CULTURAL REPLICATION AND MICROBIAL EVOLUTION Bence Nanay The aim of this paper is to argue that cultural evolution is in many ways much more similar to microbial than to macrobial biological evolution. As a result, we are better off using microbial evolution as the model of cultural evolution. And this shift from macrobial to microbial entails adjusting the theoretical models we can use for explaining cultural evolution. Introduction We have a very elegant and effi cient theory for explaining certain biological changes from population to population: the theory of natural selection. The theory of natural selection has a remarkable explanatory power: it can explain something very complex, such as the structure of the human eye or the fi t between the organism and the environment in terms of something very simple, the dumb causal processes of births and deaths. A tempting idea is that the same explanatory scheme could be used to explain some complex non-biological, more precisely, cultural phenomena (Richerson and Boyd 2005; Sterelny 2006a; Lumsden and Wilson 1981; Hull 1988, 2001; Fracchia and Lewontin 1999; Dawkins 1976, 1982a, 1983; Cavalli-Sforza and Feldman 1981; Aunger 2000; Dennett 1995; Campbell 1956, 1960, 1974; Toulmin 1967, 1970, 1972; Kantorovich 1989; Bradie 1986; Popper 1963, 1972, 1974, 1978; Nanay 2011b – the list is obviously far from being complete). This is exactly what theories of cultural evolution attempt to do. There are numerous important differences between biological and cultural evolution. The question is whether the explanatory scheme of the theory of natural selection could be applied in the cultural domain in spite of these dissimilarities. In this paper, I want to focus on three salient differences between biological and cultural evolution: 1) Cultural evolution is very fast, much faster than biological evolution. 2) In the case of biological evolution, information transmission is vertical: we inherit our genes from two individuals only (our parents). This is not true for cultural evolution, where information is also transmitted laterally, from peers to peers. 3) The fi delity of information transmission in the case of biological evolution is much higher than in the cultural case. Pleh_Naturalistic-2-Normal.indd 122 2014.02.13. 15:02:59 123 There are many more widely discussed differences: it has been claimed that cultural evolution is Lamarckian, whereas biological evolution is Weismannian, etc. (Hull 1980, 1981). But I will focus on (1), (2), and (3) here. The aim of this paper is to argue that when it comes to (1), (2), and (3), cultural evolution is much more similar to microbial than to macrobial biological evolution. As a result, we are better off using microbial evolution as the model of cultural evolution. And this shift from macrobial to microbial entails adjusting the theoretical models we can use for describing cultural evolution. Macrobial versus microbial evolution as a model for cultural evolution Here is an odd fact about the literature on cultural evolution. It invariably takes macrobial evolution to be the model of cultural evolution. When it compares the biological and the cultural domain, it really compares the domain of macrobial biology and culture. But macrobes are not the only biological entities – in fact, they are not even the most widespread ones. Evolutionary biologists and philosophers of biology have been actively ignoring microbes, and this, arguably, has been a mistake. We may be able to understand important facts about biological evolution if we understand the microbial world (see, for example, O'Malley and Dupré's 2007 manifesto). And my proposal in this paper is that we may be able to understand something important about cultural evolution if we take microbial evolution, and not macrobial evolution, as its model. 1) Microbial evolution is in many respects very different from macrobial evolution. Here are three important (and conspicuously numbered) differences: Microbial evolution is very fast, much faster than macrobial evolution (see e.g., Lawrence 2002). 2) In the case of macrobial evolution, information transmission is vertical: we inherit our genes from one or two individuals (the parent[s]). This is not true for microbial evolution, where information is also transmitted laterally – this is called lateral gene transfer, where the transfer of genetic material from one organism to another happens by conjugation, transduction, or transformation (Bushman 2002; Thomas and Nielsen 2005; see O'Malley and Dupré 2007: 167–168 especially for a philosophical analysis of this phenomenon). 3) The fi delity of information transmission in the case of macrobial evolution is much higher than in the microbial case (see e.g., Lawrence 2002; O'Malley and Dupré 2007). In other words, the three differences I considered in the last section between biological and cultural evolution were in fact differences between macrobial and cultural evolution. And we fi nd the exact same differences between macrobial and microbial evolution. The conclusion is that we would be much better off using microbial evolution for modeling cultural evolution. Pleh_Naturalistic-2-Normal.indd 123 2014.02.13. 