Free A Troublesome Inheritance: Genes, Race and Human History by Nicholas Wade Page A

different challenges. These variations, or races, are fluid, not fixed. If the selective pressure that brought them into being should disappear, they will merge back into the general gene pool. Or, if a race should cease to interbreed with its neighbors through the emergence of some barrier to reproduction, it may eventually become a separate species.
    People have not been granted an exemption from this process. If human differentiation were to continue at the same pace as that of the past 50,000 years, one or more of today’s races might in the distant future develop into a different species. But the forces of differentiation seem now to have reversed course due to increased migration, travel and intermarriage.
    Races develop within a species and easily merge back into it. All human races, so far as is known, have the same set of genes. But each gene comes in a set of different flavors or alternative forms, known to geneticists as alleles. One might suppose that races differ in having different alleles of various genes. But, though a handful of such racially defining alleles do exist, the basis of race rests largely on something even slighter, a difference in the relative commonness, or frequency, of alleles, a situation discussed further in the next chapter.
    The frequency of each allele of a gene changes from one generation to the next, depending on the chance of which parent’s allele is inherited and whether the allele is favored by natural selection. Racesare therefore quite dynamic, because the allele frequencies on which they depend are shifting all the time. A good description is provided by the historian Winthrop Jordan in his history of the historical origins of racism in the United States. “It is now clear,” he writes, “that mankind is a single biological species; that races are neither discrete nor stable units but rather that they are plastic, changing, integral parts of a whole that is itself changing. It is clear, furthermore, that races are best studied as products of a process; and, finally, that racial differences involve the relative frequency of genes and characteristics rather than absolute and mutually exclusive distinctions.” 10
    Races emerge as part of the process of evolutionary change. At the level of the genome, the driving force of evolution is mutation. Mutation generates novelty in the sequence of DNA units that comprise the hereditary information. The new sequences are then acted on—either eliminated, made more common or ignored—by the evolutionary processes of natural selection, genetic drift and migration.
    The chemical units of which DNA is composed are long lasting but not permanent. Every so often, from spontaneous decay or radiation, a unit will disintegrate. In every living cell, repair enzymes constantly patrol up and down the strands of DNA, proofreading the sequence of chemical units, or bases, as chemists call them. The four bases are known for short as A (adenine), T (thymine), G (guanine) and C (cytosine). The structure of a DNA molecule consists of two strands that spiral around each other in a double helix, with each base on one strand lightly cross-linked to a base on the other strand. The cross-linking system requires that where one strand of the double helix has A, there will be a T at the same site on the opposite strand, with G and C being similarly paired. If the base opposite a T is missing, the repair enzymes know to insert an A. If a C is missing its partner, the enzymes will provide a G. The system, though amazingly efficient, is not perfect. A wrong base is occasionally inserted bythe proofreading system, and these “typos” are called mutations. When the mutations happen to occur in a person’s germ cells, whether eggs or sperm, they become evolutionarily significant, because they may then get passed on to the next generation.
    Other kinds of mutation occur through copying errors made by the cell in manipulating DNA. All these types of mutation are the