A caste system then restricts marriage to within-caste pairings, which in turn alters the course of genetic evolution. After a thousand years of inbreeding within caste, castes will diverge slightly on a few genetic traits—for example, shades of skin color—which might in turn lead to a growing cultural association of caste with color rather than just with occupation. (It only takes twenty generations of selective breeding to create large differences of appearance and behavior in other mammals.)50 In this way, genes and cultures co-evolve;51 they mutually affect each other, and neither process
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