Race as a biological concept has had a variety of meanings. In the taxonomic literature, a race is any distinguishable type within a species, such as dark-bellied and light-bellied variants of small mammals. In 1937, Theodosius Dobzhansky introduced the idea of geographical races — populations of species that differ in the frequencies of one or more genetic variants. But as no two populations have identical gene frequencies at variable (polymorphic) loci, Dobzhansky's definition of race becomes synonymous with that of population.

The classical definition of race, as applied to our species, is based on phenotypes such as skin colour, facial features and hair form that clearly differ between native inhabitants of different regions of the world. An underlying assumption is that all of these defining features (all largely genetic traits, although few of their genes have been identified) are characteristic of the genome in general. In other words, just as there are large differences between races in genes for skin colour, so there should be large genetic differences between races in general. In the previous absence of data to confirm or deny this assumption, it was not an unreasonable one to make.

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