100
individuals
84
individuals
0.84
0.4286
70
%
100
individuals
84
individuals
0.84
0.4286
70
%
The Effective Population Size Calculator computes Ne, the number of individuals in an idealized population that would experience the same rate of genetic drift as the actual population. Effective population size is almost always smaller than the census size due to unequal sex ratios, variation in reproductive success, and fluctuating population size.
Enter the number of breeding males and females to calculate the effective population size using the unequal sex ratio formula, which is one of the most common reasons Ne is reduced.
When the number of breeding males and females is unequal, the effective population size is:
Ne = (4 × Nm × Nf) / (Nm + Nf)
Where Nm = number of breeding males and Nf = number of breeding females. This formula gives the harmonic mean weighted for diploid reproduction. When Nm = Nf, Ne equals the census size. When the sex ratio is skewed, Ne is pulled toward the smaller sex, reducing effective size substantially.
Inputs
Results
With only 5 breeding males and 95 females, Ne is just 19 despite 100 total breeders. The extreme sex ratio reduces effective size to only 19% of census size.
Inputs
Results
Equal numbers of males and females breeding gives Ne = N = 100. This is the ideal scenario for maintaining genetic diversity in conservation programs.
Ne determines the rate of genetic drift, inbreeding, and loss of genetic diversity. A population of 10,000 with Ne of only 500 loses diversity as fast as an idealized population of 500. For conservation, Ne > 50 is a minimum to avoid short-term inbreeding depression, and Ne > 500 is needed to maintain long-term evolutionary potential.
Several factors reduce Ne below the census count: unequal sex ratios, variation in family size (some individuals produce many more offspring than others), overlapping generations, and fluctuating population size. In most natural populations, Ne is estimated to be 10-20% of census size, sometimes even less.
Conservation programs aim to maximize Ne by equalizing the sex ratio of breeders, equalizing family sizes (each pair contributes the same number of offspring), and rotating breeding pairs across generations. Even small increases in Ne can significantly slow the loss of genetic diversity in endangered species.
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