It is sometimes argued that human genetic variation is continuous and so grouping people into discrete categories is illegitimate. The flaw in the first argument is simple: organizing continuous variation into discrete categories can help us predict and explain the world, and, so, can be valid scientific categories. In fact, scientists often group continuous variation into discrete categories. Consider, for instance:
- Medical researchers break continuous blood pressure variation into discrete categories such as “high” and “low”.
- Physicists group continuous variation across the color spectrum into discrete colors such as “blue” and “green”.
- Social scientists break continuous variation in income into discrete categories such as “poor” and “rich”.
In each of these cases, scientists are taking continuous variation and breaking it down into discrete categories because it is useful to do so. The usefulness of discrete categories comes from their power to predict. For instance, knowing that someone has “high” blood pressure, or knowing that someone is “poor”, can help you predict various other things about them. Similarly, knowing someone’s race can allow you to make predictions about any variable in which the races differ. Thus, racial categories are useful for prediction, which is a key function of science.
It’s also worth noting that human genetic variation is not, in fact, continuous. Or, at least, not across racial lines. Genetic variation becomes continuous when there is a large amount of gene flow between populations. However, gene flow can be significantly hindered by geographical features such as mountains and seas. Such geographic obstacles often stood in the way of the inter-racial mating pre-historically. For instance, Native Americans were separated from the rest of humanity by an ocean, Blacks were separated by the sub-Saharan desert, and Europe was separated from many Asian peoples by the Ural and Caucus Mountain ranges. Of course, cultural practices also limited inter-racial mating and explain way, for instance, even after 300 years of living in same areas, roughly 96.5% of American whites have basically no African admixture in their genomes (Bryc et al. 2015).
That human genetic variation is not continuous across racial lines was shown byRosenberg et al. 2005 who found that two populations of the same race are, on average, more genetically similar than two populations of different races, even when both population pairs are equally far from one another geographically. If human genetic variation was continous across racial lines, a computer program told to group human genetic variation into clusters such that intra cluster genetic similarity is maximized and inter cluster genetic similarity is minimized would not group people into races nearly 100% of the time. But it does (Tang et al 2005, Rosenberg et al. 2005, and Rosenberg et al. 2002 ).
It is also importance to realize that racialists have virtually always known that, to some degree, the human variation is continuous. Below are quotes from three important thinkers, all writing before 1900, who explicitly recognized this in their writings.
Johan Bumlenbach was a founder of modern anthropology and, in the late 1700’s, devised a 6 race categorization of humans that still matches common sense today. He was one of the first writers to speak of a Causation race which included both Europe and north Africa and an early proponent of the idea that the Races all had a common ancestor. In 1775 he described the way that traits vary geographically from one race to another as an “imperceptible transition”.
“No variety exists, whether of color, countenance, or stature, so singular as not to be connected with others of the same kind by such an imperceptible transition, that it is very they are all related, or only differ from each other in degree.” – Blumenbach 1775
Comte de Buffoon is the most important popularizer of the idea that a species should be defined as a population that can produce fertile offspring. He was also another early proponent of the idea that all the races have a common ancestor. Here, we can see Buffoon describing populations as differing by “imperceptible degrees”. This is clearly consistent with the modern terminology describing genetic variation as continuous.
“Man descends, by imperceptible degrees, from the most enlightened and polished nations, to people of less genius and industry; from the latter to others more gross, but still subject to kings and laws, and these, again, to savages” – Buffon 1753 (page 186)
Finally, we have Darwin. Darwin’s significance in general biology hardly needs to be explained. It is worth noting, though, that Darwin was probably the single most important historical advocate of the idea that human races are subspecies, rather than species. Here, we can see him using the continuous nature of human genetic variation as evidence for this claim:
“But the most weighty of all the arguments against treating the races of man as distinct species, is that they graduate into each other, independently in many cases, as far as we can judge, of their having intercrossed.” – Darwin 1871 (page 226)
In summary, human genetic variation is not continuous across racial lines. But even if it was, this would not mean that we can’t put that continuous variation into discrete categories. In fact, many of histories most important racial thinkers believed that human variation was continuous across racial lines, and this did nothing to sway them from their race realism.