One of the most annoying aspects of talking about human evolution is the rather misguided idea that cultural evolutionary processes operate in a zero-sum environment in relation to biological evolutionary processes. The colloquial rendering of this idea is that because humans are a highly cultural plastic species, we are “beyond” biological evolution. Many researchers though suspect that on the contrary, because of cultural variation and plasticity we may be buffeted by even greater evolutionary pressures than is the norm for a relatively slow-breeding species with a small effective population size. Probably the best example of this is the ability of adults in several human populations to digest lactose sugar. This is, to not put too fine a point on it, a freak ability. Why would a mammal need to digest milk sugar as an adult after all? Well, you know why, the human mammal is wont to consume the milk of other mammals, which it has taken into bondage. Viewed from the outside the whole process is rather weird and Frankenstein-like, but we’ve been habituated to the normalcy of this sort of thing because of the diversity of cultural forms on evidence in H. sapiens (though in some societies the initial exposure to the fact that Europeans, for example, consume milk and milk products into adulthood was perceived to be highly strange).
A new paper in PNAS implicitly makes this point, Cultural diversification promotes rapid phenotypic evolution in Xavánte Indians. To be sure, I think it does set up some strawmen as well. For example, the authors suggest that their results depart “from the classic view that human evolution is the sole result of adaptation to the external environment.” “Classic” is the wrong word. Outmoded is perhaps better. I doubt any evolutionary minded anthropologist would espouse this viewpoint. Rather, the idea that culture drives evolution is I believe a null hypothesis (this may not be the case for cultural anthropologists). In other words, this paper supports and adds detail to our prior expectations, it does not shift a paradigm.
All that being said, what did they do? The authors used a set of variables amongst groups of indigenous Amazonian populations, and analyzed how the variables related to each other. In particular, they found that one tribe seems to have undergone a great deal of phenotypic divergence from a genetically and linguistically related population (last common ancestors ~1,500 years B.P.). The phenotypic variables were head circumference, facial height, nasal height, nasal breadth, and glabello-occipital length. They also constructed a phylogeny using mtDNA, and related that and the phenotype to geography, and climatic 6 × 6 distance matrix. One assumes that variables like phylogeny, geography and climate should be robust predictors of phenotypic divergence (i.e., in a random drift model phenotypic divergence would be proportional to genetic distance).
The primary descriptive result in illustrated to the left. The Xavánte are outliers in both genotype and phenotype. But, they do cluster with the Kayapó on genotype. The phenotypic and genotypic pattern simply does not align. Why? One rationale would be local adaptation, which drives between group divergence out of sync with total genome genetic distance. But recall that the authors attempted to take into account these particular exogenous variables into their model. In other words, the phenotypic distance can not be explained by variation in genetic distance, or conventional exogenous variables such as geography and climate. By a process of elimination one is then left with the position that endogenous cultural factors are driving the phenotypic separation of the outgroup.
First, how plausible are these results? I have little to say about the geographic, climatic, or phenotypic variables. But, as the authors observe mtDNA is a single locus. That’s the only genetic data they had, but it may not be very reflective of the average phylogeny when you draw at random from the broader genome, which would be a much better reflection of population genetic history. One can easily imagine this sort of study being subject to false positive bias. Many researchers have databases of mtDNA genetic distance, as well as other variables, and the only ones which get published are those which show the statistically significant deviation noted above. So replication of the same sort of result in other populations is essential when it comes to lending credit to the plausible model of culture-driven evolution.
A bigger issue for me is the theoretical assumption that between society gene flow will rapidly eliminate differences sans very strong cultural pressures. Hostile neighbors still tend to exchange genes (e.g., kidnapping of women for brides, or slaves which are eventually assimilated into the enslaving tribe). Only a small amount of gene flow is necessary to prevent the accumulation of group-level differences. So you need strong between group selection to maintain those differences. In contrast, cultural differences can easily manifest in large between group variation, and little within group variation. An accent is the most obvious illustration. A tribe can easily have a distinctive accent which immediately separates it from its neighbors, and only manifests modest within group variation (e.g., along generational lines). The model posited here is that these between group cultural differences are powerful enough to driven biological differences. Are they? I am not sure that they are at this fine a scale, but am open to the proposition.
What we need are cultural forms which are resistant to stochastic forces. In other words, something which is not a fad or fashion, but will be maintained for generations. In a literate society one can imagine such a thing (e.g., Jewish circumcision has persisted over 2,000 years, while the Zulu only gave up the practice during the time of Shaka). But what about pre-literate societies? I’m not so sure. On the other hand, I also expect that between group differences and hostilities are greater amongst pre-literate groups, so that works in favor of the model (societies characterized by literate elites and elaborated ideologies generally have systems and justifications for assimilation and absorption of outsiders in a coherent and systematic manner; those without may not, though often they do as well). In the final sum: more study needed!
Citation: Cultural diversification promotes rapid phenotypic evolution in Xavánte Indians, doi:10.1073/pnas.1118967109
Image credit: Wikipedia
Razib Khan’s degrees are in biochemistry and biology. He has blogged about genetics since 2002, previously worked in software development, is an Unz Foundation Junior Fellow and lives in the western US. He loves habaneros.
December 20th, 2011 at 3:11 pm
Your piece reminded me of the recent paper in PNAS looking at the effect of diet (hunter gathering vs farming) on mandibular shape and eventually tooth crowding. This is bound to create new evolutionary pressures – maybe leading to loss of wisdom teeth – due to more frequent tooth decay or ulcerations. (see paper at http://www.pnas.org/content/108/49/19546) Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies, PNAS, Published online before print November 21, 2011, doi: 10.1073/pnas.1113050108
From Wikipedia: “About 35% of the population do not develop wisdom teeth at all” and there is quite a bit of population variation worth exploring: “Agenesis of wisdom teeth in human populations ranges from practically zero in Tasmanian Aborigines to nearly 100% in indigenous Mexicans.[20] The difference is related to the PAX9 gene (and perhaps other genes).”
