50 whys to look for genes: 42. Depict human genealogical and genetic relationships

Using genome-wide variation it is possible to depict the variation among humans in ways that indicate the divergence from a common ancestral group. The following diagram comes from the work of Tishkoff and collaborators on genetic variation among humans in and out of Africa.

(Source: Michael C. Campbell1 and Sarah A. Tishkoff, 2010, “The Evolution of Human Genetic and Phenotypic Variation in Africa,” Current Biology 20, R166–R173)

The diagram shows that there are many more branches leading from the ancestral within-Africa human population to current African groups than there are to groups in the rest of the world, that is, to groups derived from people who migrated out of Africa at some point after 100,000 years ago.  This branching pattern suggests that, if we were to divide human genetic diversity into a small number of groups of similar diversity, say, five groups, then most of these groups would be African.  (Indeed, four would be from Africa and the fifth would be a combination of an East African group and the non-African groups.)  This finding seems to discredit the genetic reality of traditional races, where “traditional” has varied greatly but in general separates Africans, Asian, Australian, and Europeans or “Caucasians”.

Complications
1. The diagram places the non-Africans out on the right, taking up 40% of the horizontal scale, and colored lighter, all suggesting that the rest of the groups can still be lumped together.  Would someone who promotes (or promoted) a traditional racial classification would be at all troubled by learning that their African race was really a collection of 13-14 races?  If they had the idea that something special genetically happened in the branch that left Africa, then those groups left in Africa are united in lacking that something.

2. Can any depiction of genetic relationships among humans allow simultaneously for similarity, diversity, ancestry, and admixture (i.e., groups that had split mixing again)?

3. Our familiarity with phylogenetic trees invites us to think—even if subconsciously—about human genetic ancestry as if the branches are like separate species. This resonates with a long history of scientific arguments that human races are separate species, or that the branches of the human tree achieved human status at different rates. We might ask, therefore, whether the very methodology of generating and depicting human ancestry privileges a racialized view of human diversity?

——
This post is adapted from the first of a series of previous posts exploring depictions of human genetic relationships.

(Introduction to this series of posts)

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