Fst (Fixation index)

The Fst index quantifies genetic differentiation between subpopulations relative to the total genetic variation in a species. Calculated as (H_T - H_S)/H_T, where H_T is the total expected heterozygosity under random mating and H_S is the average heterozygosity within subpopulations. Fst ranges from 0 (no differentiation) to 1 (maximum differentiation), reflecting population structure intensity.

High Fst values (e.g., 0.3) indicate 30% of genetic variation occurs between subpopulations. This arises from limited gene flow, genetic drift, or differential selection. For example, geographically isolated populations show high Fst, while well-connected ones approach 0. The metric is widely used in population genetics to study gene flow and evolutionary processes.

To compute Fst, ensure data meets Hardy-Weinberg equilibrium and subpopulations are representative. Errors in H_T or H_S lead to misinterpretations. Note that Fst only measures differentiation magnitude, not its cause. Conservation studies use this index to design strategies for preserving genetic diversity.

Common precautions include avoiding small sample sizes, which skew H_S, and contextualizing Fst. Low values don't exclude local differentiation, while high Fst may stem from positive selection in specific markers. Combining Fst with Fis and Fit provides a more comprehensive view of population dynamics.