= 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. <Talking Glossary of Genomic and Genetic Terms by NHGRI>[Genetic Variation entry]{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.
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