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The Fixation Index (Fst) Calculator measures genetic differentiation between subpopulations. Fst quantifies the proportion of total genetic variance that is due to differences between subpopulations rather than within them. It is one of the most widely used statistics in population genetics and conservation biology.
Enter the total heterozygosity across the entire population and the mean heterozygosity within subpopulations to compute Fst and estimate gene flow.
Fst is defined as the reduction in heterozygosity due to population subdivision:
Fst = (Ht − Hs) / Ht
Where Ht is the expected heterozygosity of the total population (if it were a single random-mating unit) and Hs is the average expected heterozygosity within subpopulations. Fst ranges from 0 (no differentiation) to 1 (complete fixation of different alleles). Gene flow (Nm) is estimated as: Nm = (1 − Fst) / (4 × Fst).
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Results
Fst of 0.20 indicates great genetic differentiation between subpopulations. Estimated gene flow of 1 migrant per generation is the threshold below which drift dominates.
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Results
Fst of 0.042 indicates little genetic differentiation, with an estimated 5.75 migrants per generation maintaining genetic connectivity.
Wright's qualitative guidelines: Fst of 0-0.05 indicates little genetic differentiation, 0.05-0.15 moderate differentiation, 0.15-0.25 great differentiation, and above 0.25 very great differentiation. However, interpretation depends on the organism and markers used. Highly polymorphic markers tend to produce lower Fst values.
Under the island model, Fst is inversely related to the number of migrants per generation (Nm): Nm = (1−Fst)/(4×Fst). When Nm > 1 (more than one migrant per generation), gene flow prevents substantial divergence by drift. When Nm < 1, drift can cause subpopulations to diverge significantly.
Fst (Wright) and Gst (Nei) are conceptually similar measures of genetic differentiation based on heterozygosity. Gst extends Fst to multiple alleles. Rst is an analog designed for microsatellite data that accounts for allele size differences under a stepwise mutation model. For SNP data, Fst is most commonly used.
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