As a molecular anthropologist, my research involves using genetic data to address questions of anthropological interest about the origins, history, migration, structure, and relationships of human populations. I frequently am asked to give lectures to nonspecialist audiences on insights from genetics into human evolution, and invariably during the ensuing discussion period the viewpoint will be expressed that while yes, humans may have evolved during the distant past, surely humans have stopped evolving, because of culture: if something changes in our environment, we respond culturally, not biologically. For example, if the ozone layer continues to disappear, and levels of ultraviolet radiation reach life-threatening levels, we will most likely respond by developing protective skin creams and clothing, moving our cities underground, etc., and not by evolving thicker skin or hair. Conversely, many research groups (including my own) have become interested in detecting and analyzing recent biological evolution in humans, which would seem to contradict the above viewpoint. I therefore have been thinking a lot lately about the role of culture in human evolution, and I thought this might make an interesting topic for this forum (at least, I would be interested in the responses I get).
First, some terminology and background, especially for the nonspecialist. “Evolution” has different meanings to different scientists; a population geneticist, for example, views evolution simply as changes in allele frequencies (that is, the frequencies of the variant forms of a gene) over time. Such changes are usually random, reflecting the fact that not everybody leaves offspring, so by chance some alleles increase in frequency and others decrease in frequency over time. These random fluctuations, known as genetic drift, occur more rapidly in small populations than in large ones. Genetic drift results in loss of genetic variation within populations and increases in genetic differences among populations over time, and is countered by migration among populations, which restores genetic variation within populations and decreases genetic differences among populations. Thus, to a population geneticist, since allele frequencies are always changing because of drift and migration, by definition evolution is always happening, and it therefore makes no sense to say that humans are no longer evolving.
But to most people who are not population geneticists, biological evolution means natural selection, in the Darwinian sense: increase in the frequency of an inherited trait which enhances the survival and/or reproductive success of individuals with that trait, also referred to as genetic adaptation. Often, this is expressed as a response to a change in the environment, which in turn leads to a change in those traits that confer enhanced survival/reproduction. Familiar examples of genetic adaptations that resulted in human evolution include bipedality, increased brain size, loss of body hair, and variation in skin pigmentation. To say that humans have stopped evolving, then, is to say that such inherited traits no longer matter when it comes to how humans respond to their environment. This is the view that I often hear: culture acts as a barrier or a buffer between us and the environment, thereby preventing human evolution.
However, if culture is a buffer, it is an imperfect one. For example, humans are plagued by a variety of infectious diseases, and for every success story (e.g., eradication of smallpox and polio) there are diseases that resist our efforts at finding vaccinations or cures (e.g., malaria and AIDS). And you can be sure that if our culture is unable (or unwilling) to do what it takes to prevent or cure a disease, then genetic resistance will indeed occur and will increase in frequency. Some classic examples of natural selection in humans involve genetic variants that increase resistance to malaria, such as sickle-cell anemia. Genetic variants that increase resistance to AIDS have been identified, and it is a safe bet that such variants will increase in frequency if there is no cure/vaccination for AIDS – but such increase comes at the expense of individuals who do not carry such genetic variants. Evolution in response to infectious disease is thus an ongoing story in humans.
But there is an alternative view to that of culture as a (leaky) barrier to human evolution, which can be expressed as follows: humans have been evolving and continue to evolve, not just in spite of culture, but because of culture. That is, cultural practices have actually caused humans to evolve, and a classic example is lactose tolerance. The story goes as follows: lactose is the major sugar present in mammalian milk, and most mammals stop making lactase, the enzyme that digests lactose, shortly after weaning because they are never again exposed to lactose in their diet. This, incidentally, is a nice example of the evolutionary principle of “use it or lose it”: there is no need to continue making lactase if there is no lactose in the diet. Some humans are weird, however, in that they retain the ability to digest lactose into adulthood. It turns out that the frequency of this trait, known as lactose tolerance (or lactase persistence), is highly correlated with milk-drinking populations in Europe and Africa, and was apparently driven to high frequency by natural selection in those populations. Thus, a human cultural trait – domestication of cattle, thereby providing cow’s milk as a new source of nutrition – resulted in human evolution (namely, an increase in lactose tolerance).
Even more provocatively, recently the view has been put forth that not only has culture influenced human evolution, culture has actually increased the rate of human evolution. According to this view, cultural traits such as the invention and spread of agriculture, domestication of animals, increasing population density and urbanization, etc., have influenced recent human evolution much more dramatically than has the environment. The evidence for this view comes largely from studies that find numerous signals of selection in the patterns of genome-wide genetic variation in human populations. That is, we expect that selection for an inherited trait in a particular population will alter patterns of variation at the responsible gene(s): in general, we expect larger than average genetic differences between populations, and unusually long haplotypes (chromosomal segments), to be associated with such genes. There have been numerous studies looking for such signals of selection in genome-wide data from various human populations, and invariably numerous signals of selection are claimed to have been found. Moreover, some analyses indicate that such signals of selection in our genomes have been accumulating recently, indicating that selection has become more prevalent of late, rather than less prevalent as might be expected if culture is increasingly acting as a barrier to human evolution. The most likely explanation for an increase in recent times in genomic signals of selection would appear to be that culture is indeed driving human evolution.
However, some caveats are in order. First and foremost, there is an ongoing controversy over the reliability of genome scan approaches for detecting selection. It turns out that demographic processes – in particular, population growth and geographic expansion, both of which have certainly been important in human history – can mimic the expected genomic signals of selection. Thus, at least some signals of selection are likely to be false positives and not due to selection at all – and there are those who would argue that this holds for the majority of such signals. If the critics are right and the majority of such signals are indeed false positives, then the evidence for culture driving human evolution disappears. My own view is that the role of demographic processes in producing spurious genomic signals of selection certainly deserves more attention. However, I am fairly confident that the genome-wide approaches do provide at least some evidence for selection in humans, for two reasons. Firstly, one of the predictions we would make is that if a signal of selection on a particular gene is real, then there should be a functional difference between the putatively-selected and non-selected variants of that gene. We have tested that prediction in three cases, and in all three cases we do indeed find a functional and/or phenotypic difference (for further details, see Hughes et al. 2008, Bryk et al. 2008, and Ryan et al., 2009) . This is a necessary, albeit not sufficient, indication of selection – it still needs to be demonstrated that the functional difference has resulted in a trait subject to selection – but I emphasize that in all three cases that we tested, the gene was selected for further study solely on the basis of exhibiting a strong signal of selection in a genome-wide study. So this makes me think that some of the candidates identified by genome-wide studies have indeed experienced selection. Secondly, there is good reason to think that there are many more false negatives than false positives in genome-wide studies of selection. We know, from computer simulations, that genome-wide methods only detect very strong selection – weak selection, in which the fitness advantage provided by those with the trait is only slightly larger than for those lacking the trait, will be missed. And since it is quite likely that weak selection is far more prevalent than strong selection, our present genome-wide studies are detecting only the tip of the iceberg. Thus, I do think that that genome-wide studies are, if anything, underestimating the role of selection.
