Nei's decomposition of total expected heterozygosity in subdivided populations into within- and between-subpopulation components,H(S)andD(ST), respectively, is a classical tool in the conservation and management of genetic resources. Reviewing why this is not a decomposition into independent terms of within- and between-subpopulation gene diversity, we illustrate how this approach can be misleading because it overemphasizes the within-subpopulation component compared to Jost's nonadditive decomposition based on gene diversity indices. Using probabilistic partitioning of the total expected heterozygosity into independent within- and between-subpopulation contributions, we show that the contribution of the within-subpopulation expected heterozygosity to the total expected heterozygosity is notH(S), as suggested by Nei's decomposition, butHS/s, withsbeing the number of subpopulations. Finally, we compare three possible approaches of decomposing total heterozygosity in subdivided populations (i.e., Nei's decomposition, Jost's approach, and probabilistic partitioning) with regard to independence between terms and sensitivity to unequal subpopulation sizes. For the conservation and management of genetic resources, we recommend using probabilistic partitioning and Jost's differentiation parameter rather than Nei's decomposition.