15:02:59 124 The aim of this paper is to cash out what this shift of emphasis from macrobial to microbial evolution in the analysis of cultural evolution would entail in terms of the theoretical framework we can use to model cultural evolution. Two ways of thinking about natural selection There are two distinct ways of conceiving of selection processes. According to one, selection is the heritable variation of fi tness. According to the other, it consists of repeated cycles of replication and interaction. These two models of selection75 give us very different ways of formulating evolutionary explanations, and they even yield different kinds of evolutionary explanations. According to the fi rst model (Lewontin 1970; Maynard Smith 1987), selection should be described as the heritable variation of fi tness. A typical formulation is the following (see also Lewontin 1970: 1; Endler 1986: 4; Ridley 1996: 71–72; Godfrey-Smith 2007: 515). A suffi cient mechanism for evolution by natural selection is contained in three propositions: 1) There is variation in morphological, physiological, or behavioral traits among members of a species (the principle of variation). 2) The variation is in part heritable, so individuals resemble their relations more than they resemble unrelated individuals, and, in particular, offspring resemble their parents (the principle of heredity). 3) Different variants leave different numbers of offspring either in immediate or remote generations (the principle of differential fi tness). (Lewontin 1980: 76).76 According to the alternative concept, selection consists in repeated cycles of two separate processes. As Ernst Mayr says, "natural selection is actually a two-step process, the fi rst one consisting of the production of genetically different individuals (variation), while the survival and reproductive success of these individuals is determined in the second step, the actual selection process" (Mayr 1991: 68; see also Mayr 1982: 519–520; Mayr 2001: 117; Mayr 1978). David Hull calls these two steps replication and interaction (Hull 1981; Hull 1988; Hull et al. 2001). Hull defi nes selection as "[t]he repeated cycles of replication and environmental interaction so structured that environmental interaction causes replication to be differential" (Hull et al. 2001: 53). In turn, Hull (1988: 408) defi nes the unit of replication, the replicator, as "an entity that passes on its structure largely intact in successive replications" (see Hull 1980: 318 for a slightly different defi nition). The unit of interaction, the interactor, on the other hand, is de75 I will refer to these two ways of conceiving of selection as two models of selection, acknowledging that my use of the concept of models is different from the way this term is used in biology. 76 According to Lewontin (1980: 76), each of these three propositions is necessary for evolution by natural selection (besides being jointly suffi cient). Pleh_Naturalistic-2-Normal.indd 124 2014.02.13. 15:02:59 125 fi ned as the "entity that interacts as a cohesive whole with its environment in such a way that this interaction causes replication to be differential" (Hull 1988: 408; see Hull 1980: 318). This replication–interaction model of selection was introduced as an improvement on the heritable variation of fi tness model, and it is supposed to clarify a number of details left implicit therein. More precisely, the replication–interaction model has been thought to help us to understand what is at stake in the units of selection debate: if selection is replication plus interaction, then we should not talk about the units of selection, but rather about the units of replication and the units of interaction, which may not be (and in fact most often are not) the same. The thought is that the replication–interaction distinction in itself will not solve this problem, but it is supposed to help us to formulate the problem in such a way that would make it possible to tackle it (see e.g., Lewontin 1970: 7; Brandon 1982, 1988, 2006; and especially Lloyd 2001). In the last decade or so, more and more evolutionary biologists and philosophers of biology have been arguing against the replication–interaction model. Their main claim is that replication is not necessary for evolution by natural selection, or, as I will put briefl y, for selection.77 As a result, the heritable variation of fi tness model has become more and more widely used. In the cultural evolution literature, both of these models are used. The most famous, but not the only, example of the replication–interaction model in the domain of cultural evolution is meme theory. These two models are also often applied to the cultural domain without a clear attempt to distinguish the two – as in Richerson and Boyd (2005: Chapter 3), where