Micro-evolution in a Human Population: A Study of Social Endogamy and Blood Type Distributions among the Western Apache

Abstract

T HIS paper has a two-fold objective: 1) to apply modern population genetics to human breeding groups, and 2) to demonstrate certain aspects of the micro-evolutionary process in these groups. Population genetics, applied to man, must take into consideration a form of environment not encountered with other organisms; namely, culture. The rules of marriage and kinship structure play significant roles in the patterning of mate selection, the size and stability of breeding groups, and the degree of intra and intergroup mobility. After a brief introduction to the pertinent work already done in this area of research, we describe the population to be studied in terms of family, clan, phratry, and band organization. We then analyze some current and historic marriage records to determine if correlations exist between geographic origin of spouses and band affiliations. It is shown that the bands are endogamous but that random mating occurs within the band. Band interbreeding is apparently too infrequent to constitute a significant source of inflowing genes. The size of the bands within historic times has been too great for genetic drift to have operated as an effective evolutionary agent. Differences between bands in frequencies of the genes for the ABO and Rh antigens are demonstrated to exist and to have remained relatively constant in the past three generations. It is contended that these differences arose in prehistoric times when the band sizes were sufficiently small to allow the effective operation of genetic drift. Relative isolation and increased size in historic times tended to fix the gene frequencies as we find them today. Certain changes in gene frequencies that are observed might best be explained as the result of natural selective forces.