50 whys to look for genes: 39. Assign humans to groups that arose after dispersal from Africa

Population geneticist, Richard Lewontin “found that the majority of the total genetic variation between humans (i.e., of the 0.1% of DNA that varies between individuals), 85.4%, is found within populations, 8.3% of the variation is found between populations within a ‘race’, and only 6.3% was found to account for the racial classification. Numerous later studies have confirmed his findings” (wikipedia). Critics of Lewontin,

including philosopher Sesardic (2010), have observed that the fact that variation within a group is of larger than variation between (the average of) the groups, does not mean that the groups cannot be distinguished .

Let me affirm this last point with an example from a course I once took in multivariate statistic. We could not say with confidence whether a student was male and female on the basis of their height—there was too much overlap of the ranges—or, for the same reason, on the basis of their hip circumference. Yet a simple linear function that subtracted hip from height was very reliable in discriminating male from female students. In Sesardic´s figure, rotated 90 degrees below, height would be the x-axis, hip the y-axis; the squares the males, the triangles the females.


This point is not, however, sufficient to rehabilitate a biological picture of race.

Suppose we imagine an original human gene pool that dispersed at some point of time from its origins in Africa around the world and was not subject to subsequent breeding among widely dispersed parts of the pool. Cluster analysis techniques could be used on genetic data to divide humans into, say, N groups. Such clustering techniques are sensitive to assumptions that determine whether groups are of roughly equal size or are a mix of a few large groups. If we looked for groups that had similar within-group genetic variation, most of the N groups would be in Africa. In other words, the traditional subdivisions of human races would have to be reformulated.

Of course

there has been considerable migration and cross-breeding subsequent to the initial dispersal from the place of human origin in Africa, including but not confined to the recent centuries of cross-Atlantic slavery and master-slave relations. How well could we recover from current individuals the one or more groups (as delineated [above]) that make up the individuals´ ancestry? [T]his is an empirical question. Biomedical researchers [might well] judge that research efforts might be more fruitfully directed along other avenues, such as those indicated by biomedical correlates of socially defined race (i.e., not the groups that would emerge from the cluster analyses….

Even if we were to put aside the issues above and

imagine a world in which we were able to use genetic information to assign humans to original post-dispersal groups as reliably as in the statistics class we were able to assign individuals to male and female groups. What could we do with that knowledge that there is a difference between the average genetic profiles for groups A and B when there is large within-group variation for most genetic loci (at least, for those that vary within the human species)?

Let me accentuate this question with using the IQ test score case Sesardic has paid considerable attention to (2005). Suppose we knew (which we do not) that only a certain small set of genes influenced IQ test scores. What could we do with the knowledge that there is a large difference between the average IQ test score for two groups and this difference is smaller than the within-group variation? (To visualize this situation, imagine one of the axes in the Figure is IQ test score.) [Who would use the] ability to assign humans to original post-dispersal groups based on genetic profiles as grounds for using an individual´s membership in a group to make educational or employment decisions for the individual?


The quotes are from a previous pair of posts, which became Taylor, P. J. (2011) “Rehabilitating a biological notion of race? A response to Sesardic,” Biology and Philosophy, 26 (3):469-473.

Lewontin, R. C. (1972). “The Apportionment of Human Diversity”. “Evolutionary Biology”. pp. 381–398. doi:10.1007/978-1-4684-9063-3_14.

Sesardic N (2005) Making Sense of Heritability. Cambridge University Press, Cambridge.

Sesardic N (2010) Race: A Social Destruction of a Biological Concept.  Biology and Philosophy 25:143-162.


(Introduction to this series of posts)


One thought on “50 whys to look for genes: 39. Assign humans to groups that arose after dispersal from Africa

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s