the fi rst half of the chapter uses the heritable variation of fi tness model, whereas the second half uses a version of the replication–interaction model, without any explicit acknowledgement of the difference between the two. My aim is to point out that regardless of whether the heritable variation of fi tness model or the replication–interaction model is better suited for describing macrobial evolution, the heritable variation of fi tness model faces serious problems when applied to microbial evolution. And it faces the same problems when applied to cultural evolution. In other words, we are better off using the replication–interaction model for describing microbial and cultural evolution. The heritable variation of fi tness and microbial evolution The heritable variation of fi tness model may look straightforward, but in fact it is not. What this account of selection entails very much depends on the way in which we interpret the concept of fi tness. And there is no agreement on a number of important features of this concept. 77 There is an important terminological difference in the way the concept of selection is being used in the literature. Some ask whether replication is necessary for evolution by natural selection (Okasha 2007; GodfreySmith 2007), others ask whether replication is necessary for selection itself (Hull 1988; Neander 1995; Hull 2001; Nanay 2005). I assume that these are two different ways of asking the same question (the question of whether replication is necessary for evolution by natural selection) and I will use the latter formulation because it is simpler. If the reader prefers the former one, he/she should read 'evolution by natural selection' instead of 'selection' in what follows. Pleh_Naturalistic-2-Normal.indd 125 2014.02.13. 15:02:59 126 Is fi tness a causal or a statistical concept (Matthen and Ariew 2002)? Is it a populationlevel or an individual-level concept (Millstein 2006)? What entity do we attribute fi tness to, individuals or to trait types (Sober 1981; see also Nanay 2010b; Nanay 2011c)? If the former, is an individual's fi tness the same throughout its life (Ramsey 2006)? If the latter, how should we individuate these trait types (Nanay 2010a)? The two most infl uential questions about fi tness and about selection are whether they should be taken to be population-level or individual-level phenomena, and whether they are causal or statistical concepts (Matthen and Ariew 2002; Walsh et al. 2002; Millstein 2006; Brandon 2006; Bouchard and Rosenberg 2004; Rosenberg and Bouchard 2005; Stephens 2004). It has been pointed out that the concept of fi tness is used in two different ways: as an "ecological descriptor" and as a "mathematical predictor" (Sober 2001: 319; this distinction may be traced back to Kitcher 1984: 50). Building on Sober's distinction, Mohan Matthen and André Ariew (2002) made a distinction between "vernacular" and "predictive" fi tness.78 Vernacular fi tness is a measure of the "overall competitive advantage traceable to heritable traits" (Matthen and Ariew 2002: 56). Predictive fi tness, in contrast, is the "expected rate of increase (normalized relative to others) of a gene, a trait, or an organism's representation in future generations" (Matthen and Ariew 2002: 56). Vernacular fi tness plays a role in the informal presentations of natural selection, whereas predictive fi tness is used in mathematical formulations of population genetics. Vernacular fi tness is a comparative measure, whereas predictive fi tness is a quantitative one. Vernacular fi tness is usually taken to be a cause of selection, whereas predictive fi tness is taken to be a measure of selection, not its cause. Matthen and Ariew (2002) argue that we should only use the concept of predictive fi tness. Others defend the concept of vernacular fi tness and insist that it is an individual-level concept (Bouchard and Rosenberg 2004; Rosenberg and Bouchard 2005). Yet another group of philosophers concede that it is a population-level concept, but maintain that it is a causal one (Stephens 2004; Millstein 2006). There are some further questions about fi tness. Is it fi xed throughout the organism's lifetime (Ramsey 2006)? In what way does it depend on the environment and how can we characterize the environment it depends on (Abrams 2007)? Before we get entangled in the Byzantine debates surrounding the concept of fi tness, we should take a step back and ask: why should we conceive of selection as the heritable variation of fi tness at all? There are important cases of natural selection where it is not clear how the heritable variation of fi tness account could even be formulated.79 An important aspect of the heritable variation of fi tness account is that it talks about parents and offspring. Both what Lewontin calls "the principle of variation" and what he calls "the principle of differential fi tness" (Lewontin 1980: 76) are principles about the parent– offspring relation. But there are cases of natural selection where it is unclear what should be considered as the parent and what should be considered as the offspring. Here are two such 78 Ariew and Lewontin (2004) refer to these two concepts of fi tness as "Darwinian" and "reproductive" fi tness. 