Archive for June, 2014

A Letter to Dr. Michael White

There are debates in linguistics about how to categorize languages and dialects; Nicholas Wade has reignited the debate over how to categorize human populations.

Michael White’s recent article is titled  “Why Your Race Isn’t Genetic,” although at the end of his essay, he writes, “Without natural genetic boundaries to guide us, human racial categories remain a product of our choices. Those choices are not totally arbitrary, biologically meaningless, or without utility.” So, perhaps a better title would have been “Why Your Race Isn’t Only Genetic.”

I recommend you read the article before reading the following letter.

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Dr. White,

In your article, you cite Templeton’s “Biological races in humans,” where Templeton argues that all humans share a common lineage and that races are not sub-species because the five major ‘races’ of humans account for only 4.3% of cross-population human genetic variation—well below the 25% threshold set for sub-species categorization. But, later, Templeton himself writes that “this finding does not mean that all human populations are genetically identical. Past founder events, isolation-by-distance, and other restrictions on gene flow ensure that human populations are genetically differentiated from one another, and local adaptation ensures that some of these differences reflect adaptive evolution to the environmental heterogeneity that our globally distributed species experiences” (9).

So, there are genetic differences between human populations, but the argument offered by you and Templeton is that those differences don’t meet the standard for classifying different human populations as sub-species. I accept this argument completely insofar as “sub-species” is given an objective cut-off point, but it still doesn’t tell us how to classify (or whether we should classify) the differences that do exist between human populations. With that point in mind, here’s my first extended question:

I mentioned dog breeds on Twitter, and it seems that variation across breeds is near 27%, while human genetic variation has been found to be somewhere on the range of 5-10% (Parker et al. 2004), though, as just noted, Templeton puts it at 4.3%.

However, all of these numbers take large swaths of humanity (or dog breed-dom) into consideration. Ostrander and Wayne 2005 note (Figure 2) that within certain clusters of breeds, there is considerably less variation between one breed and the next. Two other papers (Erdogan et al. 2013 and Ye et al. 2009) have shown that cross-breed genetic variation drops well below 25% in certain contexts. Genetic variation between labs and springer spaniels, for example, is set at 0.09.

That dog breeds are the results of artificial breeding is inconsequential for this discussion about categorization. We know a priori that the notion of “breed” in dogs is a valuable classification system. So, if it’s true that among certain breeds, cross-breed variation drops well below 25%, then why isn’t it possible to have such a classification system to describe variation among human populations, which likewise drops below the 25% threshold for sub-species categorization?

My next extended question is related to this idea of variable genetic distance:

You make much of the fact that human populations are fuzzy and not distinct; the reticulating nature of our human family tree makes any kind of intra-human categorization moot:

. . . . But as it turns out, our species’ family history is not so arboreal. Geneticists have methods for measuring the “treeness” of genetic relationships between populations. Templeton found that the genetic relationships between human populations don’t have a very tree-like structure, while chimpanzee populations do. Rather than a family tree with distinct racial branches, humans have a family trellis that lacks clear genetic boundaries between different groups.

But doesn’t the truth of this statement wax and wane depending on which parts of the human family tree you’re talking about? I could be wrong here, so it’s an honest question. There can be fuzzy boundaries in Northern Europe and fuzzy boundaries in Southwestern Africa, but does that mean that the boundary between populations in Northern Europe and populations in Southwestern Africa is equally fuzzy as when comparing within those geographic boundaries? Isn’t this the point of Figure 2 in Templeton’s paper?

There’s a lot of fuzziness between dialectical boundaries in English and dialectical boundaries in Ojibwe, but not nearly as much fuzziness between English and Ojibwe. If boundaries are as universally unclear as you imply here, what’s the use of FST scores, and how is it that scientists manage to know a person’s ancestry down to a small geographic area?

Templeton argues for an isolation-by-distance model of human genetic variation, and I don’t disagree at all. But isolation-by-distance plus small but not negligible amounts of allele frequency variation between populations . . . . sounds a bit like allopatric speciation to me, at least when you’re comparing the far ends of that isolation cline?  But then, I’m a linguist, not a biologist, so I’m willing to be corrected.

Final question:

We can all agree, I think, that there is variation in allele frequencies between human populations, and that geography is a decent proxy for the occurrence of those frequencies. (This is all that I, and most people, mean by “race.”)  Where we disagree is on whether or not that variation is worth codifying with a classification system. Some people think it is; you think otherwise.

Not all human populations are genetically identical, and insofar as some of us think the study of genetic differences in human populations is interesting, we need a word for those differences. If you want to abandon “race,” fine, but what word would you use? Or would you not use any word because you don’t think these differences are meaningful or worth studying?

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