Human evolution presents special problems for Neo-Darwinian theory because a second system of inheritance, culture, is an important determinant of phenotype in our species. An elementary theory to explain how cultural inheritance affects the evolutionary process is developed from three basic postulates: (1) both genes and culture evolve by natural selection; (2) the reproductive fitness optimum as a function of phenotype is different for genes and culture because the rules of inheritance of the two systems are different; and (3) a genetic capacity for culture is assumed to be optimized by selection with respect to genetic fitness. The theory is formally a two-person variable sum game in which genes and culture compete to control phenotype, although the conservatively Neo-Darwinian capacity-for-culture assumption ensures that culture will benefit genotype. Simple mathematical models are used to deduce the general properties of equilibrium phenotypes. Results include the possibility that under some circumstances phenotype may be at the cultural rather than the genetic fitness optimum. Particularly if it is assumed that the capacity for culture is a general trait permitting many specific cultural ones, the culture capacity will be like a pleiotropic gene and many cultural traits are likely to be at the cultural optimum. The fact that in a majority of human societies, people bias their kinship behavior in ways unexpected from degree of genetic relatedness may be an example of the effect of selection on culture.
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