79 I leave aside some further potential problems with the heritable variation of the fi tness account, for example, that it presupposes that the parent and offspring generations do not overlap (see Ariew and Lewontin 2004). I assume that the heritable variation of the fi tness account could be modifi ed in such a way that it could deal with this potential problem. Pleh_Naturalistic-2-Normal.indd 126 2014.02.13. 15:02:59 127 cases: selection among clonal organisms and in the microbial world. For the purposes of this paper, I will focus on microbial evolution (but see Nanay 2011a on clonal selection). It is important to note that these are not marginal cases of natural selection (on how widespread and important clonal reproduction is, see Godfrey-Smith 2009: 71–72, and Bouchard 2008; on the importance and relevance of the microbial world, see O'Malley and Dupré 2007's manifesto). As we have seen, a striking feature of most microbial population is lateral gene transfer, the transfer of genetic material from one organism to another by conjugation, transduction, or transformation (Bushman 2002; Thomas and Nielsen 2005; see O'Malley and Dupré 2007: 167–168 especially for a philosophical analysis of this phenomenon). Lateral gene transfer makes natural selection (and evolutionary change in general) in the microbial world more rapid and more frequent than it is among macrobes (see e.g., Lawrence 2002). But how can we talk about the heritable variation of fi tness in the case of lateral gene transfer? Lateral gene transfer is not from parent to offspring. It is from offspring to offspring. This, again, makes it diffi cult to even formulate the principle of variation and the principle of differential fi tness of the heritable variation of fi tness account (see O'Malley and Dupré 2007 for a summary of how lateral gene transfer in the microbial world challenges our existing evolutionary accounts). Could we not defend the heritable variation of fi tness account by arguing that lateral gene transfer should be considered a simple mutation from the point of view of the organism that is on the receiving end of the transfer? This move is indeed open to the proponents of the heritable variation of fi tness account, but it is diffi cult to see how it will help. Lateral gene transfer can have varying degrees of fi delity. Thus, it can, in principle, give rise to bona fi de evolution by natural selection that may even lead to adaptation. But lateral gene transfer is (by defi nition) not an intergenerational change. And this makes it impossible to talk about the change of fi tness values, as fi tness is defi ned with reference to (some features of) the parent generation and (some features of) the offspring generation. When lateral gene transfer gives rise to evolution by natural selection, this cannot be described with the help of the heritable variation of fi tness account. It seems then that, while the heritable variation of fi tness account may or may not be the right model for macrobial evolution, it is unlikely to be the right way to describe microbial evolution. But, because of the structural similarities between microbial and cultural evolution, it is also unlikely to be the right way to describe cultural evolution. The argument I gave in the last couple of paragraphs can be easily rephrased with regards to horizontal information transfer in the case of the cultural domain. If we want to understand cultural evolution (and microbial evolution), we are well advised not to use the heritable variation of fi tness account. We should turn to the replication–interaction model. The replication–interaction model and microbial evolution The replication–interaction account of selection is a genuine alternative to the heritable variation of fi tness account, but it has different versions and the most widespread of these is widely assumed to be highly problematic. We can distinguish two versions of this account, the replicator–interactor account and the property-replication account. The former Pleh_Naturalistic-2-Normal.indd 127 2014.02.13. 