But there is a more important – and less widely-appreciated – caveat to the assertion that culture is driving recent human evolution, and that is that our current methods for detecting signals of selection based on genome-wide studies can only detect recent selection. The genomic signature of selection that has happened in the distant past will be erased by subsequent mutations and recombination, and after some time will no longer be detected by our methods. So a crucial question is: how far back in time can selection be detected reliably? The answer is we don’t know for sure, but a best guess would be on the order of 10,000 – 20,000 years for our current methods of detecting selection, which happens to coincide with the period of time when the rate of selection has supposedly increased during human evolution. Thus, the apparent recent increase in signals of selection that supposedly has been driven by culture could in fact be just an artifact of our methods for detecting selection; maybe there was just as much or even more selection in the distant past, that was driven by the environment and not by culture, but our methods cannot detect the signal of such older selective events in our genome.
To conclude, it is clear that humans have been evolving recently and are continuing to evolve. It is also clear that humans have evolved because of culture, as witness the lactose tolerance trait. However, whether or not culture has been the main driving force in recent human evolution remains to be seen.
Some Selected References:
- Balter, M. (2005). Are humans still evolving? Science 309, 234-237.
- Bryk, J., Hardouin, E., Pugach, I., Hughes, D., Strotmann, R., Stoneking, M., and Myles, S. (2008). Positive selection in East Asians for an EDAR allele that enhances NF-kappaB activation. PLoS One 3, e2209.
- Cochran, G., and Harpending, H. (2009). The 10,000 Year Explosion: How civilization accelerated human evolution. New York: Basic Books.
- Hancock, A., and Di Rienzo, A. (2008). Detecting the genetic signature of natural selection in human populations: models, methods and data. Annu Rev Anthropol 37, 197-217.
- Hawks, J., Wang, E.T., Cochran, G.M., Harpending, H.C., and Moyzis, R.K. (2007). Recent acceleration of human adaptive evolution. Proc Natl Acad Sci U S A 104, 20753-20758.
- Hofer, T., Ray, N., Wegmann, D., and Excoffier, L. (2009). Large allele frequency differences between human continental groups are more likely to have occurred by drift during range expansions than by selection. Ann Hum Genet 73, 95-108.
- Hughes, D.A., Tang, K., Strotmann, R., Schoneberg, T., Prenen, J., Nilius, B., and Stoneking, M. (2008). Parallel selection on TRPV6 in human populations. PLoS One 3, e1686.
- Kelley, J.L., and Swanson, W.J. (2008). Positive selection in the human genome: from genome scans to biological significance. Annu Rev Genomics Hum Genet 9, 143-160.
- Lopez Herraez, D., Bauchet, M., Tang, K., Theunert, C., Pugach, I., Li, J., Nandineni, M.R., Gross, A., Scholz, M., and Stoneking, M. (2009). Genetic variation and recent positive selection in worldwide human populations: evidence from nearly 1 million SNPs. PLoS One 4, e7888.
- Pickrell, J.K., Coop, G., Novembre, J., Kudaravalli, S., Li, J.Z., Absher, D., Srinivasan, B.S., Barsh, G.S., Myers, R.M., Feldman, M.W., et al. (2009). Signals of recent positive selection in a worldwide sample of human populations. Genome Res 19, 826-837.
- Ryan, A.W., Hughes, D.A., Tang, K., Kelleher, D.P., Ryan, T., McManus, R., and Stoneking, M. (2009). Natural selection and the molecular basis of electrophoretic variation at the coagulation F13B locus. Eur J Hum Genet 17, 219-227.
- Sabeti, P.C., Varilly, P., Fry, B., Lohmueller, J., Hostetter, E., Cotsapas, C., Xie, X., Byrne, E.H., McCarroll, S.A., Gaudet, R., et al. (2007). Genome-wide detection and characterization of positive selection in human populations. Nature 449, 913-918.
- Voight, B.F., Kudaravalli, S., Wen, X., and Pritchard, J.K. (2006). A map of recent positive selection in the human genome. PLoS Biol 4, e72.
Mark Stoneking’s argument that evolution continues in modern human populations runs counter to one of the false tendencies in early forms of evolutionary psychology (EP)—the tendency to envision human evolution as fixed at some point in the Pleistocene. As I argued in a previous post on this site, the EP conception of a massively stable past corresponds with a conception of a human mind consisting largely of “modules,” that is, automatic, species-wide bits of neural machinery adapted to regularities in the ancestral environment. I’ll call this vision—the massively regular ancestral environment and the massively modularized brain—the “narrow-school” version of EP.
In recent years, the narrow-school EP fixation on massive modularity has been successfully challenged by several thinkers who register the human capacity for “general intelligence” (Geary; Sterelny). Narrow-school EP had tended to dismiss the achievements of culture over the past 50,000 years or so as a set of “mismatches” with the Pleistocene mind. Broader school EP, encompassing the idea of conserved motives but linking them with the powers of general intelligence, makes it easier to assimilate these achievements to our understanding of human nature. A still broader school (Boyd; Carroll; Dissanayake; Dutton; Wilson) recognizes that dispositions for “imaginative culture”—literature and the other arts—also form part of the adapted mind. Humans use their cultural contrivances to fulfill basic, conserved human needs, and they use their imaginative vision of the world to regulate their behavior.
As Mark Stoneking suggests, gene-culture co-evolution should be a central feature in our thinking about what makes us human. He mentions lactose tolerance as the standard example. In a recent book, Catching Fire: How Cooking Made Us Human, Richard Wrangham convincingly argues that cooking is a much earlier and more fundamental instance of the way culture and natural selection have intertwined causally in human evolution. Not all humans are lactose tolerant, but all humans have evolved to eat cook food. Cooking made food consumption much more energy efficient, reducing the size of the gut and releasing metabolic resources for a larger brain. Provisioning the metabolically expensive human brain required dual parental care and a sexual division of labor, with males doing the hunting and females doing the gathering and cooking. Hunting provided important but irregular supplies of animal protein. Gathering and cooking insured that the family group received regular provisioning despite unsuccessful days spent in hunting. The hunter-gatherer way of life thus formed a complex of interdependent causal forces in which specifically human cognitive capabilities co-evolved with specifically human strategies for nutrition, reproduction, and social organization.
