gene-culture evolution

The interplay of genetic and cultural evolution

In my last post, I discussed the framework for cultural evolution laid out by Claire El Mouden and colleagues in a new article in the Journal of Evolutionary Biology (ungated pdf and supporting information). By setting out clear definitions for the analysis of cultural evolution, such as cultural relatedness and fitness, a workable framework using evolutionary biology’s Price equation can be developed.

As I noted in that post, it is when the biological and genetic frameworks are laid on top of each other, as is the focus of the dual-inheritance literature, that things get interesting. With their framework, El Mouden and friends tackle a couple of the prominent gene-culture evolution questions.

The first question is to what extent cultural evolution increases or decreases genetic fitness. The authors note that a theme of the gene-culture evolution literature is that cultural evolution made the scale and distribution of today’s human population possible. How does this work?

For there to be a positive correlation between cultural and genetic fitness, those who have the most cultural influence must also leave the most offspring. It is easy to see circumstances where this holds, with those of high social status tending to have both cultural influence and more offspring (look at the number of partners of rock stars).

An example to the contrary is low fertility in developed countries. El Mouden and colleagues reference work by Richerson and Boyd, who suggest that high status is given to professions with high investment in education, which would allow the behaviour to spread despite the negative relationship between education and fertility.

However, El Mouden and friends show that this mismatch between cultural and genetic fitness is evolutionarily unstable as genetic natural selection acts to align genetic and cultural fitness. They suggest two reasons for this, one involving transmission and the other selection.

Their transmission explanation relies on the already evolved propensity for people to avoid behaviours that are harmful to their genetic interests. For example, the nausea produced when consuming toxins would fight against any cultural pressure to eat toxic food. However, it is an interesting question as to how general this transmission effect would be, as many cultural forces are novel and without precedent in our evolutionary history. Transmission may be unlikely to constrain our desire for high status professions that require large investments in education any time soon.

The genetic selection explanation would seem to have broader power. In the supplementary information, the authors ask us to consider a population where cultural and genetic fitness were not aligned. Now, imagine a mutant in that population that causes people to pay attention to a cultural trait that is more highly correlated with genetic fitness. As these mutants have higher genetic fitness, they increase in proportion of the population, and cultural and genetic fitness are now more correlated. Cultural fitness now promotes genetic fitness. In the long-run, the two will be perfectly correlated (the exception being where cultural traits are neutral to genetic fitness).

The catch in that last sentence is the “long-run”. As cultural evolution can be so much faster than genetic evolution, systems can be far from genetic equilibrium until the genetic response evolves. Fertility in developed countries would be an example of this. There may also be some constraints that prevent perfect alignment, such as the presence of appropriate learning mechanisms.

This interaction of genetic and cultural evolution gets most interesting is when we turn to the evolution of altruism. In examining this question we must remember that genetic and cultural fitness are distinct. Cultural altruism reduces the altruist’s cultural fitness; that is, their influence. As a result, the claim that cultural evolution increases genetic altruism (the more common claim in the gene-culture evolution literature) needs to be made carefully.

As an illustration, consider this interesting example from the paper. A stranger is being attacked, so a good Samaritan steps in to defend them and dies as a result. Whether this is culturally altruistic would depend on whether the Samaritan’s deed was copied. If so, then the Samaritan’s act would actually have been increase cultural fitness as it would have increased their influence in respect of that cultural trait.

Conversely, their death is genetically altruistic. As a result, genetic selection would tend to act against it. Those who ignore this cultural trait will have higher genetic fitness, grow in proportion of the population and eventually bring cultural and genetic fitness into alignment.

So what of behaviours that are both culturally and genetically altruistic? Whether the behaviour spreads will depend on the degree of cultural and genetic relatedness.

Evidence suggests that cultural relatedness within ethnic groups is higher than genetic relatedness (although it is still not high in absolute terms, with more within group than between group variation). This means that there are a wider range of circumstances for which cultural altruism can emerge than for genetic altruism. However, that domain in which cultural but not genetic altruism is likely to emerge will be subject to the forces described above to align cultural and genetic fitness.