15:02:59 128 has been repeatedly criticized. I argue that we should use the latter when modeling microbial and cultural evolution. The replicator–interactor account According to the fi rst version of the replication–interaction account, replication is the copying of an entity, the replicator. Hull defi nes the replicator as "an entity that passes on its structure largely intact in successive replications" (Hull 1988: 408; see also Godfrey-Smith 2000; Brandon 1990; see Nanay 2002 on the concept of replicator). The unit of interaction, the interactor, on the other hand, is defi ned as the "entity that interacts as a cohesive whole with its environment in such a way that this interaction causes replication to be differential" (Hull 1988: 408). I will call this version of the property-replication account the replicator–interactor account as it identifi es replication with the copying of an entity, the replicator. In the last decade or so, many philosophers and biologists have argued against this replicator–interactor account of selection (Okasha 2007: 15–16; Godfrey-Smith 2007: 515; Godfrey-Smith 2009; Avital and Jablonka 2000: 359; Jablonka and Lamb 1995; Richerson and Boyd 2005: Chapter 3; Griesemer 2000: 74–76; Griesemer 2002: 105). Their main claim is that the copying of replicators is not necessary for selection; hence, selection cannot consist of repeated cycles of replication (conceived of as the copying of replicators) and interaction. There are ways of transmitting information (for example, extragenetic inheritance) that do not count as replication but that are (given other conditions) suffi cient for selection (Okasha 2007: 15; Avital and Jablonka 2000: 359; Jablonka and Lamb 1995: 3). Samir Okasha summarizes this line of objection: "evolutionary changes mediated by cultural and behavioural inheritance cannot be described as the differential transmission of replicators" (Okasha 2007: 15). To put this objection in more general terms, selection can happen if there is suffi cient phenotypic parent–offspring resemblance. Replication is not needed (Okasha 2007: 15). One example is maternal effects, i.e., cases in which large mothers have large offspring as a result of laying eggs with larger food reserves (Uller 2008). The property-replication account It is important that these problems are problems for the replicator–interactor account and not for the replication–interaction account in general. Remember that the original alternative to the heritable variation of fi tness account was the view that selection consists of repeated cycles of replication and interaction. It is an additional requirement that replication should be thought of as the copying of an entity, namely, the replicator. We may be able to salvage the general gist of the replication–interaction account if we deny that replication is the copying of an entity. We could conceive of replication as the copying of property-instances (Nanay 2011a; see also Nanay 2002: 113). The hope is that this version is not vulnerable to the objections raised against the replicator–interactor account. I will use the term property-replication account for this version of the original replication–interaction account to contrast it with the replicator–interactor account. Pleh_Naturalistic-2-Normal.indd 128 2014.02.13. 15:03:00 129 It is important to clarify the difference between these two versions, i.e., what is meant by entities and properties here. The cup in front of me is an entity. It has lots of properties, some interesting, some others less so. Its color is one property, its shape is another one, etc. Thus, the copying of an entity and the copying of one of the properties of this entity are very different processes. Properties are always properties of entities, of course. But it is possible to copy a property of an entity without thereby copying the entity itself. The claim is that replication is the copying of properties: we can have a replication process without there being a replicator that is being copied.80 The defi nition of replication would then be the following (Nanay 2011a: Section 4): property P of object (a) is a replica of property Q of object (b) if and only if: (1) P is similar to Q and (2) Q is causally involved in the production of P in a way responsible for the similarity of P to Q. An important feature of this defi nition is that (a) and (b) are not necessarily objects of the same kind. Object (b) may be an apple, and object (a) a color photograph of this apple. The color of the photograph can be a replica of the color of the apple under my defi nition, but this does not mean that the objects themselves are replicas or copies or replicators in the old sense of the word. This notion of replication is very weak: many non-biological copying processes, like photocopying, will also qualify as replication. Is this a problem? No. The same is true of the traditional concept of replication as the copying of replicators (Godfrey-Smith 2000; Nanay 2002). Importantly, any account that conceives of selection as the repeated cycles of replication and interaction needs to acknowledge that not every replication process will be particularly interesting from an evolutionary point of view. But this is what we should expect: the notion of replication is only the starting point for an account of selection. Further additional criteria need to be met in order for replication to lead to selection: replication needs to give rise to an interaction process that makes the next round of replication differential. How can this property-replication account handle the objections to the replicator–interactor account? First, according to the property-replication account, both