Animals of other species make tools, share information, and learn behaviors from observing each other. Because humans have exceptionally large brains, they have been able to expand these rudimentary capabilities in three ways unique to human culture: (1) they produce art (Carroll); (2) they retain and develop social, mechanical, and intellectual innovations, adding new innovations to old (Sterelny); and (3) they extrapolate general ideas (Geary). Animals of other species produce emotionally expressive vocalizations and engage in play. Humans alone produce oral narratives and visual artifacts designed to depict objects and actions, evoke subjective sensations, give aesthetic pleasure, and delineate through symbols the salient features of their experience. Through cumulative innovation, humans have transformed techniques into technology, tribes into civilizations, discoveries into progressive sciences, and artistic novelties into aesthetic traditions. By extrapolating general ideas, they have produced theology, philosophy, history, the sciences, and theories about the arts.
Works Cited
Boyd, Brian. On the Origin of Stories: Evolution, Cognition, and Fiction. Cambridge: Harvard UP, 2009.
Carroll, Joseph. “An Evolutionary Paradigm for Literary Study.” Style 42 (2008): 103-35.
Dissanayake, Ellen. Art and Intimacy: How the Arts Began. Seattle: U of Washington P, 2000.
Dutton, Denis The Art Instinct: Beauty, Pleasure, and Human Evolution. New York, Bloomsbury, 2009.
Geary, David C. The Origin of Mind: Evolution of Brain, Cognition, and General Intelligence. Washington, D. C.: APA, 2005.
Sterelny, Kim. Thought in a Hostile World: The Evolution of Human Cognition. Malden, MA: Blackwell, 2003.
Wilson, Edward O. Consilience: The Unity of Knowledge. New York: Knopf, 1998.
Wrangham, Richard. Catching Fire: How Cooking Made Us Human. Philadelphia: Basic Books, 2009
Perhaps the most relevant ways in which culture may drive human evolution are left undiscussed. One: weapons of mass destruction (or their environmental equivalents). Once humans acquire the power to destroy most of their own populations, and use it, the surviving gene pool may show significant statistical variations with respect to the previous one. Perhaps hypothetical yet, but don’t rule it out. Two: Differential reproduction in different cultures. E.g. here in Spain slightly more than one child per couple; African populations have other habits. Chinese policies about number of children, etc.- All of that is relevant. Three: Increased communications mean an increased choice of mates for sexual selection. Some people likely to get married in a small agricultural community are likely to stay unmarried, with their genes de-selected, in a different régime of communications. And there may be other issues as well.
The answer to the question of whether culture prevents or drives human genetic evolution is definitely not yes or no, because humans are probably the only species having integrated culture into their phenotype. Indeed humans are not just naked apes, and their fitness already depends to a large extent on their cultural, societal and technological status, in addition to the classical interaction between their environment and their genes. Human culture is therefore an integral part of human evolution, and is the subject of another form of selection occurring at a much more rapid rate than genetic evolution. Human evolution has thus been tightly linked to cultural and technological evolution, at least for say the last 100,000 years, and this interaction has certainly also modified our genotype, physiology, physical appearance and cognitive abilities. A new transition may soon occur when humans will modify their own genotype to heal some genetic disease, become resistant to pathogens, extend their life expectancy, or just modify their global appearance. The consequences of this plastic gene therapy are difficult to predict, but these modifications will be certainly common by the end of this century.
Coming back to a more population genetics focused view of recent human evolution, culture has certainly changed the target of selection in our species. Humans have adapted to their changing environment by means of cultural adaptations, which has made unnecessary the need of (slower) genetic adaptations, but which has also certainly led to the relaxation of purifying selection pressure, enabling us to accumulate mutations that would have been otherwise deleterious and thus to become genetically more diverse. The greater consequence of human cultural evolution has certainly been the massive demographic increase that accompanied our spread out-of-Africa. As mentioned by Mark Stoneking, this demographic and range expansions can lead to genetic signatures that are similar to those of genetic adaptations (Excoffier et al. 2009), and the distinction between these two processes is an active area of research.
A large demographic increase has another direct consequence on the genetic evolution of a population: slightly advantageous or slightly deleterious mutations which were not evolving under the effect of selection in small populations can become the target of selection in large populations, where the effect of genetic drift is reduced. This is why there are much more signs of selection in species with large population sizes (e.g. fruit flies) than in humans. It has thus been postulated that there could have been a recent burst of adaptive mutation during the Neolithic (Hawks et al. 2007). However, it is still highly unclear if the local size of Neolithic populations was really that large a few thousand years ago, since food producing communities rarely exceeded a few hundred individuals. Indeed, what is important for selection to operate is local population size and not the total number of individuals living in a country, most of them never being in direct contact with each other. Therefore, the real increase in genetic population size that is sufficient to make selection operational might have been much more recent than previously believed, and have been driven either by increasing mobility between small communities or by urbanisation leading to large random mating populations. The mobility of individuals has increased dramatically over the past hundred years. Current European populations have incorporated many new immigrants and are now a mixture of individuals of very diverse geographical origins (Novembre et al. 2008). Recent cultural changes like higher mobility have thus certainly enabled new categories of mutations to be the target of positive or negative selection in these large modern populations, but it is unlikely that we’ll be able to see the genetic consequence of this new demographic transition, which will anyway not affect the individuals unless new forms of eugenics are implemented. Deleterious mutations not counteracted by medicine will be just more effectively removed from populations by selection, and new beneficial mutations not surrogated by culture will only (slowly) increase in frequency. What is currently happening is that human populations certainly accumulate genetic variability through mutations and migrations, and this increased genetic diversity could reveal beneficial if natural selection would again become crucial for humans. Until that time, cultural evolution will drive human evolution.
References
Excoffier L, Foll M, Petit RJ (2009) Genetic Consequences of Range Expansions. Annual Review in Ecology, Evolution, and Systematics 40:481-501.
Hawks J, Wang ET, Cochran GM, Harpending HC, Moyzis RK: Recent acceleration of human adaptive evolution (2007) Proc Natl Acad Sci U S A 104:20753-20758.
Novembre J, Johnson T, Bryc K, Kutalik Z, Boyko AR, Auton A, Indap A, King KS, Bergmann S, Nelson MR et al. (2008) Genes mirror geography within Europe. Nature 456:98-101.
