Human Races: Classifying People vs Understanding Diversity

Guido      Barbujani

Abstract

The idea that all humans naturally belong to one of a few biological types or races that evolved in isolation was unchallenged for centuries, but large-scale modern studies failed to associate racial labels with recognizable genetic clusters. Recently, the conclusions of those studies have been questioned by authors who argue that racial classification has objective scientific bases and is indispensable in epidemiology and genetics. However, no classification is useful if the classification units are vague or controversial, and no consensus was ever reached on the number and definition of the human races. The available studies show that there is geographic structure in human genome diversity, and that it is possible to infer with reasonable accuracy the continent of origin from an individuals multilocus genotype. However, clear-cut genetic boundaries between human groups, which would be necessary to recognise these groups as relatively isolated mating units which zoologists would call races, have not been identified so far. On the contrary, allele frequencies and synthetic descriptors of genetic variation appear distributed in gradients over much of the planet, which points to gene flow, rather than to isolation, as the main evolutionary force shaping human genome diversity. A better understanding of patterns of human diversity and of the underlying evolutionary processes is important for its own sake, but is also indispensable for the development of diagnostic and therapeutic tools designed for the individual genotype, rather than for illdefined race-specific genotypes.

Keywords: human diversity, population structure, geographic variation, gene flow, selection, isolation, risk factors