extragenetic inheritance and cultural transmission can count as replication. Nothing in the defi nition of replication suggests that the replicated property needs to be a property of the DNA. Thus, extragenetic properties can replicate as much as the properties of the DNA can. If property P of the offspring is similar to property Q of the parent, and the latter is causally responsible for this similarity, then we do have replication, regardless of whether these properties can be called genotypic or not. Crucially, the transfer of cultural information also counts as replication if we understand replication in the way that property-replication suggests: cultural properties are being replicated. Remember that the defi nition of replication was the following: property P of object (a) is a replica of property Q of object (b) if and only if: (1) P is similar to Q and (2) Q is causally involved in the production of P in a way responsible for the similarity of P to Q. As P and Q can be any property in this defi nition, cultural information transfer would qualify as replication, as long as both (1) and (2) are satisfi ed. 80 Biologists call the properties of organisms 'traits.' If someone prefers this concept to the concept of properties, he/she can rephrase my defi nition of replication as 'the copying of traits.' But as the replicated properties are not necessarily properties of an organism, I will talk about properties, rather than traits, in what follows in order to preserve generality. Pleh_Naturalistic-2-Normal.indd 129 2014.02.13. 15:03:00 130 More generally, if we accept the property-replication account, then phenotypic traits can replicate. Take the maternal effects example I mentioned in the last subsection. According to the property-replication account, there is a property that replicates in this case: the property of being large. The offspring's instantiation of this property is similar to her mother's (in as much as the degree of similarity between the size of the two individuals is higher than it is between the size of two randomly chosen individuals in the population), and her size is causally responsible for this similarity. Thus, we do have selection in this population, but we also have replication. We do not have replicators though. Property-replication and lateral gene transfer So far, everything looks promising: the property-replication account is not susceptible to the objections that were raised against the replicator–interactor account. But the real question is whether the property-replication account is a genuine alternative to the heritable variation of fi tness account. More precisely, can it handle the cases of selection in the microbial world that were problematic for the heritable variation of fi tness account? If we accept the property-replication account, then microbial evolution will pose no problem as lateral gene transfer will count as a replication process. Lateral gene transfer is the copying of an entity (and its many property-instances) from one organism to the other. And this counts as replication under any account of replication: both the replication–interaction conception and the property-replication conception. Some replication processes will happen from parent to offspring, some others from offspring to offspring. If either kind of replication processes gives rise to environmental interaction that makes the next round of replication (again, either parent to offspring, or offspring to offspring replication) differential, we have a selection process, conceived as the repeated cycles of replication and interaction. We can talk about selection in microbial populations without running into the problems that the concept of fi tness poses in this context. And the same goes for horizontal information transfer in the case of cultural evolution: it counts as replication in the sense that the property-replication view uses the term. Some replication processes will happen from parent to offspring, some others from offspring to offspring. If either kind of replication processes gives rise to environmental interaction that makes the next round of replication (again, either parent to offspring, or offspring to offspring replication) differential, we have a selection process, conceived as the repeated cycles of replication and interaction. We then get the following picture: there are three ways of modeling natural selection, the heritable variation of fi tness account, and two versions of the replication–interaction account, the replicator–interactor account and the property-replication account. We have seen that the heritable variation of fi tness account is unlikely to be able to be the right way to think about cultural and microbial evolution because it cannot handle lateral gene transfer and horizontal information transfer. The replicator–interactor account has been facing various objections. The best bet for those who want to understand cultural and microbial evolution is then the property-replication account. Pleh_Naturalistic-2-Normal.indd 130 2014.02.13. 