Understanding Effective Population Size

The effective population size is not simply the total number of animals, but a measure that takes into account the number of breeding males (NmNm) and females (NfNf). It determines how genetic drift and inbreeding affect the population, and is especially important for conservation biology and breeding program management.

Formula for Effective Population Size

For a population with separate sexes, the standard formula is:

Ne=4NmNfNm+NfNe=Nm+Nf4NmNf

Where:

• NmNm = Number of breeding males

• NfNf = Number of breeding females

This formula shows that unequal sex ratios significantly reduce the effective population size, increasing the risk of genetic drift and loss of genetic diversity.

Calculation Example: 40 Males and 10 Females

Given:

• Nm=40Nm=40

• Nf=10Nf=10

Plug these values into the formula:

Ne=4×40×1040+10=160050=32Ne=40+104×40×10=501600=32

Therefore, the effective population size for a polygamous species with 40 males and 10 females is 32.

Why Is Effective Population Size Important?

• Genetic Health: A lower NeNe means greater risk of inbreeding and loss of genetic diversity, which can affect the population’s long-term survival.

• Conservation: Conservation programs use NeNe to assess the vulnerability of endangered species and to design breeding strategies that maximize genetic variation.

• Evolution: Effective population size influences the rate of genetic drift and the potential for adaptation.

Key Takeaways

• The effective population size is not the same as the census size, especially in species with skewed sex ratios or polygamous mating systems.

• In this example, although there are 50 individuals, the effective population size is only 32 due to the uneven number of males and females.

• Maintaining a balanced sex ratio is critical for maximizing NeNe and preserving genetic diversity in both wild and captive populations.

Correct answer:
(2) 32