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| Many methods exist for assessing biological variation within and between human populations. A number of biochemical traits, such as red blood cell polymorphisms, provide a means of assessing genetics at the level of the molecular structure of blood types and blood proteins and enzymes. Other measures of genetic variation focus specifically on the underlying genetic code by directly examining differences in DNA sequences. A number of physical traits, such as anthropometrics and skin color, provide data on phenotypic variation, which reflects both underlying genetic differences and environmental and developmental influences. Throughout much of history, human biological variation has been discussed in terms of race and racial classification, with an emphasis on identifying discrete groups of human populations that differ widely in genetics. In the past, race was used as a crude means by which to describe patterns of human variation. A major problem in using race as a concept is that distinct "races" take on a reality of their own in people's minds. The race concept has limited use in analyses of biological variation, particularly for widespread species such as human beings, because it uses arbitrary classifications of predominantly continuous traits. Studies of genetic variation in the human species do not produce results consistent with the race concept. Instead, we find evidence of geographic differences between human groups, but with no clear boundaries between them. Genetic variation is greatest within local human populations, such that for many traits only 10 percent of the total genetic variation is found between major geographic groups. The pattern of genetic distances between human populations is strongly related to geography and its effect on gene flow. |