Effects of demographic and ethnohistorical factors on average heterozygosities of South Amerindians.

Recent investigations have shown that average within-Tupi genetic distances differ from within-Carib distances, which is possibly due to differences in effective size of the populations belonging to these two linguistic stocks of South American tribes. The aim of this paper is to verify the influence of demographic factors and of interpopulation contacts on the degree of intragroup genetic variability of 18 South American native groups (eight Carib, seven Tupi, and three Gê). The mean per locus per individual heterozygosity (Hm) was studied for seven polymorphic systems and the distance from the centroid rii on Hm was evaluated with regression analysis according to Harpending and Ward's model. Data on intergroup contacts among the populations since the end of the eighteenth century were collected from an extensive review of the ethnohistorical literature. The level of Hm of the three linguistic stocks did not differ, which suggests a homogeneous within-group variation for these groups. In addition, there was no association between gene diversity and population size. On the other hand, demographic size correlated with rii, which suggest that gene frequencies in groups of larger populations more closely resemble the average gene frequencies of native South Americans. Values of rii differed between the stocks, and the average distance from the centroid of the Carib was about 2.6 times greater than that of the Tupi, in accordance with previous genetic distance analysis. It should also be emphasized that there was an important correlation between mean heterozygosity and the degree of historical intertribal contacts. This constitutes a particularly significant finding, suggesting the basic role of intertribal gene flow of the past two centuries on the level of present genetic variability of South American tribes.