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: Concept Overview:
This question highlights natural selection in humans. The persistence of the harmful sickle-cell allele (HbS) is due to the survival advantage it provides in the heterozygous state within malaria-affected regions.
Step 2: In-Depth Analysis:
- AA individuals (two normal hemoglobin alleles) are highly vulnerable to severe malaria.
- SS individuals (two sickle-cell alleles) develop often-fatal sickle-cell anemia.
- AS individuals (one normal and one sickle-cell allele) are mostly symptom-free for sickle-cell disease and gain substantial malaria resistance.
- In malaria-endemic regions, AS heterozygotes exhibit the greatest fitness (survival and reproduction) compared to AA and SS homozygotes.
- This scenario, where heterozygote advantage drives natural selection and sustains allele diversity at equilibrium, is termed balancing selection. This equilibrium prevents the eradication of the harmful allele and fixation of the advantageous allele.
Step 3: Conclusion:
The co-occurrence of sickle-cell anemia and malaria, resulting in superior fitness for the heterozygous genotype, exemplifies balancing selection.