The Genetic Structure of a Tribal Population, the Yanomama Indians. VI. Analysis by F-Statistics (Including a Comparison with the Makiritare and Xavante

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RESUMO

The infra-structure of three relatively undisturbed tribes of American Indians (Yanomama, Makiritare, Xavante) has been investigated by means of the F-statistics of Wright, using 8, 9 and 6 codominant systems respectively. The data for the first two mentioned tribes are much more extensive (37 and 7 villages) than for the third (3 villages), and much of the argument is based on the first two. An additive model partitioning FIS into an average effect (FA) and deviations due to deme size, systems effects, village effects, and random error has been employed. The Cannings-Edwards formulation suggests that the small size of the demes alone would result in an FIS of —0.008 for the Yanomama and —0.007 for the Makiritare. There is no evidence for significant village or systems effects. Despite considerable scatter, FA values are not significantly heterogeneous and tend to be negative (—0.012 to —0.023). On the basis of a computer simulation model, it appears that there is an excess of consanguineous marriage over random expectation, i.e. the negative FA values are probably not due to avoidance of close inbreeding in a subdivided population in which demes are small. Aspects of population structure which could contribute to negative FA values are identified. These include unequal gene frequencies in the sexes and occasional marked differential fertility. It is at this point unnecessary to introduce overdominance as a cause of the negative FA values, since a computer simulation program which does not incorporate selection satisfactorily reproduces the observed FIS values. If population breeding structure alone can result in negative FIS values, then this may constitute a mechanism for retarding random fixation.—Mean FST values are 0.063 for the Yanomama and 0.036 for the Makiritare. While truly comparable data are lacking, it seems likely these will be found to be relatively high values for human populations. FIT values have been calculated by both direct and indirect approaches. The direct approach yields a value of 0.045 for the Yanomama and —0.009 for the Makiritare; the respective indirect values are 0.085 and 0.017. The primary identifiable reason for this difference between tribes is the greater genetic heterogeneity among Yanomama villages. The assumptions underlying the indirect approach to the calculation of FIT do not appear to be met in these populations.

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