Mark Stoneking’s perspective is an interesting and thoughtful one; but it is very much that of a population geneticist, concerned with the endless sloshing-around of gene frequencies within our species Homo sapiens. And in the limited sense that those frequencies will never cease to slosh, Stoneking is absolutely right to say that human evolution is ongoing. But to a paleontologist this looks like only part of the story, even if an essential one. The larger evolutionary patterns that we see as macroevolution are almost certainly due in large part to processes acting at a higher level than that of the genes whose frequencies we measure. Indeed, a moment’s thought is enough to reveal that natural selection cannot act on individual genes, for it is the entire individual that thrives or fails in the reproductive stakes. The individual is an enormously complex integrated whole, genetically and morphologically; and as the estimated numbers of coding genes steadily drops, each one evidently has more jobs to do. Selection cannot pick and choose among those functions. Most importantly here, this means that we cannot look at evolution only in terms of individual characters, or genes, or gene complexes. For one thing, entire species are also triaged out there on the constantly shifting ecological stage. It is hardly of much use to be the most splendidly-adapted exemplar of your species if that entire species is being outcompeted into extinction. This may not, of course, have much relevance to human beings, who have been busily eliminating all the competition ever since they became symbolic creatures. But one consequence of the radically new human lifestyle is population growth; and today the single species Homo sapiens is unprecedentedly widely distributed around the world, in densities unimaginable for any other mammal of comparable body size. Among mammals such as us, it is generally accepted that the fixation of genetic novelties is only probable within small, isolated populations – exactly the conditions known to have obtained at the point when our extraordinary species emerged. But all of that changed with the adoption of settled lifestyles. And as long as the demographic trends established at that fateful point in human history continue (no sure bet, certainly), the probability of any biologically significant evolutionary change in our species must be very close to zero. In which case, we are going to have to learn to live with ourselves as we are. This reply was solicited.
Just a few brief comments. The distinction between “natural” selection and social selection underlies much of the above commentary, but is not tagged specifically in the various examples, and this leaves conclusions a little vague. Yes, we adapt and have adapted to the conditions of our physical environment, and it is this evolution that is usually considered to have been interrupted by culture. But, of course, we also adapt and have adapted to the conditions of our social environments, and these have become denser and more complex over time, while amplified by the advantages culture has provided (to adapt to the physical environment). So culture has not and cannot have interrupted our social evolution–where we adapt to competition within our own species, rather than with the physical environment. It is more likely to have increased it over time. I think this is where art now plays its biggest adaptive role. The selection pressures on human intelligence, and the resulting alterations of cultural adaptations, destabilize the foundations of any such argument. But social evolution almost certainly exerts more pressure now than in the past, and, because culture has in a sense insulated us from some “natural” selection, plays a larger role than adaptation to physical environments and physical changes or threats. I think. jt
A few informal thoughts from a cultural historian with regard to the influence of culture on genetic selection: culture and the information and attitudes involved in culture’s influence on individuals do not spread uniformly throughout the population but depend on a variety of factors including access to information and willingness to act on it. Thus, for example, individuals in a population who have regular access to scientific information learn sooner about the various effects of harmful behavior such as smoking or sun exposure and those individuals within that group who are disposed to act upon new information will modify their behavior sooner and thus, presumably, have a greater chance of surviving and reproducing, as well as inculcating similar values in their offspring, who themselves presumably…etc. It is amazing how many individuals simply don’t care to know about the physical world and how they might best deal with it, both to their own advantage and to the advantage of the world at large. Others know, but continue with harmful behaviors. Part of this refusal is cultural: they were raised to believe that x is the way to do things because those were the cultural norms surrounding them. I could cite smokers of a certain age who were raised in a smoking culture who explicitly refuse to hear that smoking is harmful and keep on smoking. The question is to what extent does this impair their reproductive fitness.
On the question of individuals with a greater tendency to aggression: a series of wars, as for example those occurring in Italy in the late fifteenth and early sixteenth centuries, resulted in the early deaths of many of the relatively aggressive members of the population; in addition, the Italian states lost those wars, which produced widespread depression. One can make a case that this was at the root of many cultural developments of the following century or two as Italians used cultural activities to produce new images of appropriate and praiseworthy behavior, so here the paradigm is more complicated: certain cultural values led to the wars, but the wars were lost and high-energy individuals killed, which led to a new set of cultural values as well as affecting the gene pool via limits on those left to reproduce.
Humans have a highly developed ability to reflect on what other species have that they do not and provide themselves with it mechanically, for example to fly or to navigate by the stars, staying only within the realm of birds. There are many physical capabilities that animals have that humans have not yet figured out; for example, engineers have not yet been able to model how the horse, a very large animal, can remain upright and mobile on small hooves. What do we really know about animal communication systems? I venture to say, not yet very much. The more we learn, the more we find they have. Similarly with their social architecture. It is a wonderful world waiting to be discovered, that will also teach us much about ourselves.
Does social evolution increase informational complexity of genetic evolution?
In what way has culture influenced genetic evolution of human beings? It is an illuminating perspective that Stoneking develops. It opens doors to new insights and raises questions about received wisdom. Or does it?
First, it is useful to clarify that culture is not the constant factor Stoneking’s description suggests. Culture evolves. The interesting question is therefore how cultural evolution influences genetic evolution of human beings. This perspective is well established in the literature on “dual inheritance” or gene-culture coevolution (Boyd and Richerson 1985, Durham 1991). Durham (1991) has a beautiful account of the lactose tolerance example where he shows how cultural instructions in a population directly influence the differential reproduction of genes and genotypes. In his example culture evolves in a separate inheritance track.
Cultural evolution can be described in the following way (Hodgson and Knudsen, forthcoming). From a prior state of instinct-driven behavior, habits made it possible to form and transmit dispositions to engage in useful behaviors in response to particular situations. The evolution of habits facilitated the coding and transmission of experiential learning relating to essential tasks such as hunting and farming. The medium was learning by imitation in human populations and the advantage was that it became possible to change behavioral repertoires on time-scales much shorter than those accounting for the genetic evolution of instincts. Cultural evolution involves a replication-machinery where we spread our habits of action and thought rather than our genes. So far so good.
But social evolution has led to much greater complexity than is usually considered in the literature on gene-culture coevolution. There are many important features of this story. Among these are a number of major transitions in the way information (above the genetic level) is transmitted, stored and utilized. Examples include the emergence of language, custom, law, and institutionalized science and technology.
Social evolution includes the evolution of culture. The potential for complexity in social evolution has greatly increased. But it is unclear in what ways social evolution has influenced the potential for complexity of genetic (human) evolution. In part, the answer to this problem depends on the relevant time-scales for the evolutionary dynamics. It is commonly understood that social (and cultural) evolution occurs at much short time-scales than genetic evolution. Are there good reasons to revise received wisdom in that regard? Or does social evolution primarily influence the developmental potential of genes associated with phenotypic plasticity, e.g. a potential to increase life-span because of improved food-preparation and nutrition? These are exciting problems, and Stoneking should be commended for triggering our thoughts about them.
References
Boyd, Robert and Richerson, Peter J. (1985) Culture and the Evolutionary Process (Chicago: University of Chicago Press).
Durham, William H. (1991) Coevolution: Genes, Culture, and Human Diversity (Stanford: Stanford University Press).
