Razib Khan One-stop-shopping for all of my content

December 20, 2011

Culture evolves our bodies!

Filed under: Anthroplogy,Gene-Culture Coevolution,Human Evolution — Razib Khan @ 2:46 pm

Human cultural diversity

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

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