Measurement of genetical differentiation among subpopulations.

The basis for measuring differentiation among subpopulations is discussed and a number of conditions formulated that are desirable for an appropriate measure. These conditions imply that each subpopulation is characterized by the difference in genetic composition between it and its complement. A direct method of determining this difference is described and shown to result from a known measure of genetic distance between populations. The weighted average of the genetic distances between subpopulations and their complements constitutes a measure of differentiation among subpopulations which fulfills all of the desirable conditions and has the additional advantage that its values are directly interpretable. This measure (δ) is equally applicable to gene (δ ge ), gametic (δ ga ) or genotypic (δ go ) frequencies, which guarantees an unequivocal multilocus treatment, provided that the sets of genetic entities to which the frequencies refer are properly defined. The general relationship δ ge ≤ δ ga ≤ δ go is consistent with the principle that increasing complexity of organization of genetic material results in increased opportunity for differentiation. It is demonstrated that Wright's F st (G ST in Nei's notation), which is often used to measure subpopulation differentiation, meets some but not all of the conditions formulated for a desirable measure.