Fixation Index (FST) Calculators

0 calculators tagged with “Fixation Index (FST)”

The fixation index (FST) is a measure of population genetic differentiation — it quantifies what proportion of total genetic variation is due to differences between populations rather than within them. Developed by Sewall Wright, FST ranges from 0 (no genetic differentiation; all populations have identical allele frequencies) to 1 (complete fixation; populations are fixed for different alleles). FST is widely used in population genetics, conservation biology, and human genetics to assess gene flow, detect natural selection, infer population history, and identify recently diverged populations.

All Calculators

No calculators found for this topic.

What Is FST?

FST is defined as the variance in allele frequencies among subpopulations relative to the maximum possible variance:

FST = (HT − HS) / HT

Where:

  • HT = expected heterozygosity in the total pooled population (based on overall allele frequencies)
  • HS = average expected heterozygosity within subpopulations

Alternatively: FST = σ²p / [p̄(1 − p̄)], where σ²p = variance in allele frequency p across subpopulations and p̄ = mean allele frequency.

Interpretation of FST Values

  • FST = 0: No genetic differentiation — all populations have identical allele frequencies; extensive gene flow
  • FST = 0.05–0.15: Low to moderate differentiation — moderate gene flow
  • FST = 0.15–0.25: Moderate to great differentiation — limited gene flow
  • FST > 0.25: Great differentiation — very limited gene flow or long isolation
  • FST = 1: Complete fixation — populations fixed for different alleles; no shared alleles

Human populations typically show FST ≈ 0.10–0.15 across global continental groups — most human genetic variation (~85%) is within populations, not between them.

FST and Gene Flow

Wright showed that FST relates to gene flow (Nm = migrants per generation per population) approximately as:
FST ≈ 1 / (4Nm + 1)

High gene flow → low FST; low gene flow or isolated populations → high FST.

Applications

  • Identifying population structure in conservation genetics (minimum viable populations)
  • Detecting loci under natural selection (outlier FST values)
  • Forensic population assignment of unknown samples
  • Inferring migration history in human population genetics

Glossary

Fixation Index (FST)
A measure of genetic differentiation between populations: FST = (HT − HS)/HT. Ranges 0–1. FST = 0 means no differentiation; FST = 1 means populations fixed for different alleles. Reflects gene flow, drift, and selection.
HT (Total Heterozygosity)
Expected heterozygosity calculated from allele frequencies of the pooled total population. Used in FST calculation as the denominator representing total variation.
Gene Flow (Nm)
The movement of alleles between populations via migration. Related to FST by Wright's formula: FST ≈ 1/(4Nm + 1). High gene flow reduces FST; low gene flow allows populations to diverge through drift and selection.

Frequently Asked Questions

FST measures the proportion of total genetic variation that is due to differences between populations: FST = (HT − HS)/HT, where HT is total heterozygosity and HS is average within-population heterozygosity. FST = 0 means populations are genetically identical (no differentiation); FST = 1 means populations are fixed for completely different alleles. It quantifies how much populations have diverged due to genetic drift, selection, or limited gene flow.

Wright's guidelines: FST 0–0.05 = little differentiation; 0.05–0.15 = moderate differentiation; 0.15–0.25 = great differentiation; >0.25 = very great differentiation. Human global FST across continental populations is ~0.10–0.15, meaning ~85–90% of genetic variation is within populations. Highly isolated populations (island populations, endangered species in fragmented habitats) often show FST > 0.25 reflecting little gene flow.

Wright's island model predicts FST ≈ 1/(4Nm + 1), where Nm = number of migrants per generation. High gene flow (Nm >> 1) → FST near 0. Very limited gene flow (Nm << 1) → FST near 1. This relationship provides a rough estimate of historic gene flow from observed FST values, though it assumes idealized conditions (equal-sized populations, symmetric migration, equilibrium). Real populations often violate these assumptions.

Genome scans compare FST values across thousands of genetic markers (SNPs, microsatellites). Most loci are affected only by demographic history and drift, producing a background distribution of FST values. Loci under divergent selection (different adaptive pressures in different populations) show unusually high FST — 'outlier' loci that are more differentiated than expected by drift alone. These outliers are candidates for locally adaptive genes. Conversely, unusually low FST may indicate balancing selection maintaining diversity across populations.