. All that can happen is that pre-existing types are retained, or not retained. The long-lived entities may still change via developmental processes; a population that has no reproduction need not be static. (This is what Lewontin 1983 called a “transformational” mode of evolution.) But the particular way in which natural selection can reshape a population in a way that makes otherwise improbable new variants accessible is a process that requires reproduction.

I can think of some replies that might be available to Bouchard here. He might argue that even if novelty is only arising by developmental processes that do not involve reproduction, still differential persistence may be important in Origin explanations. The longer an entity persists, the better chance it has of taking a developmental path that leads to a given novel state. That effect does seem real, but less powerful than the distinctive way in which selection is relevant to Origin events in the normal Darwinian cases. In those cases, the idea of reproduction of countable offspring is essential, because the role of selection is to increase the number of independent “slots”, often by a very large factor, at which a novel variant can arise. (And I think that Bouchard's interesting example involving mud does have reproduction in this sense.)

What does this mean for culture? It might turn out that real cases of cultural change are only Darwinian in the (for me) very marginal sense that does not involve reproduction. If it is also true that this precludes selection from figuring in Origin explanations, then selection would have a much less important role in cultural cases than it has in biological ones. The other possibility, discussed in the previous section, is that some cases of cultural phenomena can be usefully treated as Darwinian in the richer sense that does involve reproduction.

Peter Godfrey-Smith

References

Bouchard, F. 2007. Ideas that Stand the [Evolutionary] Test of Time. A&R webconference: http://www.interdisciplines.org/adaptation/papers/12

Bryson, J. 2007. Representational Requirements for Evolving Cultural Evolution. A&R webconference: http://www.interdisciplines.org/adaptation/papers/13

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