Another important point is that each cultural trait should be considered separately. Even though a group may have the same language, giving them high relatedness for this cultural trait, this does not mean that they have the same views on giving their lives for strangers, for which they may have low cultural relatedness. Consideration of the conditions for altruism need to consider the specific cultural trait.

There are many other interesting points in the article – I recommend reading the whole thing – but I will close with a point on the practicality of modelling cultural evolution in this way. El Mouden and friends note that there is a host of complications not present in the genetic case. Cultural relatedness can vary wildly across cultural traits, whereas the nature of genetic transmission means that relatedness is similar across most of the genome. Recognising the pattern of inheritance is also a challenge, as ancestor numbers can vary in number and be of vastly different biological ages. In that context, there is no such thing as a standard length of generation.

So although this paper presents a nice approach to cultural evolution, it does not present an approach that is easily applied to empirical observation. However, given the lack of clarity across much of the gene-culture evolution literature, particularly when examined across authors and papers, it is nice to see an attempt to achieve some conceptual coherence.

Boyd and Richerson's The Origin and Evolution of Cultures

The Origin and Evolution of CulturesWhen I asked for suggestions for my evolutionary biology and economics reading list earlier this year, Boyd and Richerson’s The Origin and Evolution of Cultures was one of the most recommended. Their exploration of cultural evolution has many elements that are relevant to economics, including the development of institutional frameworks, the evolution of cooperation and the transmission of technology.

The book comprises 20 papers (published between 1987 and 2003) that are grouped into five thematic groups: the evolution of social learning; ethnic groups and markers; human cooperation, reciprocity and group selection; archaeology and culture history; and links to other disciplines. Each chapter was a stand-alone paper, so rather than going into any of them in further detail, I will save that for some later posts and give some more general observations here.

First, Boyd and Richerson are clear in arguing that “culture” is a distinct feature from “environment”, and that it should be examined through an evolutionary lens:

[C]ultural variation is transmitted from individual to individual, it is subject to population dynamic processes analogous to those that effect genetic variation and quite unlike the processes that govern other environmental effects. Combining cultural and environmental effects into a single category conceals these important differences.

Having been sceptical before reading the book, this is one issue on which I am a convert. I am still not convinced that it is always (or often) possible to identify practically which cultural trait is subject to selection or to differentiate it from the environment, but drawing this distinction led to some interesting and parsimonious models. Further, an evolving cultural trait may be the environment for another cultural trait.

Their exploration of cultural evolution often contains a genetic element, usually in the context of “gene-culture coevolution”. For example, they describe a process whereby cultural institutions might result in people with certain genetic predispositions beings weeded out.

Mechanisms by which cultural institutions might exert forces tugging in this direction are not far to seek. People are likely to discriminate against genotypes that are incapable of conforming to cultural norms (Richerson and Boyd, 1989; Laland, Kumm, and Feldman, 1995). People who cannot control their self-serving aggression ended up exiled or executed in small-scale societies and imprisoned in contemporary ones. People whose social skills embarrass their families will have a hard time attracting mates. Of course, selfish and nepotistic impulses were never entirely suppressed; our genetically transmitted evolved psychology shapes human cultures, and, as a result, cultural adaptations often still serve the ancient imperatives of inclusive genetic fitness. However, cultural evolution also creates new selective environments that build cultural imperatives into our genes.

However, Boyd and Richerson’s exploration of gene-culture coevolution does not usually extend to developing models with where genes and culture simultaneously evolve. At times this is problematic, particularly where they incorporate cultural group selection into the picture, as it can be difficult to understand how the process would actually work from the often loose verbal descriptions. Conversely, a model incorporating these multiple evolving elements would lose the clarity and simplicity that allows most of the models in the book to be useful.

