Geographic and genetic distance

In the previous post, we saw how Cavalli Sforza was criticizing the concept of “racial purity”. He then spends some time in his book to clarify the concept of “genetic distance” between populations. C. Sforza describes the “genetic distance” as a difference between percentage frequencies of the form of a gene.

In order to contrast the populations, there is a need to synthetize a vast amount of genetic information, in other words it is essential to average the distance between two populations over many genes if one wants reproducible conclusions. But, it is worth noting that most of the formulas currently used to calculate genetic distances provide very similar results overall.

The main conclusion of the various studies is that the genetic distance between two populations generally increases in direct correlation with geographic distance separating them. This derives from the observation that while most spouses are selected from within their own villages or town, or part of a city, a small proportion is chosen from neighbouring ones.

But, while the increase of genetic distance with geographic distance may be linear at first, over a greater geographic distance, the increase in genetic distance slows sharply. The two characteristics of the curve – the rate (or slope) of the initial increase, and the maximal value reached by the genetic distance over a great geographic distance- are different in every continent. However, studies show that they are slightest in Europe, which is the most homogeneous continent. The maximal genetic distance within Europe is three times smaller the on the least homogeneous continents. Despite political fragmentation, migration within Europe has been sufficient to create a greater genetic homogeneity than elsewhere.

What is a race, then?

A race is a group of individuals that could be recognized as biologically different from others. To be scientifically “recognized” the differences between a population that we would like to call a race and neighbouring populations must be statistically significant according to some defined criteria.

C. Sforza’s experiments have shown that even neighbouring populations (villages or towns) can often be quite different from each other. But, classifying the world’s population into several thousands or a million different “races” would of course be completely impractical. So, what level of genetic divergence would be necessary to determine boundaries for the definition of a “racial difference”? Because genetic divergence increases in a continuous manner, it seems obvious that any definition or threshold would be completely arbitrary. C. Darwin had understood that geographic continuity would frustrate any attempt to classify human races. The US census is itself a proof of the theoretical difficulties of race classification, mixing ethnicity, history and language.

The observation has been made that almost any human group, from a village in the Pyrénées or the Alps to a Pygmy camp in Africa, displays almost the same average distance between individuals: any small village typically contains about the same amount of genetic variation as another village located on any other continent. Regardless of the type of genetic markers used (selected from a very wide range), the variation between two random individuals within any one population is 85% as large as that between two individuals randomly selected from the world’s population.

It seems therefore wise to C. Sforza to abandon any attempt at racial classification along the traditional lines. But he then points out that from a genetic point of view, the most meaningful social group is the one in which one is likely to find a spouse. This social group has, for anthropologists an average or “ideal size” of more or less five hundred individuals. The geneticist looks for its part at groups of 5000 to 500.000 individuals. Within this group, people would have greater genetic similarity than two random individuals because they would share significantly more ancestry, even more if the group has been defined on the basis of endogamous behavior (tendency to marry within the group).