Written by: Stephen Hsu
Primary Source: Information Processing
I wrote this to help a journalist who is trying to understand the current controversy over A Troubled Inheritance, the new book by NYTimes genetics correspondent Nicholas Wade. (Link above goes to earlier discussion on this blog, with additional useful links and figures.)
The anthropologist Ashley Montagu advanced the idea that race is a social construct rather than a biological reality. For Wade, Montagu is a foil against which to benchmark recent advances in human genomics.
Wade: … So I decided that I would write a book that explained what we know about race and what the consequences might be, and I think Montagu made a terrible mistake, though I share his motives.
Note the discussion below avoids using the term “race” and focuses instead on groups of humans that share ancestry. The degree of sharing can now be directly measured through genotypes.
What’s New Since Montagu?
Two modern humans differ at about 1 in 1000 loci (out of ~ 3 billion in the human genome). There are a few million differences between any two individuals across their entire genome.
A common argument is that 99.9 percent genetic similarity cannot leave room for “consequential” differences. But modern humans and Neanderthals are almost as similar (~ 99.8 percent; we have high accuracy sequences now for Neanderthals), and there are significant differences between us and them: both physical and cognitive. However, because humans and Neanderthals are known to have interbred, we are still part of the same species. (Would it be fair to refer to them as a separate “race”? Is the modern-Neanderthal difference merely a social construct?) Furthermore, this 0.1 percent genetic variation accounts for human diversity encompassing Confucious, Einstein, Shaq and Shakespeare.
Genetic variation is patterned — two individuals who trace their ancestry to the same geographical region (e.g., two Japanese) will have about 15 percent fewer total differences between them than if we were to compare individuals from widely separated ancestries (e.g., a Nigerian and a Japanese). Gene variants (alleles) which are common in one population (e.g., 90 percent of Japanese have version A) can be rare in another (e.g., only 20 percent of Nigerians have version A). Differences in allele frequencies are correlated across populations. From these correlations one can easily identify a genome (or even a small chunk of DNA as long as it includes many alleles) as belonging to a particular ancestral group. To oversimplify: one simply asks whether the DNA chunk in question has mostly the variants that are common in one group as opposed to another. Even if the differences in allele frequency are small — e.g., allele X is 62 percent likely in Japanese, versus 57 percent likely in Nigerians — once we consider thousands of such alleles the statistical signal becomes apparent. Each individual (or chunk of DNA) can be associated with a particular ancestral group.
Is this genetic difference consequential? Does it make two Nigerians more similar, on average, to each other than to a random European? Obviously, on some superficial phenotypes such as skin color or nose shape, the answer is yes.
But what about more complicated traits, such as height or cognitive ability or personality? All of these are known to be significantly heritable, through twin and adoption studies, as well as more modern methods.
We can’t answer the question without understanding the specific genetic architecture of the trait. For example, are alleles that slightly increase height more common in one group than another? We need to know exactly which alleles affect height… But this is challenging as the traits I listed are almost certainly controlled by hundreds or thousands of genes. Could population averages on these traits differ between groups, due to differences in allele frequencies? I know of no argument, taking into account the information above, showing that they could not.
In fact, in the case of height we are close to answering the question. We have identified hundreds of loci correlated to height. Detailed analysis suggests that the difference in average height between N and S Europeans (about one population SD, or a couple of inches) is partially genetic (N Europeans, on average, have a larger number of height increasing alleles than S Europeans), due to different selection pressures that the populations experienced in the recent past (i.e., past 10k years).
Many who argue on Montagu’s side hold the prior belief that the ~ 50k years of isolation between continental populations is not enough time for differential selection to produce group differences, particularly in complex traits governed by many loci. This is of course a quantitative question depending on strength of selection in different environments. The new results on height should cause them to reconsider their priors.
It is fair to say that results on height, as well as on simpler traits such as lactose or altitude tolerance, are consistent with Wade’s theme that evolution has been recent, copious, and regional.
Further extrapolation to behavioral and cognitive traits will require more data, but:
1) The question is scientific — it can be answered with known methods. (I estimate of order millions of genotype-phenotype pairs will allow us to extract the genetic architecture of complex traits like cognitive ability — perhaps sometime in the coming decade.)
2) There is no a priori argument, given what we currently know, that such differences cannot exist. (Cf. Neanderthals!) Note this is NOT an argument that differences exist — merely that they might, and that we cannot exclude the possibility.
An honest Ashley Montagu would have to concede points 1 and 2 above.
The second part of A Troublesome Inheritance covers controversial topics such as genetic group differences in behavioral and cognitive predispositions, and their societal implications. Wade is mostly careful to present these as speculative hypotheses, but nevertheless his advocacy leaves him vulnerable to easy attack. What I have summarized above are the incontestable (albeit, in some circles, perhaps still controversial and poorly understood) new results that have accumulated through the last decade of genomic research.