15:03:00 131 Property-replication and cultural evolution: Cultural replication without memes As the most famous account of applying the replication–interaction model to cultural evolution is meme theory, one may worry that the shift from the heritable variation of fi tness model to the replication–interaction model I am encouraging in the context of cultural evolution would amount to a return to meme theory. Much of the recent discussion of cultural evolution has been revolving around the concept of meme. The distinction between replicator and interactor was originally famously introduced "as a generalization of the traditional genotype–phenotype distinction" (Brandon 1990: 125). This means that there can be, and supposedly there are, entities other than the gene that would count as replicators. The main candidates for such replicators have been memes. Memes are defi ned as the "units of the cultural transmission" (Dawkins 1976/1989: 192; see also Dawkins 1982a, 1982b). According to meme theory, cultural phenomena can be explained, at least partially, with the help of the following evolutionary model: memes are pieces of information, and they compete for survival in a way quite similar to genes; the difference is that they compete for the capacity of our minds. A meme can be a tune, the idea of liberalism, or the habit of brushing one's teeth. Those tunes will survive that can get into and stay in many minds. The ones that fail to do so will die out. Meme theory is clearly a way of applying the replicator–interactor model to the cultural domain. Meme theory is still extremely popular (see Blackmore 1999; Dennett 2003, 2006; Aunger 2002; Distin 2005), but it has been severely criticized for various reasons, partly for worries about the ontological status of memes (Sperber 1996; Wimsatt 1999; Fracchia and Lewontin 1999; Richerson and Boyd 2005; Sterelny 2006a, 2006b). What are these cultural replicators supposed to be? There have been various attempts to answer this question (Dennett 2006: 80–81, and 349–350; Dennett 2003; Aunger 2002: 311–322; Distin 2005). An infl uential strategy is to say that both genes and memes are really just pieces of information, and there is nothing ontologically worrying about the concept of information (this is Dennett's and Distin's response; but see Aunger's more restrictive version). Note that this view violates the concept of replicator the original replication–interaction model was presupposing.81 It is important to note that meme theory applies the replicator–interactor model to cultural evolution. My proposal, in contrast, has been that we should apply the property-replication model instead. If we do so, we can bypass the ontological worries meme theory faces. This move would replace the notion of cultural replicators, that is, memes, with replicated cultural properties. It has been argued that whether or not we buy into meme theory, there are processes in the cultural domain that can be described as replication (Richerson and Boyd 2005; Sterelny 81 They are not reproducers either: a meme and its copy do not have any material overlap. Pleh_Naturalistic-2-Normal.indd 131 2014.02.13. 15:03:00 132 2006a, 2006b).82 If we accept my defi nition of replication, then we can explain these processes without postulating ontologically suspicious entities, like memes.83 It is important to note that if we acknowledge that there are processes that could count as cultural replication, we do not need to be thereby committed to allow for cultural selection (as replication is not suffi cient for selection), let alone cumulative cultural selection that could explain why certain cultural features are the way they are. If we accept my defi nition of replication, this will not salvage meme theory, or even the very idea of memes. But it would make it possible to talk about cultural replication, without specifying what the replicated entities would be, or without positing the existence of memes. Conclusion The aim of this paper has been to argue that we should use microbial, rather than macrobial evolution as the model for understanding cultural evolution. And the emphasis on the similarities between microbial and cultural evolution as well as on the differences between microbial and cultural evolution should persuade us to abandon both the heritable variation of fi tness model and the replicator–interactor model when it comes to understanding cultural evolution, and use the property-replication view instead. References Abrams, M. (2007) Fitness and propensity's annulment? Biology and Philosophy 22, 115–130. Ariew, A. and Lewontin, R. (2004) The confusions of fi tness. British Journal for the Philosophy of Science 55, 347–363. Aunger, R. (2002) The Electric Meme: A New Theory of How We Think and Communicate. New York: Free Press. Aunger, R. (ed.) (2000) Darwinizing Culture: The Status of Memetics as a Science. 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