A two-step test for the heterogeneity of Fst values at different loci.

Under genetic drift and in the absence of selection, the Fst values are expected to be equal at all loci, and heterogeneity among such values is considered as an evidence for different systematic pressures affecting the different genetic systems considered. A two-step test is here proposed. Fst values at various loci are classified into two nonarbitrary groups through the analysis of the association between pairs of genetic and environmental variables; the heterogeneity is then assessed by Wilcoxon's two-sample test. Estimation of the theoretical variance of Fst, which causes major shortcomings in the test proposed by Lewontin and Krakauer in 1973, is thus avoided.