Second, by far the largest amount of that variation, about 85%, is among individuals within local national or linguistic populations, within the French, within the Kikuyu, within the Japanese. There is diversity from population to population in how much genetic variation each contains, depending upon how much immigration into the population has occurred from a variety of other groups and also on the size of the population. The United States, with a very large population whose ancestors came from all over the earth including the original inhabitants of the New World, is genetically very variable whereas small populations of local Amazonian tribes are less genetically variable, although they are by no means genetically uniform. Despite the differences in amount of genetic variation within local populations, the finding that on the average 85% of all human genetic variation is within local populations has been a remarkably consistent result of independent studies carried out over twenty-five years using data from both proteins and DNA.
Of the remaining 15% of human variation, between a quarter and a half is between local populations within classically defined human “races,” between the French and the Ukrainians, between the Kikuyu and the Ewe, between the Japanese and the Koreans. The remaining variation, about 6% to 10% of the total human variation is between the classically defined geographical races that we think of in an everyday sense as identified by skin color, hair form, and nose shape. This imprecision in assigning the proportion of variation assigned to differences among population within ”races” as compared to variation among “races,” arises precisely because there is no objective way to assign the various human populations to clear-cut races. Into which “race” do the Hindi and Urdu speakers of the Indian sub-continent fall? Should they be grouped with Europeans or with Asians or should a separate race be assigned to them? Are the Lapps of Finland and the Hazari of Afghanistan really Europeans or Asians? What about Indonesians and Melanesians? Different biologists have made different assignments and the number of “races” assigned by anthropologists and geneticists has varied from 3 to 30.
Third, a small number of genetic traits, such as skin color, hair form, nose shape (traits for which the genes have not actually been identified) and a relatively few proteins like the Rh blood type, vary together so that many populations with very dark skin color will also have dark tightly curled hair, broad noses and a high frequency of the Rh blood type R0. Those who, like Leroi, argue for the objective reality of racial divisions claim that when such covariation is taken into account, clear-cut racial divisions will appear and that these divisions will correspond largely to the classical division of the world into Whites, Blacks, Yellows, Reds and Browns. It is indeed possible to combine the information from covarying traits into weighted averages that take account of the traits' covariation (technically known as "principal components" of variation). When this has been done, however, the results have not borne out the claims for racial divisions. The geographical maps of principal component values constructed by Cavalli, Menozzi and Piazza in their famous The History and Geography of Human Genes show continuous variation over the whole world with no sharp boundaries and with no greater similarity occurring between Western and Eastern Europeans than between Europeans and Africans! Thus, the classically defined races do not appear from an unprejudiced description of human variation. Only the Australian Aborigines appear as a unique group.
A clustering of populations that does correspond to classical continental "races" can be acheived by using a special class of non-functional DNA, microsatellites. By selecting among microsatellites, it is possible to find a set that will cluster together African populations, European populations, and Asian populations, etc. These selected microsatellite DNA markers are not typical of genes, however, but have been chosen precisely because they are "maximally informative" about group differences. Thus, they tell us what we already knew about the differences between populations of the classical "races" from skin color, face shape, and hair form. They have the added advantage of allowing us to make good estimates of the amount of intermixture that has occurred between populations as a result of migrations and conquests.
The every-day socially defined geographical races do identify groups of populations that are somewhat more closely similar to each other genetically. Most important from the standpoint of the biological meaning of these racial categories, however, most human genetic variation does not show such "race" clustering. For the vast majority of human genetic variations, classical racial categories as defined by a combination of geography, skin color, nose and hair shape, an occasional blood type or selected microsatellites make no useful prediction of genetic differences. This failure of the clustering of local populations into biologically meaningful "races" based on a few clear genetic differences is not confined to the human species. Zoologists long ago gave up the category of "race" for dividing up groups of animal populations within a species, because so many of these races turned out to be based on only one or two genes so that two animals born in the same litter could belong to different "races."
In his article, Leroi is inconsistent and shifting in his notion of race. Sometimes it corresponds to the classical social definitions of major races, but elsewhere he makes “race” coincident with a small local group such as the Negritos or Inuit. In this shifting concept of “race” he goes back to the varying use of the term in the 19th century. Then people spoke of the “Scots race,” “the Irish race” and the “race of Englishmen.” Indeed “race” could stand for a family group defined by male inheritance, as in the description of the last male in a family line as “the last of his race.” This inconsistent usage arises from the fact that there is no clear criterion of how much difference between groups of genetically related individuals should correspond to the category “race.” If it had turned out that groups of related populations were clearly different in the great majority of their genes from other groups, then racial categories would be clear and unambiguous and they would have great predictive power for as yet unstudied characters. But that is not the way it has turned out, at least for the human species.
The fourth and last fact about genetic differences between groups is that these differences are in the process of breaking down because of the very large amount of migration and intergroup mating that was always true episodically in the history of the human species but is now more widespread than ever. The result is that individuals identified by themselves or others as belonging to one “race,” based on the small number of visible characters used in classical race definitions, are likely to have ancestry that is a mixture of these groups, a fact that has considerable significance for the medical uses of race identification.