Hodgson, Geoffrey M. and Knudsen, Thorbjørn (forthcoming), Darwin’s Conjecture: The Search for General Principles of Social and Economic Evolution (Chicago: University of Chicago Press).
Is the Occurrence of ADHD Affected by Culture?
Several years ago, in the course of researching a book on music, I read a book review in Science that recommended Russell A. Barkley’s review and synthesis of the ADHD literature, ADHD and the Nature of Self-Control. Barkley argued that ADHD does not involve inattention as much it involves poor self-control, which Barkley argues is a failure of some central executive function. In turn, Barkley asserts that the “nature of this central executive . . . is time. More specifically, it is the conjecturing of the future that arises out of reconstruction of the past and the goal-directed behaviors that are predicated on these activities. Such activities . . . permit self-regulation relative to time” (p. 202).
Barkley went on to point out that “time is an integral, inseparable part of the physical world” (p. 204), that “our will, therefore is . . . at time’s beck and call” (p. 205) and thus that “time, timing, and timeliness . . . become important concepts in understanding . . . goal-directed behavior and in determining it” (p. 209).
Musical performances are, of course, exquisitely timed. One of the major theories about our response to music specifically focuses on the way it plays with our expectations, sometimes satisfying them, sometimes not (Meyer 1956). Beyond that, the brain itself necessarily operates in time; its activities must be well-timed or they will fail. Thus, it seemed to me that music might be an activity that the brain uses to adjust and train its own timing so that expectation (of future sounds) and fulfillment (through motor execution of the musical sounds) are properly matched.
So, when I completed the book (Benzon 2001) I decided to explore possible connections between ADHD in music. I had argued, on the one hand, that music-making is fundamental to human psychobiology. However, over that last century or so music-making has diminished in advanced industrial societies because of the ready availability of recordings. Has that had a deleterious effect on us, the members of such societies? Is the rise of ADHD, in part, an effect of the diminished presence of active music-making in our lives?
I can’t say I found strong evidence on the question one way or another, though I have written up some informal notes in the form of a working paper (Benzon 2009). The ADHD literature is large, contentious, and, at the time (roughly late 2002), not terribly conclusive: something’s going on, but just what is rather obscure. While there is some anecdotal evidence about the efficacy of music therapy in ameliorating the effects of ADHD, very little attention has been given to the possible relationship between music-making and ADHD, and that despite the fact that one of the leading ADHD researchers, Barkley, sees timing problems at the heart of ADHD.
For what it’s worth, here’s a summary of what I found out about the genetics of ADHD. Bear in mind that I did this review seven years ago and haven’t looked at the literature since then. There is some evidence of some kind of cultural linkage, but just what that is, that’s not clear.
Barkley reviewed a number of studies involving families, adopted children, and twins indicating that there is a hereditary component to ADHD (pp. 37-40). There is no evidence that ADHD is caused by chromosomal abnormalities (p. 37), but it is clear that a variety of genes predispose an individual for ADHD. We do not yet have any clear picture of these genetic factors (Comings et al. 2000).
Much of the genetic work centers on various dopamine genes. Barkley reports that initial work focused on the gene for the D2 dopamine receptor, but that there has been trouble replicating that work (cf. Todd and Lobos 2002). More recent work has focused on the dopamine transporter gene and a particular allele of the gene for the D4 dopamine receptor (DRD4). The dopamine transporter clears dopamine from the intercellular space once it has been secreted; thus a deficiency in the transporter would result in abnormally high levels of dopamine in the synaptic space. A recent article (Schmidt et al. 2001) on DRD4 concludes that variation in its alleles accounts for 5-7% of the variance in reported attention problems in a sample of children (primarily white and middle-class) that has been followed for seven years. That is to say, the gene certainly has some effect on attention, but we cannot claim to have found the key that opens the door on this particular mystery. Another study found DRD4-associated attention problems in 1 year olds (Auerbach et al. 2001).
More recently Ding et al. (2002) reported on the world-wide distribution of the different alleles of the D4 dopamine receptor gene. They suggest that a particular allele (known as 7R) “originated as a rare mutational event that nevertheless increased to high frequency in human populations by positive selection” (p. 309). That is to say, once this allele appeared, it spread through the population because it contributed to the fitness of individuals having it. They further suggest that cultural differences might be involved, though they don’t suggest what those differences might be. With respect to disorders such as autism and ADHD they “suggest entertaining the possibility that predisposing alleles in fact are under positive selection and only result in deleterious effects when combined with other environmental/genetic factors” (p. 314).
In an accompanying commentary, Harpending and Cochran (2002) discuss two hypotheses about the possible cultural influence. One suggestion is that 7R is a “dispersal morph,” increasing “the likelihood that its bearers migrate.” Their own hypothesis, however, is that “7R bearers enjoy a reproductive advantage in male-competitive societies, either in competition for food as children or in face-to-face and local group male competition. Societies in which this advantage would be present were rare before the spread of agriculture, but common after it.”
I have no comment to make about either of these hypotheses.
References:
Auerbach, J. G., J. Benjamin, et al. (2001). “DRD4 related to infant attention and information processing: a developmental link to ADHD?” Psychiatric Genetics 11: 31-35.
Barkley, R. A. (1997). ADHD and the Nature of Self Control. New York, The Guilford Press.
Benzon, W. L. (2001). Beethoven’s Anvil: Music in Mind and Culture. New York, Basic Books.
Benzon, W. L. (2009). Music and the Prevention and Amelioration of ADHD: A Theoretical Perspective. Available at SSRN: http://ssrn.com/abstract=1527090
Comings, D. E., R. Gade-Andavolu, et al. (2000). “Multivariate analysis of associations of 42 genes in ADHD, ODD and conduct disorder.” Clin Genet 58(1): 31-40.
Ding, Y.-C., H.-C. Chi, et al. (2002). “Evidence of positive selection acting at the human dopamine receptor D4 gene locus.” PNAS 99(1): 309-314.
Harpending, H. and G. Cochran (2002). “In our genes.” Proceedings of the National Academy of Sciences 99(1): 10-12.
Meyer, L. B. (1956). Emotion and Meaning in Music. Chicago, University of Chicago Press.
Schmidt, L. A., N. A. Fox, et al. (2001). “Association of DRD4 with attention problems in normal childhood development.” Psychiatric Genetics 11: 25-29.
Todd, R. D. and E. A. Lobos (2002). “Mutation screening of the dopamine D2 receptor gene in attention-deficit hyperactivity disorder subtypes: Preliminary report of a research strategy.” Am J Med Genet 114(1): 34-41.
Darwin’s mission was to explain speciation and answer the question of how the many species observed today could have evolved from a smaller set (he thought there were several common ancestors eg one for vertebrates, one for invertebrates, one for microbes ~ only Robert Chambers thought that all life came from a common source, but then he thought that the universe evolved as well…). Darwin did not concentrate on what evolution was up to when speciation was not occurring.
