Question

Simultaneous presence of Sickle cell anemia and P. falciparum malaria is an example of ____________
 

• A. Directional selection
• B. Normalizing selection
• C. Balancing selection
• D. Neutral selection

Hint:

Remember the sickle-cell and malaria connection as the number one example of "heterozygote advantage," which is a form of "balancing selection." The key idea is that selection is maintaining a balance of both the normal and the sickle-cell alleles in the population.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This question addresses a classic example of natural selection in human populations. The persistence of the sickle-cell allele (HbS), which is harmful in the homozygous state, is explained by the advantage it confers in the heterozygous state in malaria-prone environments.
Step 2: Detailed Explanation:
- Individuals with two copies of the normal hemoglobin allele (AA) are highly susceptible to severe malaria.
- Individuals with two copies of the sickle-cell allele (SS) suffer from sickle-cell anemia, which is often fatal.
- Individuals who are heterozygous (AS) have one normal and one sickle-cell allele. They are largely asymptomatic for sickle-cell disease and also have significant protection against malaria.
- In areas where malaria is endemic, the heterozygotes (AS) have the highest fitness (survival and reproductive success) compared to both homozygotes (AA and SS).
- This situation, where natural selection favors the heterozygote and maintains both alleles in the population at a stable equilibrium, is called balancing selection (specifically, heterozygote advantage). It prevents the harmful allele from being eliminated and the advantageous allele from becoming fixed.
Step 3: Final Answer:
The co-existence of sickle-cell anemia and malaria, where the heterozygous genotype has the highest fitness, is a textbook case of balancing selection.