The indeterminate nature of the culture-environment distinction I alluded to above is also highlighted by this gene-culture evolution quote. Cultural evolution creates new selective environments. While a cultural trait is evolving, it is effectively creating an environment in which other cultural traits or genes evolve. This is similar to the idea that genes effectively create the environment in which other genes evolve, whether those other genes be in the same individual or in other individuals and species.

Boyd and Richerson’s work shares some similarity with that of Sam Bowles and Herb Gintis, particularly in their approach to model development. Simulations are used as illustrations, the focus is more on demonstrating ideas than providing hard proofs, and agent based models are a common tool.  However, Boyd and Richerson have a stronger sense than Bowles and Gintis of the limitations of their models, and generally recognise their illustrative and not determinative nature. Bowles and Gintis have a habit of making a model and arguing that, since a certain feature didn’t work in their model (such as the evolution of cooperation by reciprocal altruism), their model is evidence that it can’t work at all. The problem with this approach is that the model only examines such a small subspace of the possibilities. Boyd and Richerson tend to be more constrained in their conclusions, although not always so.

One of the groups of papers focuses on group selection. I am more open to analysing the transmission of cultural traits through the lens of group selection (or multilevel selection) than I am for the transmission of genes, largely because cultural group selection is not necessarily undermined by migration between groups in the same way as genetic group selection. Boyd and Richerson note this when they state:

[S]ocial extinction does not mean physical elimination of the entire group. Quite the contrary, most people survive defeat but flee as refugees to other groups, into which they are incorporated. This sort of extinction cannot support genetic group selection because so many of the defeated survive and because they would tend to carry their unsuccessful genes into successful groups, rapidly running down variation between groups. However, the effects of conformist cultural transmission combined with moralistic punishment makes between-group cultural variation much less subject to erosion by migration and within-group success of uncooperative strategies than is true in the case of acultural organisms.

However, I am still not convinced that the cultural group selection approach provides the clearest method of analysis. I’ll save my specific issues with their approach in a separate post.

My favourite chapter of the book was the least theoretical. Boyd and Richerson (with Joseph Soltis) asked whether observed rates of group extinction could be sufficient for group selection to drive rapid cultural evolution. Based on an examination of hunter-gather tribe extinction rates, they concluded that group selection could not be responsible. It was refreshing to see some empirical analysis applied to this issue. For all the noise around group selection (both genetic and cultural), it is rare that the debates are accompanied by increasingly available data.

*My later post with my thoughts on their approach to group selection can be found here.

Haidt's group selection

Having given my thoughts on Haidt’s generally excellent The Righteous Mind in my last post, I want to turn to Haidt’s use of group selection in the last third of his book. The central themes of his book don’t rest on group selection (in my opinion), but Haidt is at the centre of the reemergence of group selection in the social sciences and his points are worth discussing.

Haidt uses the more modern phrase “multilevel selection” in addition to “group selection” through the book. Multilevel selection refers to a method of accounting for selection at the different levels (e.g. the gene, individual, group etc.), while the older concept of group selection usually refers to natural selection between groups and the evolution of group-level adaptations. Multilevel selection also involves what are called “trait groups”, which may briefly form and break up, compared to the rigid reproducing groups of the old group selection.

It seems that Haidt has a reasonable grasp on these distinctions but his use of the term multilevel selection is often confusing. For example, he keeps using the phrase “product of multilevel selection”. But multilevel selection is, as the name suggests, selection at different levels. You can look at a scenario through a multilevel selection framework and conclude that all the selection occurs at the level of the gene or individual. Multilevel selection and inclusive fitness are just different accounting methods (or languages). It is not a case that one happens and the other doesn’t. What Haidt is implying, and the way he should state it, is that selection is occurring predominantly at the level of the group. Based on some passages of the book, it is clear that Haidt understands this, but at other times his language is loose.

