In brief, Lewontin ignored the fact that differences among human populations are correlated. Although population variation at any one genetic locus tends to be small, global population structure becomes clear if one examines correlated differences across loci. For a simple analogy, consider men and women’s faces. If one takes any particular characteristic, say nose size, there is likely to be more variation within a sex than between the sexes (and it would be nearly impossible to classify faces by sex with any accuracy using only nose size). However, if one considers all facial characteristics together (which is, after all, how we actually experience human faces), then sex differences become sufficiently clear that an observer can guess the correct sex more than 95 percent of the time.
Bo Winegard
The psychologist and essayist who writes on race, intelligence, and human nature.
A more plausible conception of race, one that is consistent with how careful philosophers and geneticists use the term, recognises that: When humans began leaving Africa around 75,000 years ago, they dispersed across a much greater range of environments than they had previously inhabited. The humans that settled in different geographic regions subsequently came under different selection pressures (e.g. temperature, seasonality, altitude). Natural barriers such as oceans (e.g. the Atlantic), deserts (e.g. the Sahara) and mountain ranges (e.g. the Himalayas) impeded gene flow between different populations for substantial periods of time. When there is limited gene flow between populations that have come under different selection pressures, we would expect them to gradually diverge from one another over via the processes of genetic drift and natural selection.
The primary reason that natural philosophers began to classify humans into different races is that human populations look different from one another. Their skin colors, hair textures, facial structures, and stature all differ, often in predictable ways. Furthermore, these differences reflect their divergent geographical origins. In fact, researchers can classify human variation by continent quite accurately using only data from the human skull. (They are able to correctly classify human skulls into black and white Americans with about 80% accuracy, using only two variables.) Therefore, despite the common charge that racial classification is ipso facto racist, and that most Enlightenment typologies of human variety were motivated by prejudices and a desire to rationalize imperial domination, ordinary people and Enlightenment philosophers also took to classifying human differences for the mundane reason that such differences actually exist.