Genetic Drift Calculator
Calculate allele frequency change due to random genetic drift. Find expected variance in allele frequency after n generations for a given population size. Model bottlenecks.
🧬 Population Genetics📐 Var(Δp) = p(1-p)/2Ne🧬 Biology
Initial allele frequency p
Effective population size Ne
Number of generations t
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Genetic Drift Calculator | — | Var(Δp) = p(1-p)/2Ne | frequency |
Step-by-Step Examples
Example 1
Small Population
p=0.5, Ne=50, t=20 generations.
- Var per gen = 0.5x0.5/(2x50) = 0.0025
- After 20 gen: p̄ still 0.5 but large variance
- SD(p) ~ sqrt(0.5x0.5x[1-(1-0.01)²⁰]) = sqrt(0.0865) = 0.294
- Allele could be anywhere from 0.2 to 0.8
✓ Substantial drift in 20 generations
Example 2
Bottleneck Effect
Population reduced to Ne=10 for 1 generation. p=0.5.
- Var = 0.5x0.5/(2x10) = 0.0125
- SD ~ sqrt(0.0125) = 0.112 in one generation
- High risk of allele loss at this size
✓ Bottleneck causes significant drift even in 1 generation
Example 3
Large Population
Ne=10000, p=0.5, t=100 generations.
- Var per gen = 0.0000125
- After 100 gen: small cumulative drift
- p likely still near 0.5
✓ Minimal drift in large populations over 100 generations
Real-World Applications
Conservation Biology
Small reserves and fragmented habitats cause genetic drift, reducing diversity in endangered species.
Population Genomics
Drift affects neutral SNP frequencies, used to infer population history and Ne.
Evolutionary Biology
Drift explains why non-adaptive alleles can become fixed (neutral theory of evolution).
Conservation Genetics
Minimum viable population size calculations aim to minimize drift effects.
Common Mistakes to Avoid
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Drift is stronger in smaller populations
Variance = p(1-p)/(2Ne). Smaller Ne = larger variance per generation = faster allele frequency change.
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Genetic drift is random, not directional
Unlike selection, drift can increase OR decrease any allele frequency. Cannot predict direction, only variance.
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Effective Ne is often less than census N
Ne accounts for unequal sex ratio, variance in reproductive success, and population size fluctuations. Ne is typically 0.1-0.5 of census N.
Frequently Asked Questions
What is genetic drift? ▾
Random changes in allele frequency due to chance sampling of alleles each generation. More powerful in small populations. Drives evolution even without selection.
What is effective population size (Ne)? ▾
The size of an ideal population that would experience the same genetic drift as the actual population. Accounts for unequal sex ratios, variance in offspring number. Ne < N usually.
What is the bottleneck effect? ▾
A dramatic reduction in population size causes rapid genetic drift, potentially eliminating alleles and reducing diversity. Example: cheetah low diversity from historical bottleneck.
What is the founder effect? ▾
A small group establishes a new population, carrying only a subset of the original gene pool. Extreme drift. High frequency of some rare alleles in isolated populations.
What is neutral evolution? ▾
Kimura theory: most DNA changes are neutral (no fitness effect). Neutral alleles evolve by drift alone. Rate of neutral evolution = mutation rate (independent of Ne).