When Haidt argues for the importance of selection at the level of the group (I’ll refer to it as group selection for rest of this post), he offers four lines of evidence: the role of group selection in the major evolutionary transitions; the shared intentionality of humans; gene-culture evolution; and the potential for fast evolution.

The major evolutionary transitions, such as the emergence of eukaryotic cells from the combination of bacteria, are one of the few areas where many evolutionary biologists will agree that group selection occurred. Haidt characterises the major evolutionary transitions as times where methods to control freeriding evolved at one level, allowing superorganisms to arise at the next. Haidt then follows in the footsteps of biologists such as David Sloan Wilson and E. O. Wilson and argues that the evolution of “ultrasociality” in humans is a similar transition.

I don’t want to rehash this argument in-depth, but it goes back to the classic group selection debate. In the evolutionary transitions of the past, a reproductive bottleneck was present. Once two bacteria are combined in a cell, the only way they can reproduce is if the “group” reproduces. But that bottleneck does not exist in human groups, so there is opportunity for freeriding. We then get to the old debate about the level of freeriding and whether the level of group extinction and the degree of gene flow between groups allows group selection to outweigh this freeriding.

While Haidt follows in others’ footsteps in referring to the major evolutionary transitions, his other arguments are more his own. On shared intentionality, Haidt argues that the ability to share intentions between people allows collaboration, the division of labour and shared norms. While Haidt claims this is group selection, this is a case where the multilevel selection framework should be properly applied. How much benefit does one get as an individual from understanding what someone else is thinking, versus the benefit you get from pairing with someone who also has that ability and working together to succeed as a group? While having more people in your group who are able to share intentions will help you defeat other groups, shared intentionality is clearly beneficial to an individual. Being in a group of mind readers when you have no idea what is going on is suboptimal. Which level the selection predominantly operates at needs to be analysed (and will depend upon assumptions about what makes up a group). This is the type of scenario that I have argued before is simpler to analyse in an inclusive fitness framework.

Haidt’s third line of evidence is a somewhat confusing take on gene-culture evolution. Haidt argues that cultural group selection supported “prototribalism”, which led to an environment that then supported genetic evolution. However, Haidt’s examples do not sound like group selection. For example, Haidt writes:

[I]ndividuals who found it harder to play along, to restrain their antisocial impulses, and to conform to the most important collective norms would not have been anyone’s top choice when it came time to choose partners for hunting, foraging, or mating. In particular, people who were violent would have been shunned, punished, or in extreme cases killed.

This sounds like individual level selection against violent, non-conformist individuals. I am not sure why Haidt was so keen to covert Boyd and Richerson’s arguments on cultural group selection into genetic group selection, but try he did.

Haidt’s biggest reach, however, comes with his argument that the potential for fast evolution supports group selection. Haidt notes that gene-culture evolution reached fever pitch in the last 12,000 years, and that is an assessment I would agree with. He refers to the group selection experiments conducted by William Muir, in which Muir rapidly improved egg laying by selecting groups of successful chickens (achieved, of course, through the effective creation of a reproductive bottleneck in the experimental design). Haidt then pushes the rapid evolution argument to the limits when he seeks to implicate group selection in the emergence of religion. As large-scale religion only emerged since the dawn of agriculture, Haidt suggests the rapid recent evolution identified by the likes of John Hawks, Greg Cochrane and Henry Harpending provides scope for recent group selection. He writes:

[G]roup selection can work very quickly (as in the case of those group-selected hens that became more peaceful in just a few generations). Ten thousand years is plenty of time for gene-culture coevolution, including some genetic changes, to have occurred. And 50,000 years is more than plenty of time for genes, brains, groups, and religions to have coevolved into a very tight embrace.

The problem is that group extinction and reproduction generally occurs more slowly than individual level selection. At the individual level, we see large differences in fertility every generation. For many people, it is the end of the genetic line. To the extent heritable traits underlie this variation, we can see rapid changes in genotype. In contrast, studies of rates of group extinction suggest it is slow. Further, groups tend not to be simply wiped out, but the “loser” groups tend to merge into the victor, bringing their genes with them.