And this presents a problem because speciation is slow and rare in multicelled animals. So most of the time speciation is not occurring, just a few variations on a phenotypic theme that may be quickly reversed if the local conditions changed, as Darwin discovered when he observed pigeon varieties reverting to the wild type in just a few generations when human mediated selection was removed.
But in any population in a relatively stable environment we can observe exactly what evolution is up to. Variation begins to accumulate in the population, in the genome if there is little or no sexual selection and in the phenome if there is strident sexual selection and the population is robust enough to tolerate less than optimal variations.
If we imagine that instead of evolution itself evolving it was a planned mechanism then we might proffer that this strategy trades optimal performance in the current environment for maximal future proofing of the population ie there is a phenotypic variation for every conceivable environmental variation that may occur. Populations may retain variations that were successful in the past, but there is no way for a population to anticipate the future. Thus all variations that can be supported are retained.
Few species have had the luxury to retain variations both in genomic and phenomic forms. Finches in the Galapagos island appear to be able to draw on stored variations within a generation or two if the food source changes and a different beak shape is required.
But humans are enormously successful and can retain variation both in the genome and phenome, and this form of evolution is robustly evident today.
In the past few centuries there has been a rapid adaptation to global conditions with those individuals not able to withstand ‘common diseases’ being wiped out.
Lactose tolerance can not, however, be considered an evolved ‘human’ trait as lactose tolerance is not a species wide trait:
“The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Sicily, to more than 90% in some African and Asian countries” [from a Wikipedia entitled ‘Lactose Intolerance’, referencing “Correlation between lactose absorption and the C/T-13910 and G/A-22018 mutations of the lactase-phlorizin hydrolase (LCT) gene in adult-type hypolactasia”
]
For Darwin’s evolution to work the population must be trimmed of the fat I mentioned above, the retained phenotypic variations that are not optimised for the current environment. The death of those who could not tolerate diseases like common influenza is an example of evolution at work in response to the globalisation environment. No Darwinian evolution will occur until the next cull as variations that are unsuccessful in the current environment are retained both in genomic and phenomic forms. But evolution (but not natural selection nor speciation) is clearly in evidence.
Culture in humans has too many functions for any generalisations to be made. One must at least state whether the informational content of culture is being considered, the artefacts of culture (tools and fire through to cities and industry), the prescriptive behaviour modulators (laws and law makers including governments, religious moral codes etc) or the human imperative (eg currently, humans think of themselves as a species with the right to choose to dominate and utilise or protect any other species). Would information alone be sufficient for a human population to rebuild? (if so, what would stop a slightly more advanced chimp or computer from claiming the rights and privileges of ‘a human’?)
Culture is most usefully understood as a distinctive process of information transmission, which involves the transmission of information (under the guise of beliefs, desires, norms, behavioral practices, etc.) from individual to individual (in contrast to individual learning) within generations (in contrast to the transmission of genetic information by reproduction) and across generations. Although culture is present in several non-human species (e.g., chimpanzees), its role and importance in our species has no counterpart in any other species.
The interaction between cultural evolution (change in beliefs, norms, behaviors, etc.) and biological evolution (change in phenotype due to genetic changes—whether these are due to selection or drift) is bound to take many forms, just like niche construction. As is well illustrated in numerous species, niche construction sometimes buffers organisms from changes in their environment and, as a result, from new selective pressures—a point noted by Stoneking in his thought-provoking essay. But, by modifying the environment in which organisms live, niche construction also creates new selective pressures, resulting in the biological evolution of the relevant populations—a point also made by Stoneking. Similarly, culture is bound to have worked as a buffer during human evolution, while also creating new selective pressures. Both effects of culture (culture as a buffer and culture as an engine) should be kept in mind when one speculates about the importance of culture for the evolution of the human mind.
Hypotheses about the role of culture on human evolution in general, and on the evolution of the human mind in particular, abound in the human evolutionary behavioral sciences. As Boyd and Richerson have noted, one can identify across cultures, epochs, and social systems a particular form of social organization (involving several thousands members who share some distinctive social norms as well as some markers identifying their group membership), and here culture has plausibly been working as a buffer, preventing the emergence of new selective pressures that would have modified fundamental aspects of our social cognition. On the other hand, following others, I have hypothesized that culture allowed for the development of large social groups, which itself resulted in the selection of a distinctive form of social cognition (Boyd and Richerson; Gil-White; Machery and Faucher). Here culture was the engine of biological evolution.
Thus, hypotheses about the role of culture as a buffer or as an engine in the evolution of our species abound. Some are reasonably supported, while others are frankly speculative. However, one should honestly acknowledge that there are few uncontroversial examples. In this respect, it is stunning to see Stoneking give the example of lactose tolerance! Not because this is a controversial example: In fact, it is a well-established example of culture causing biological evolution (e.g., Tishkoff et al.). But because this example has been discussed for at least three decades, and by now I wished we had other well-supported examples!
But still, despite the scarcity of uncontroversial examples, I do not doubt for a minute that culture has played a major role in human evolution, particularly in the evolution of the human mind.
But it should also be acknowledged that when people say that cultural evolution has replaced biological evolution, they often have a distinct thought in mind—i.e., they typically do not mean to deny that the advent of a distinctive form of information transmission has had a major influence on the biological evolution of humans. Rather, they want to highlight the fact that for at least a few millenaries the dramatic behavioral changes in our species have very little to do with biological evolution (if anything at all).
In addition, culture might have a weaker and weaker influence on the biological evolution of human behavior and mind. It is plausible that cultural changes are taking place at an increasing rate, and this for several distinct reasons. Among others: a large segment of our societies is dedicated to invent new ideas, norms, fads, practices, etc., while another segment of our societies is dedicated to transmit these ideas, norms, etc., to others; the strength of social norms seems weaker in large, modern societies than in smaller societies, which allows for faster, more common changes. As a result, it might be that the rate of cultural evolution is too fast for biological evolution to catch up, since biological evolution is bound to be slower than cultural evolution. If this is true, then culture might have stopped creating new selective pressures on human behavior mind!
The biggest influence on evolution between speciation events, especially in the culturally rich modern human population, is the provisioning of those individuals that have favoured traits.
Natural and sexual selection are well known in small populations. Those individuals best adapted to the local environment or that have traits favoured by potential mating partners are favoured. In larger populations, provisioning can occur for traits that do not map directly on to sexual selection or fitness for survival in the current environment.
Any human individual can contribute to this provisioning by endorsing favoured traits recognised in individuals or groups of individuals. Consuming music, artwork and other creative output an individual may display, voting for favoured politicians or vocally supporting a position held by a particular individual all contribute to that favoured individual’s eventual fitness either directly or via the prominence of the favoured trait.
