Wednesday, November 4, 2009

Is Race Genetic? Part II - Lewontin's Fallacy

Part I here

First, a quick reminder of the last post: Lewontin's study statistical analysis involved looking at the genetic diversity of 17 different loci and he concluded that most of the genetic diversity was due to individuals being individuals, not because of any racial grouping. Therefore, racial groupings are obsolete.

In the 2003 paper Human genetic diversity: Lewontin's fallacy1, A. W. F. Edwards wrote:

It is not true that "racial classification is... of virtually no genetic or taxonomic significance". It is not true, as Nature claimed, that "two random individuals from any one group are almost as different as any two random individuals from the entire world", and it is not true, as the New Scientist claimed, that "two individuals are different because they are individuals, not because they belong to different races" and that "you can't predict someone's race by their genes".

Them's fightin' words! Let's see what guns he brings to the duel.

Edwards wrote, "There is nothing wrong with Lewontin's statistical analysis of variation, only with the belief that it is relevant to classification." In other words, he doesn't dispute Lewontin's findings but rather how they should be interpreted. Edwards claims that, while diversity can be important, it's the correlations between the genes that differentiates races, not the diversity. If you correlate multiple genes, it becomes possible to reliably determine to which population a person belongs, and those populations can correlate with what we call race. In other words, you really could look at someone's genes and figure out their race.

Let's make up a simple example. In Population A, half the people have blue hair while the other half have green.* In Population B, this is true as well. This means that if you've got someone standing in front of you with green hair, you can't tell to which population he belongs. Now, let's add in eye colour (red and purple). Half the people of Population A have red eyes while the other half have purple eyes. This also holds true for Population B. To which population does a red-eyed, green-haired person belong? Based on Lewontin's analysis, you wouldn't be able tell. Edwards points out that you can look how eye colour and hair colour correlate with each other. If red eyes and green hair tended to correlate together in Population A while purple eyes and green hair correlate in Population B, then you could guess that the red-eyed, green-haired person belongs to Population A.

Statistical methods allows you to factor in allele frequencies as well as how alleles correlate with each other. As you add more genes with more alleles into the mix, it becomes increasingly likely that you'll be able to accurately predict to which population a person belongs. Edwards calculates that, using data which simulates Lewontin's level of within/without population diversity and assuming only two different alleles at each locus, you need to correlate about 20 traits before the chance of misclassifying someone into the wrong population becomes negligible (below 1%). Studies using actual data from individuals have shown "genetic affinities that have unsurprising geographic, linguistic and cultural parallels." In other words, researchers could look at a pool of individuals and, by using statistics, cluster the individuals into populations based on various polymorphic loci, and those clusters could correspond to what we call "race."

So, despite the high amount of within population diversity, it looks like there really is a genetic basis for race. But hold on! In 2004, Bamshad et al.2 published a paper using multilocus statistics (that is, the kind of correlation statistics that Edwards advocates) and nearly 400 polymorphic loci that showed that two individuals from different populations could often be more similar than two individuals from the same population. What gives?

Check out Part III - Witherspoon et al. Bring it Together

*I'm picking physical characteristics for this example because they're easily visualized, not because they make good loci.

1. Edwards, AW. 2003. Human genetic diversity: Lewontin's fallacy. BioEssays 25: 798-801.

2. Bamshad, M, Wooding, S, Salisbury, BA and Stephens, JC. 2004. Deconstructing the relationship between genetics and race. Natural Reviews Genetics 5: 598-609.

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