Having said all this, we might be able to build a multilevel selection model in which we allow temporary religious or other “trait groups” to form and break up in short periods and divide the degree of selection between the various levels. However, I still doubt we will see significant selection at the group level for most of these examples and I don’t feel that this was the sort of group selection Haidt was interested in. Further, I expect the inclusive fitness framework would give a clearer picture. If this trait group approach could have provided a stronger argument, Haidt might have used it.

Labelling cultural group selection

Steven Pinker’s essay on group selection (my initial post on it here) has now attracted a raft of interesting responses that are well worth reading. While it is hard to stitch together and reconcile the various arguments, in sum they confirm one part of Pinker’s argument. In his essay, Pinker wrote:

The first big problem with group selection is that the term itself sows so much confusion. People invoke it to refer to many distinct phenomena, so casual users may literally not know what they are talking about. I have seen “group selection” used as a loose synonym for the evolution of organisms that live in groups, and for any competition among groups, such as human warfare. Sometimes the term is needlessly used to refer to an individual trait that happens to be shared by the members of a group; as the evolutionary biologist George Williams noted,”a fleet herd of deer” is really just a herd of fleet deer. And sometimes the term is used as a way of redescribing the conventional gene-level theory of natural selection in different words: subsets of genetically related or reciprocally cooperating individuals are dubbed “groups,” and changes in the frequencies of their genes over time is dubbed “group selection.”

In the responses, Herb Gintis talks of gene-culture evolution. Jonathan Haidt suggests that to see group selected traits, we should look at groupishness in inter-group competition. Peter Richerson talks of cultural group selection and traits such as language. David Sloan Wilson and David Queller look at the technical alignment between inclusive fitness and multi-level selection, which is a biologically focused approach. Across the responses, most of Pinker’s varieties of group selection are covered, and many responses cannot be reconciled with the others as they are talking about different ideas.

To resolve this confusion, there needs to be greater differentiation of the various phenomena that people are trying to describe. A starting point would be removing the label of “group selection” from some of them. Daniel Dennett picks up on this point:

Pete Richerson’s comment  articulates the details well, but muddies the water by speaking of cultural group selection. There are reasons for calling these phenomena a variety of group selection, reasons ably recounted by Boyd and Richerson in many publications, but better reasons—in my opinion—for avoiding the label, precisely because it seems to give support to the vague and misguided ideas of group selection that Pinker exposes so effectively. These phenomena consist in the evolution by natural selection (both cultural and genetic) of what might be called groupishness adaptations, dispositions (or traditions) of cooperation and the punishment of defectors, and the like, but not by a process of differential reproduction of groups. What differentially reproduce in these phenomena are groupishness memes, not groups. The establishment of these memes may then enable the genetic evolution of enhancements in hosts—like the lactose toleration that evolved in response to the culturally spread tradition of dairying. This is no more group selection than the differential reproduction of the flora in our guts is group selection.

Personally, I would accept some of these phenomena being called group selection if they always had the prefix of “cultural” attached. At that point, some biologists will drop their instinctive opposition and the debates will be able to focus on the question of whether cultural group selection was important in the evolution of the trait of interest or is a useful framework for analysing it. As I argued in my initial response to Pinker’s piece, cultural group selection avoids some of the issues associated with “biological” group selection. Peter Richerson also makes this point:

Natural selection on large scale patterns of cultural variation is plausible because the cultural variation between neighboring groups that might compete is typically much larger than the genetic variation between the same groups. The reasons are not hard to see; all human groups are more or less open to immigration. Groups intermarry and intermarriage is a very effective conduit for genes. This is less true of culture. Because culture evolves more rapidly than genes, groups will continue to differ despite migration. A large body of social psychology research has characterized the active mechanisms that damp down variation within groups and protect between group variation from the effects of migration. Human social groups are psychologically very salient entities as Pinker acknowledges.