The genetic structure of a tribal population , the

To the extent that a key issue can ever be identified in biology, understanding the significance of the large amount of genetic variation encountered in all properly studied outbreeding plant and animal species is the key issue in population genetics today. A meaningful discussion of this issue is seriously hampered by lack of data on the breeding structure of natural populations. Human populations offer unusual advantages for the study of population structure. Ancestry and recent patterns of migration of individuals and groups can be determined with a precision not possible for other 'wild' populations, as can individual reproductive performances. History and archaeology supply additional data on population movements in the past also usually not available for other types of organisms. While data on all types of human populations are greatly to be desired, data on the surviving relatively unacculturated, tribal-type populations are especially critical. The parameters of these populations come closest to reflecting the circumstances under which human evolution occurred and under which the vast amount of variability now known to be present in human populations arose. Such populations will not long be available for study. Guided by these and other considerations, in 1961 we initiated a major programme directed towards the American Indian (Neel & Salzano, 1964). With particular respect to the matter of the population structure of a relatively unacculturated group, the Xavante Indians of Brazil were chosen as the objects of a pilot study (cf. Neel et al. 1964). Even while this study was in progress, an intensive effort was being made to locate a larger and less acculturated Indian tribe, but one whose study did not present insuperable logistic problems. In this effort we were greatly aided by the expert knowledge of Professors Francisco Salzano and Miguel Layrisse. The choice finally settled on the Yanornama Indians of northern Brazil and southern Venezuela, a tribe of some 10,000 individuals first touched by missionary activity in 1950. Their approximate distribution is between latitudes 1 and 5" N and longitudes 62 and 65" 30' W. Although several expeditions or individual explorers had traversed portions of the territory they occupy prior to 1950 (Schomburgk in 1837, Koch-Griinberg in 1911, Rice in 1919-20, Salath6 in 1930), as far as is known the contacts whennot actually hostile (Rice, 1921) had been fleeting. Even at the height of the natural rubber boom, tappers do not seem to have penetrated this area. At present there are about a dozen mission stations within the area occupied by the Yanomama; the clearing of air-strips at seven of the stations during the past decade was an important logistic consideration in the choice of the tribe. However, despite this intrusion of missionaries, t o whose spiritual ministrations the Yanomama have been unusually resistant, we estimate that perhaps a third of the estimated 100-125 Yanomama villages are yet to be visited by a white man. A good index