Fixation Index (Fst) Calculator
Calculate Fst to measure genetic differentiation between populations. Fst = 0 means no differentiation; Fst = 1 means complete genetic isolation. Analyze population structure.
🧬 Population Genetics📐 Fst = (HT - HS) / HT🧬 Biology
Average heterozygosity within subpops HS
Total heterozygosity across all pops HT
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Fixation Index (Fst) Calculator | — | Fst = (HT - HS) / HT | 0 to 1 |
Step-by-Step Examples
Example 1
Human Populations
Average HS = 0.632, HT = 0.663 across worldwide human populations.
- Fst = (0.663-0.632)/0.663 = 0.031/0.663 = 0.047
- ~4.7% of genetic variation is between populations
- ~95.3% is within populations
✓ Human Fst ~ 0.05: humans are one of the least genetically differentiated species
Example 2
Isolated Island Population
HS=0.40 (within-island), HT=0.80 (global).
- Fst = (0.80-0.40)/0.80 = 0.50
- 50% of variation between populations
- High differentiation due to isolation and drift
✓ Fst = 0.50: highly differentiated isolated population
Example 3
Subspecies Comparison
Two wolf subspecies: HS=0.55, HT=0.70.
- Fst = (0.70-0.55)/0.70 = 0.214
- 21% of variation between subspecies
- Moderate-large differentiation
✓ Fst = 0.21: substantial between-subspecies differentiation
Real-World Applications
Conservation Genetics
Fst identifies distinct management units for conservation of wildlife species.
Human Population Genetics
Fst reveals human migration history and relationships between populations worldwide.
Forensic Genetics
Fst corrections used in DNA evidence calculations to account for population structure.
Evolutionary Biology
Fst values reveal degree of reproductive isolation between populations or species.
Common Mistakes to Avoid
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Fst compares within vs between population diversity
Fst = 1 - (within-group diversity / total diversity). High Fst: populations very different from each other.
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Fst ranges from 0 to 1
Fst = 0: populations identical. Fst = 1: populations fixed for different alleles (complete isolation). In practice, most populations show Fst between 0.01 and 0.30.
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HS must be ≤ HT
By definition, total heterozygosity cannot be less than within-group heterozygosity. If HT < HS, check your data.
Frequently Asked Questions
What does Fst measure? ▾
The proportion of total genetic diversity that is due to differences between populations. High Fst: populations genetically distinct. Fst is one of the most widely used statistics in population genetics.
What are typical Fst values? ▾
Humans: 0.05-0.15 (little structure). Migratory birds: 0.01-0.05. Non-migratory birds: 0.05-0.25. Marine fish: very low (0.001-0.01). Isolated populations: up to 0.7-0.9.
What is Wright Fst interpretation? ▾
Wright (1978): Fst 0-0.05: little differentiation. 0.05-0.15: moderate. 0.15-0.25: large. >0.25: very large differentiation. These thresholds are widely cited though somewhat arbitrary.
How does Fst relate to migration? ▾
Fst ≈ 1/(1+4Nem) where Nem = effective number of migrants per generation. High migration: low Fst. No migration: Fst approaches 1 (determined by drift rate).
What are alternatives to Fst? ▾
Gst (Nei 1973), Jost D (2008, for high-diversity markers), Theta (Weir & Cockerham), Psi (for diploids). All measure population differentiation differently, particularly for high-diversity microsatellites.