The Effective Population Size Formula

For populations with different numbers of breeding males ($N_m$) and females ($N_f$), the standard formula is:

Ne=4NmNfNm+NfNe=Nm+Nf4NmNf

Where:

• $N_m$ = Number of breeding males

• $N_f$ = Number of breeding females

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

Calculation Example: 150 Females and 50 Males

Given:

• $N_m = 50$

• $N_f = 150$

Plug the values into the formula:

Ne=4×50×15050+150Ne=50+1504×50×150Ne=4×7,500200Ne=2004×7,500Ne=30,000200Ne=20030,000Ne=150Ne=150

Therefore, the effective population size for a population with 150 breeding females and 50 breeding males is 150.

Why Effective Population Size Matters

• Genetic Drift: Smaller $N_e$ means stronger genetic drift, leading to faster loss of genetic diversity.

• Inbreeding: Low $N_e$ increases the risk of inbreeding and the expression of harmful genetic traits.

• Conservation: Conservation programs use $N_e$ to assess population health and design breeding strategies to maximize genetic diversity.

Key Takeaways

• The effective population size is not always equal to the census size—unequal sex ratios can significantly reduce $N_e$.

• In this example, even with 200 breeding individuals, the effective population size is 150 due to the 3:1 female-to-male ratio.

• Maintaining a balanced sex ratio is important for maximizing $N_e$ and preserving genetic diversity in both wild and managed populations.

Correct answer:
(3) 150