This may not be immediately obvious when the artist, for instance, is homosexual or otherwise not contributing to the gene pool of the next generation. But let us consider a gene-centric view of this common cultural dynamic. Let us consider an imaginary gene that confers an abstract reward for the appreciation of symbolic representation eg art work, language and literature appreciation and so on.
If those who express the gene in a culturally salient manner are rewarded, then those who have the gene will also be strongly attracted to expressing that gene through culturally salient manner (you can not create good art unless you can appreciate art) and those that do, even weakly, will have a sexual selection advantage over those that do not. Even though an individual who has the gene but does not express it culturally (by doing art) nor has offspring (that would perpetuate the gene), the frequency of the gene can increase in subsequent generations if this individual favours those who also have the gene eg by buying their artwork, by teaching art to students, by donating resources to art institutions or by hailing the achievements of artists.
This dynamic has a salient manifestation at all levels from tribal groups through to modern times and represents a mechanism for influencing gene frequency outside of direct reproduction (you can influence the frequency of gene expression that produces a trait valued by you without having children).
Only human culture has the capacity to mediate this extra-sexual modulation of gene frequency, and the biggest influence on evolution that human culture has had is the consolidation of this extra-sexual form of evolution. No such extra-sexual modulation of gene frequency is seen in the culture of any other species (eg chimpanzee).
Posted by
Robert Karl Stonjek
Humans use a biocultural toolkit to interact with their surroundings, modifying and being modified by our niche. We are evolving, and culture and natural selection influence each other and our evolution. But thinking about human evolution primarily as changes in allele frequencies or primarily via behavioral responses to environmental stressors, is too simplistic and lies at the root of public, and in some cases academic, misunderstanding about human evolution.
Being clear about what we mean by evolution is important, and Stoneking rightfully notes this. When considering selection at the genetic level, individual death and quality of life do not matter as long as the genetic material in question is passed to subsequent generations at a sustainable frequency. But the idea that if “culture is unable (or unwilling) to do what it takes to prevent or cure a disease, then genetic resistance will indeed occur and will increase in frequency” is an oversimplification. Such responses to selection pressure at the genetic level assume that the appropriate underlying genetic variation exists and that fitness is significantly curtailed. However, if drugs can keep people with AIDS alive for long periods they can achieve a degree of reproductive success and there may be nothing for selection to respond to. Or, people may be dying of the disease in their 20s and 30s without any drug intervention but still reproducing (reducing selection pressure). There also might be a wide array of behavioral, ecological, physiological, and developmental variants that affect the rates, virulence, density and patterns of HIV across and within populations. Understanding human evolution involves much more than focusing on just natural selection and/or “culture”.
I agree that culture should not be seen merely as a “buffer” and that cultural activity can affect selection pressures, but that is only a small part of the story. We need to also include recent contributions in evolutionary theory that complexify basic selection scenarios and examine the roles of behavioral, epigenetic, and symbolic inheritance (Jablonka and Lamb, 2005), and niche construction and ecological inheritance (Odling-Smee et al. 2003). To envision human evolution basically as natural selection and culture affecting change is too limited a view of evolutionary processes (Fuentes 2009).
Fuentes, A. (2009) Re-situating Anthropological approaches to the evolution of human behavior. Anthropology Today 25(3):12-17
Jablonka, Eva and Lamb, Marion (2005)Evolution in four Dimensions: genetic, epigenetic, behavioral, and symbolic variation in the history of life Cambridge, MA: MIT Press
Odling-Smee, F. J., Laland, K.N., and Feldman, M.W. (2003) Niche construction: The neglected process in evolution Monographs in Population Biology 37. Princeton University Press, Princeton
Given the scope of this topic and the smart commentary that is already here, I’d like to push the boundaries of the discussion further. I want to make two points. (1) Culture is both a tolerator and innovative selector of genetic variation, and it does so via the same mechanisms. (2) As result of culture, human nature is socially constructed.
First, Stoneking’s opposition of the notions that culture prevents selection on genes and that culture drives selection on genes is of course the classic one. That many nonspecialists continue to think in this way is a continued puzzle to the specialist commentators here. On top of their specific comments, I want to make clear that a basic reason to refuse to frame the discussion in the binary opposition (culture does or does not cause selection) is because culture does both things—it both prevents selection on genes and allows novel selection on genes.
The argument that cultural adaptation—the ability of individuals and groups to socially evolve and transmit behavioral strategies that are adapted to local contexts—reduces selection on genetic variation has a lot going for it. The peoples of the arctic have not had to re-evolve fur, because clothing and the control of fire substitute in maintaining body temperature. If the existing genetic variants in the population are sufficiently close to the phenotypic optimum, then culture and other forms of behavioral adaptation can make these genetic variants essentially invisible to selection. “Sufficiently close” in this context must refer to how far socially transmitted behavior and technology can move phenotypes. In the case of clothing, socially transmitted traditions can produce tremendous insulation, meaning that genes for producing modest amounts of body hair can be nearly or just as successful as genes producing more body hair.
The opposite argument, that culture actually produces new selection pressures, arises from the same logic. When the existing genetic variants in the population are all very far from the phenotypic optimum, it is only via socially transmitted behavior that people can expose these variants to natural selection. As hominins expanded their range out of Africa, the many novel environments they encountered would have selected for new genes. But in many cases, it is likely that the existing genetic variants were all very poorly suited to a new ecology, and as a result, none were favored over others by the action of natural selection. It’s like asking which contemporary human genetic variants would have highest fitness in an aquatic environment—since we would all drown, and there are no genes for having gills, selection can’t make progress yet. Cultural adaptations, however, would have allowed populations to essentially shift all of these variants towards being more optimal. Then some of them would possibly have higher fitness and increase in frequency, allowing for selection to get leverage where previously there was little. Consider again the evolution of aquatic adaptations—if we begin with SCUBA gear and underwater domes, genetic variants for respiratory efficiency suddenly become relevant where none before would have allowed even survival.
The thought experiments above are silly in many ways. But they are chosen to illustrate the logic of how cultural dynamics both make selection weaker and stronger, depending upon how from the phenotypic optimum existing genetic variants lay. So while cultural evolution is special in many ways, it’s also just another example of genetic systems creating developmental processes that alter how selection acts on the genome. The argument I just made for culture has been made by scholars of evolutionary development for other mechanisms, notably systems for growth and the regulation of organismal development. Developmental systems are not blueprints for what an organism will look like at maturity. Instead they are highly dynamic recipes and “learn” in meaningful ways. They can adjust phenotypes and therefore accommodate genetic variants that are close to the optimum. And when no genetic variants would survive otherwise, they can shift the phenotype close enough to expose the existing variation to selection (see Gerhart and Kirschner cited at at the end for a fuller discussion of these mechanisms and pointers to the broader literature).