December 20th, 2011 at 5:28 pm
For what it’s worth as an anecdote, my upper wisdom tooth acted as a spare when I had the rear molar right in front of it extracted (due to an large crack that would have been tough to repair.) Dropped neatly into place within a year. So we’ve still got a lot of mystery – and possibly evolution – to go in regards to wisdom teeth.
December 20th, 2011 at 6:12 pm
Got all 4 wisdom teeth still, at age 50. They just act as normal molars. Probably luck in my case, but I suspect spare teeth are the function of wisdom teeth. Is bad wisdom teeth an agricultural thing, or do H/G populations also have problems? Another cultural effect if wisdom tooth trouble runs with diet.
December 20th, 2011 at 7:32 pm
It is worth recalling that these populations are quite small in absolute numbers, that the founding populations ca. 1500 years ago would have been even smaller, and that there was probably a major population contraction that could have given rise to lose genotypes in between ca. 200-500 years ago due to Columbian contact related pressures of one sort or another (not necessarily direct or demic).
It is much easier to get quirky results when you are dealing with small numbers of people than it is when you are dealing with large numbers of people.
December 21st, 2011 at 7:06 am
What sort of selection would this be called? I think it should be differentiated from natural and sexual selection. I think it should be called ‘group selection’ in that a population evolves in relation to its group structure and culture. I think this meaning is grammatically consistent with ‘natural selection’ and ‘sexual selection.,’ whereas ‘group selection’ with ‘group’ being the object of selection isn’t. After all, ‘natural’ selection doesn’t mean nature is being selected. It means an organism evolves in relation to it’s natural environment. In this case, a population is evolving in relation to it’s cultural environment.
December 21st, 2011 at 8:55 am
I’ll preface my skepticism by stating that group environments are unquestionably selection factors (sexual selection being a reduced and special case of this). Also, I have not read the paper carefully.
1. First, it seems they provide no positive evidence for selection by culture, they only claim to eliminate other factors. For example, they picked a few environmental/climatic/social factors, which were not significant in their model. This is nowhere near sufficient to argue that the phenotypic differences aren’t environmental — they have no idea what environmental factors might influence these traits, which appear to have been chosen because someone happened to measure them in the 1970s. Because these are skull morphological traits, I would think you would be interested in the physiological/chemical/dietary environment of pregnant women, birth practices, infant handling/feeding rather than the traditional preoccupations of anthropologists (kinship structure, marriage practices).
2. I’m suspicious that gene flow could be so low between these groups as to allow such rapid divergence.
3. At one point, they seem to claim seems to be that it’s drift made possible by cultural barriers to reproduction. That’s a nice hypothesis with no evidence in this paper. It’s also not selection, yet they seem to want to have it both ways.
4. mtDNA: fool me once, shame on you; fool me regularly for decades…
Steve C, that’s a terrible idea.
December 21st, 2011 at 12:23 pm
Okay, then if it turns out that cultural differences do present different selective pressures, what would you call it? Or what we you call the general line of inquiry into whether or not it even exists?
December 21st, 2011 at 1:01 pm
would, not we
December 21st, 2011 at 2:03 pm
It is a type of ecological selection. I’m not sure why it would need a separate category. The behavior of conspecifics is a selective pressure for most if not all animals; culture is a product of human behavior. Giving it a special designation just gives credence the bogus idea about human exceptionalism that Razib dismisses in the beginning of his post — that some other process or exemption can be invoked in the case of humans. The power of variation + natural selection as an evolutionary model is that it is generalizable across a wide range of parameters. We needn’t muddy the waters with extraneous terms.
December 22nd, 2011 at 9:30 am
The application of the concept of cultural influence on human evolution to the hybridization of Neanderthals and Sapiens could be quite interesting. Perhaps there are some phenotypic aspects of Neanderthals that we just can’t, won’t and don’t stop perhaps because they are culturally advantageous.
December 22nd, 2011 at 12:50 pm
Miko, it’s pretty much accepted that things like primate intelligence or human’s ‘theory of mind’ resulted from selection due to primates and humans living in complex social groups,not say as adaptations to their natural environment. I would say that making that distinction clarifies things rather than muddies them.
Also, if humans aren’t qualitatively different from other animals, i.e. human exceptionalism, then we are in some important respects quantitatively different. No other animal has extended evidence of their existence a light century from Earth, consumes energy and resources like humans or have build complex societies and economies to the scale that humans have. This type of exceptionalism is pretty obvious and it’s worthwhile to look at what forces created these abilities. Recognizing that these things resulted from humans living in complex groups allows us to understand these processes better than just calling it an ‘ecological’ adaptation. I think social group selection is sufficiently different than natural environment selection to warrant its distinction.
I’ve been following the debate for years and it still seems pretty confused, thus my modest attempt to clarify things by pointing out this confusion results partially from grammatical inconsistency.
December 24th, 2011 at 10:27 am
“What sort of selection would this be called? I think it should be differentiated from natural and sexual selection. I think it should be called ‘group selection’ in that a population evolves in relation to its group structure and culture.”
“Cultural selection” fits better imo as it’s the culture acting on the individual to select for individual traits. “Group selection” would more suit the process of how culture is selected for by the environment e.g. late marrying or early marrying, subsistence monogamy or surplus polygamy etc. Once the environment has selected aspects of the culture then the culture goes to work on individuals.