Genetic differentiation

Genetic differentiation is a difference in genetic composition between biological populations, often measured through allele-frequency differences. It can result from genetic drift, restricted gene flow or different selection pressures. Detecting differentiation alone does not identify which cause dominates. ^[ref]

One descriptive form of FST is $(H_T-H_S)/H_T$, where $H_T$ is expected heterozygosity in the pooled population and $H_S$ the average within-population value. For two equally weighted populations with allele frequencies 0.2 and 0.8, $H_T=0.5$ and $H_S=0.32$, giving 0.36. This constructed example shows how differing frequencies can reduce within-group diversity relative to pooled diversity. Sampling estimators, multiple alleles and genomic comparisons require additional choices; one locus is not a complete measure of biological separation.