Effective Population Size Calculator
Calculate effective population size (Ne) from census size, sex ratio, or variance in offspring number. Ne determines the rate of genetic drift and loss of heterozygosity.
🧬 Population Genetics📐 Ne = 4NmNf/(Nm+Nf)🧬 Biology
Number of breeding males Nm
Number of breeding females Nf
OR variance in offspring number Vk
Census population size N
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Effective Population Size Calculator | — | Ne = 4NmNf/(Nm+Nf) | individuals |
Step-by-Step Examples
Example 1
Unequal Sex Ratio
100 breeding males, 400 breeding females.
- Ne = 4 x 100 x 400 / (100 + 400)
- = 160000/500 = 320
- Census N = 500 breeding individuals
- Ne/N = 0.64 (reduced by unequal sex ratio)
✓ Ne = 320 (64% of census N)
Example 2
Elephant Seal Harem
1 male breeds with 100 females (extreme harem).
- Ne = 4 x 1 x 100/(1+100) = 400/101 = 3.96
- Census N = 101
- Ne/N = 0.039 (extreme reduction!)
- Genetic diversity determined almost by 1 male
✓ Ne = 4 (Ne/N only 4%!)
Example 3
Variance in Offspring
Population of 1000, variance in offspring Vk=8 (high).
- Ne = 1000 x (2-1)/(2-2+8/2) = 1000/4 = 250
- High variance means some individuals have many offspring while others have none
- Ne much less than census N
✓ Ne = 250 (25% of census N)
Real-World Applications
Conservation Biology
Ne determines minimum viable population size. Ne < 50 leads to rapid inbreeding depression.
Population Genetics
Ne is the key parameter governing genetic drift, rate of inbreeding, and loss of heterozygosity.
Managed Breeding
Zoo programs maximize Ne by equalizing family sizes to preserve genetic diversity.
Wildlife Management
Ne estimates from genetic markers help assess population viability of threatened species.
Common Mistakes to Avoid
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Ne is almost always less than census N
Unequal sex ratio, variance in reproductive success, population size changes all reduce Ne below census size. Ne is typically 0.1-0.5 of N.
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Sex ratio has huge effect
With Nm=1, Nf=1000: Ne=4, not 1001! A single breeding male bottlenecks the entire population genetically.
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Fluctuating population Ne is harmonic mean
Ne over time = harmonic mean of per-generation Ne values. Small populations in even one generation drastically reduce long-term Ne.
Frequently Asked Questions
What determines effective population size? ▾
Three main factors: 1) Unequal sex ratio (formula: Ne = 4NmNf/(Nm+Nf)), 2) Variance in offspring number (high variance reduces Ne), 3) Population size fluctuations (bottlenecks reduce long-term Ne).
Why is Ne important in conservation? ▾
Ne < 50: rapid inbreeding, loss of heterozygosity, accumulation of deleterious mutations. Ne < 100: cannot maintain adaptive variation long-term. IUCN uses Ne thresholds for threat classification.
What is the 50/500 rule? ▾
Ne > 50 to avoid inbreeding depression short-term. Ne > 500 to maintain adaptive potential long-term. These are minimum guidelines for conservation management.
How is Ne estimated from genetic data? ▾
LD-based methods use linkage disequilibrium between SNPs to infer Ne. Temporal methods compare allele frequencies across generations. Heterozygosity decline rate: Ne = -1/(2 x ln(1 - ΔH/H0) / t).
What is the relationship between Ne and inbreeding? ▾
F (inbreeding coefficient) increases by 1/(2Ne) per generation. After t generations: F = 1-(1-1/(2Ne))^t. After 50 generations with Ne=50: F ~ 0.40 (40% inbreeding).