These two processes—the weakening and strengthening of selection via culture—must interact over evolutionary time. When genetic variation is accommodated by cultural “silencing,” this allows selection to be more efficient when the context changes (perhaps due to culture itself) and these variants become useful. The maintenance of genetic variation is a resource for populations, and so even when culture is reducing natural selection on short time scales, it may be cultivating our futures.
Now my second point: There is a strong thesis lurking in this topic that human nature is in fact socially constructed. If people believe something, even if false, those beliefs can reconstruct the facts, over social and evolutionary time scales. This means it shouldn’t be possible to make a complete account of human biology without accounting for socially constructed beliefs, because beliefs exert tremendous effects on the fates of genes. If most North Americans believe in the category “black” and treat “blacks” differently, then this will have tremendous material effects on “blacks,” even though the biological nature of this category is famously specious. Various versions of this kind of hypothesis have been explored recently, most notably Philip Kitcher’s (1999) speculation about the possible social construction of real genetic differences and Cochran et al’s (2006) argument that social identity processes lead Ashkenazi Jews to be selected for particular mental traits, that their intelligence is really socially constructed. Whatever one makes of these particular cases, the possibility is great that the demography and behavior of human populations is always interacting with socially constructed categories like “gender” and “race” and “IQ.” The most cogent discussion of this particular possibility that I know is by Mallon (2008).
The influence of culture is so thorough that human nature can only be understood as culturally evolved. One can no more label some portion of human nature “cultural” than one can say some portion of a sexual species is “sexual.” Sexual reproduction changes how evolution proceeds, such that accounting for evolutionary change in sexual species is different than in a-sexual ones. Likewise, a highly cultural species like ourselves is fully cultural as well as fully biological. We socially construct our natures and thereby continually remake our images of ourselves, as well as our genomes.
References
Cochran, G., J. Hardy, et al. (2006). “Natural History of Ashkenazi Intelligence.” Journal of Biosocial Science 38: 659–693.
Gerhart, J. and M. Kirschner. (2007). The theory of facilitated variation. PNAS 104(S1):8582-8589.
Kirschner, M. and J. Gerhart. (1998). Evolvability. PNAS 95(15):8420-8427.
Kitcher, P. (1999). “Race, Ethnicity, Biology, Culture”. In Racism. ed. L. Harris. New York, Humanity Books: 87–120.
Mallon, Ron, “Naturalistic Approaches to Social Construction”, The Stanford Encyclopedia of Philosophy (Winter 2008 Edition), Edward N. Zalta (ed.), URL = .
Mark Stoneking highlights growing (but still incomplete) evidence that human evolution is ongoing, and sometimes driven by culture, an exciting suggestion that further complicates any simple division between biological and cultural causes of human traits. When one looks at the striking evidence of “numerous signals of selection in the patterns of genome-wide genetic variation in human populations,” one can’t help but wonder what all these genes might be being selected for, a question for which we have few good answers at the moment.
I agree with Stoneking that the case of lactose tolerance reveals something very interesting. There culture modifies the standing resources of the environment in a way to benefit those that can take advantage of it. And much of the rest of the phenomena he is referring to may represent population encounters with other foods, as well as toxins and diseases. These resources and hazards are plausible suggestions because they perdure, enabling them to drive selection over many generations.
These resources and hazards are what we might think of as “basic level” in that a sustained encounter with them will generate selective pressure, and culture can play a role in altering the probability of the sustained encounter. A different sort of resource is more thoroughly cultural: for example having a good job or a large amount of money with which one can purchase goods. These latter “symbolic” resources are beneficial only insofar as there is a society that will convert them into more basic level goods.
I mention this distinction between basic level and symbolic level goods and hazards because an intriguing question in the context of culture-driven selection (though much further along towards simple conjecture than anything Stoneking suggests) is to what extent ongoing selection is driven, in part, by access to symbolic as opposed to basic level goods and hazards.
Cochran, Hardy, and Harpending (2006) have offered the surprising argument that cultural practices of ethnic/racial classification and professional discrimination against Jews in the 9th to the 17th century in Eastern Europe created selective pressure for higher IQs among Ashkenazi Jews but also certain genetic diseases (e.g. Tay-Sachs) that are linked to high IQ. Specifically, they hypothesize that discrimination limited the available employment for Jews to certain professions that themselves required high IQs. In effect, the authors argue that culture altered the environment – altered the symbolic resources – of an entire human subpopulation that was largely reproductively isolated for a sustained period in ways that drove selection. I don’t know if this specific claim is right, but it does point to questions for cultural selection that promise to expand its scope. Questions like: has symbolic culture remained stable enough over long periods of time to represent a selective force? If so, where and when? The circumstances of cultural stability surrounding Cochran et al.’s case are such that they will not ubiquitously generalize. But they do lead us to consider what range of cultural circumstances might be relevantly similar.
My own guess (and it is just a guess) is that culture-driven selection can only generalize so far. This is because human symbolic culture is, famously, highly variable in a way that makes any evolutionary problems it poses into moving targets for the slow work of natural selection. Indeed, (and this is quite conjectural) it seems that in the last few millennia cultural change has been remarkably rapid. If that is right, the situation that Cochran et al. make reference to may be anomalously stable.
Here is another conjecture: where there is culture-driven selection, we might expect that the cultural elements in question are themselves stabilized by standing psychological biases of the sort suggested in recent evolutionary psychology and anthropology – for example, biases to think about human groups in an essentialized way (e.g. Hirschfeld 1996, Gil-White 2001; Machery and Faucher 2005). Where these biases are present are operating they may stabilize culture allowing culture to drive selection over a sustained period.
Where culture is more variable, however, the moral looks to be that when we talk about culture driving evolution, symbolic resources like good jobs probably do not play as large role in ongoing human evolution as compared to ongoing encounters with new basic level resources and hazards like food, toxins and disease.
References:
Cochran, G., J. Hardy, et al. (2006). “Natural History of Ashkenazi Intelligence.” Journal of Biosocial Science 38: 659–693.
Gil-White, F. (2001). “Are Ethnic Groups Biological ‘Species’ to the Human Brain?” Current Anthropology 42(4): 515-554.
Hirschfeld, L. A. (1996). Race in the Making: Cognition, Culture, and the Child’s Construction of Human Kinds. Cambridge, MA, MIT Press.
Machery, E. and L. Faucher (2005). “Social Construction and the Concept of Race.” Philosophy of Science 72: 1208–1219.
This conversation, while ending here, continues on Facebook. Join us there by logging on to your Facebook account and proceeding to our group: http://bit.ly/OnTheHumanFacebook.