In recent decades, medical anthropologists have been searching for evidence of feedback loops between cultural variation, population dynamics, health, and genetics in the controversial field of Darwinian medicine. The theory of evolution itself, and the role of cultural factors in adaptive change, is being re-examined. For example, co-evolution, "a theory of evolution by cultural selection" (Durham, 1991, p. 38), is believed to parallel organic, molecular evolution.
An example illustrating links between cultural variation and evolution is the unusual degree of diversity in Tibetan marriage systems, including polygyny, polyandry, polygynandry, and monogamy. This variation, in Durham's view (1991, pp. 59-70), is a solution to female and male infertility. It allows an infertile couple to bring an additional wife, husband, or p'horjag ("extra man") to the household so that heirs could be produced.
It also allows considerable economic flexibility, a benefit in a high altitude ecosystem with low and seasonal productivity of scarce arable lands (Durham, 1991, p. 71). Parenthetically, similar flexibility in household structures and marriage forms were found traditionally in Arctic peoples for many of the same reasons.
Co-evolution theory does not imply that variation in marriage forms is genetically programmed. Rather, behavioral diversity parallels genetic diversity, and both lead to optimal flexibility in environmental uncertainty. The enhanced fertility of households contributes to reproductive success, while the marriage systems themselves are preserved due to their "replicative success" in achieving benefits to the household and the community (Durham, 1991, p. 78).
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