Tag Archives: human

Depictions of human genetic relationships: Exploration 8

Exploration 8: Deeper messages in the conventional ancestral tree of human groups

The original Tishkoff diagram of human ancestry is certainly easier to read than the reticulating web of exploration 6, let alone the web overlaid with aprons in exploration 7.  We could try to remedy this by helping audiences to become familiar with the graphic conventions and by using technology like the slide show to display the branching and replacement of ancestral aprons with those of their descendants.  In this concluding post of the series I argue that it is important for all to work on being able to read reticulating webs because of an undesirable message built into the simpler branching diagram.

To expose this message, consider the horizontal links in the Tishkoff diagram, which represent gene flow between branches, that is, admixture.  The branching pattern can be extracted from the genetic data only because these flows are not so large as to obscure the genetic mutations or other differences that arose over time after each branching.  Indeed, to ensure that this is the case, some studies of human genetic variation involve data from the special subset of people who live in the same place as, say, all their great-grandparents.  The reticulating web with aprons likewise relies on a branching pattern that can be discerned despite the potentially confounding effects of gene flow.  Still, the aprons remind us of variation around the mid-point of each group—variation that may well have been enlarged by gene flow.

Now, there are some branching patterns that are subject to minimal or no gene flow, namely branching of species or higher taxa (taxonomic groups) from ancestral taxa.   We are all familiar with such evolutionary trees.  The first example is for the classes of vertebrates; the second is for liverwort species.


Source: http://www.biology.duke.edu/bryology/LiToL/LeafyII.html

Our familiarity with these trees invites us to think—even if subconsciously—about human genetic ancestry as if the branches are like separate species.  There is 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.  As Desmond and Moore (2009) have shown in Darwin’s sacred cause: How a hatred of slavery shaped Darwin’s views on human evolution, the debate was especially heated during Darwin’s adult life.  Darwin’s view of descent from a single common ancestor was a minority view, discredited to some extent by its association with literal interpretation of the bible’s account of Adam and Eve, but more so by its association with anti-slavery movements.  Yet, the debate did not disappear with the 19th century.  Carleton Coon, a physical anthropologist who died in 1981 after a long career as a professor at Harvard and University of Pennsylvania, wrote in 1962 that Homo erectus evolved into Homo sapiens five separate times “as each subspecies, living in its own territory, passed a critical threshold from a more brutal to a more sapient state”.  (http://en.wikipedia.org/wiki/Carleton_S._Coon#Polygenism)  The multiregional hypothesis is a more recent variant.

Ideas about multiple origins for humans are not the only way that biology can be invoked to explain or even justify a hierarchy of human races.  However, to the extent that we want to distance ourselves from such views, it can only help to do the work to depict genetic relationships among humans in ways that allow simultaneously for similarity, diversity, and admixture at the same time as we depict ancestry.

Depictions of human genetic relationships: Exploration 7

Exploration 7: Superimposing genetic variation on the ancestry diagram from a simulation

The following picture comes from the same random simulation used in the previous post to generate directions of branching and the distances of each branch from its most recent common ancestor.  The two dimensions stand for the genetic variation of the whole set of populations.  This time aprons are drawn around the midpoints of the groups A to R at the bottom of the ancestry tree (but not around their ancestors).  This shows that the variation of the original population (which would extend about 20% past the largest circle) is reduced after the branchings that have brought us to the present, but there is still great overlap between most groups.  In particular, the descendants A and B of the group AB, which branched off early, shows variation that subsumes that in the the rest of the groups.

A careful viewer might notice, however, that there are some circles that do not overlap at all, as if to say these groups share no genetic variation.  This is an artifact of my deciding to reduce the variation at each branching enough so that not all the circles would extend beyond the web.  In doing so, I realized that I was increasing the ratio of between groups to average within-group variation well beyond what we find in the actual human world.

Depictions of human genetic relationships: Exploration 1

Exploration 1: Rearranging the horizontal sequence of a tree diagram

(Continuing from the previous post, we consider alternative depictions of human genetic variation keeping in mind the question, “Can any depiction of genetic relationships among humans allow simultaneously for similarity, diversity, ancestry, and admixture?”)

The diagram of human ancestry from Tishkoff and collaborators branches out like an upside-down tree from a common ancestral group into 18 groups today.  (The diagram shows some cross-links that indicate gene-flow between populations, but we will ignore these for the time being.)

(Source: Michael C. Campbell1 and Sarah A. Tishkoff, 2010, “The Evolution of Human Genetic and Phenotypic Variation in Africa,” Current Biology 20, R166–R173.  Letters at the bottom added for the purposes of referring to the groups in this series of blog posts.)

Below we see the tree for the first three forks, where AR is short for a group that includes all the ancestors of groups A through R; NR for all the ancestors of groups N through R; etc.

Now, the branches at any fork can be flipped so the next diagram conveys the same information about ancestry and branching.

Notice that the second variant does not convey the impression that the branch that in ancestral to the non-Africans, i.e., NR, is more different from the branches ancestral to the African groups, i.e., AB, CC, DM, than these branches are from each other.  Although the lineage that ended up at CC (the ancestor of group C) branched off earlier than the lineage leading to NR, there is nothing in the ancestry diagram that says it should be more similar genetically to AB than to NR.

If we exclude diagrams with crossing over of branches, such as the one below, there are four distinct reorderings of the four branches that preserve the sequence of the branchings.  There are 2 to the power 16 = 65536 reorderings of the full set of the 18 current groups.  The point is not that we need to find one correct ordering from among such a large set.  The lesson is that no lessons should be drawn from the order along the bottom of a branching diagram that is not already contained in the sequence of branches above.  (In this light, diagrams with crossing over should be excluded because they suggest that the two branches at a fork are further away from each other than to one of the earlier branches, which goes against the information contained in the sequence of branches.)

It is not easy, however, to convince one’s brain not to give significance to these horizontal positions.  This cognitive weakness gives rise to the explorations in the next posts.