Explain phenomenon of natural selection taking examples of peppered moth and sickle cell anaemia. (IAS 2023/15 Marks)

Introduction

Natural selection is a key mechanism of evolution, proposed by Charles Darwin, which explains how species adapt to their environment over time. It is the process by which organisms that are better suited to their environment survive and reproduce, while those that are less suited are eliminated. 

The Case of the Peppered Moth

• Scientific Background: The peppered moth (Biston betularia) is a classic example of natural selection observed in industrialized England.
• Two Color Morphs:
  
  • Light-colored moths (typica): Well-camouflaged against the lichen-covered trees in rural, pre-industrial England.
  • Dark-colored moths (carbonaria): Initially rare, these moths were more visible against the lichen-covered trees.

Mechanism of Natural Selection in Peppered Moth

• Pre-industrial England:
  
  • The environment was rich in lichens, with trees having lighter bark.
  • Light-colored moths blended in well with the environment, making them less visible to predators.
  • Dark-colored moths were more easily spotted and eaten by birds, leading to fewer dark moths surviving to reproduce.
• Industrial Revolution:
  
  • Factories released large amounts of soot and pollution, darkening the tree barks and killing the lichens.
  • The lighter moths became more conspicuous, and their numbers decreased as they were more easily eaten by predators.
  • The dark-colored moths, which were previously at a disadvantage, now had better camouflage and were more likely to survive and reproduce.
• Natural Selection in Action:
  
  • The frequency of the dark-colored morph (carbonaria) increased significantly during the industrial period due to the higher survival rate of these moths.
  • This shift in population is an example of directional selection, where one extreme trait (dark coloration) becomes more common in the population.
• Role of Mutation and Genetic Variation
  
  • The variation in color (light vs. dark) in the moth population was due to genetic mutations.
• Mutations occur randomly, and natural selection favors the color trait that provides a survival advantage in a particular environment.

Sickle Cell Anaemia: A Case Study in Natural Selection

• Definition of Sickle Cell Anaemia:
  
  • Sickle cell anaemia is a genetic blood disorder characterized by the presence of abnormally shaped red blood cells (sickle-shaped cells), which can block blood flow, causing pain and organ damage.
  • It is caused by a mutation in the gene that codes for hemoglobin, the protein in red blood cells responsible for carrying oxygen.
• Genetic Basis:
  
  • The disease is caused by a mutation in the hemoglobin gene, where a single nucleotide change causes hemoglobin to crystallize when deoxygenated.
  • Homozygous individuals (having two copies of the sickle cell allele) suffer from sickle cell disease, while heterozygous individuals (having one sickle cell allele and one normal allele) are carriers but usually do not show severe symptoms.
• Natural Selection and Sickle Cell Anaemia
  
  • Selection Pressure from Malaria:
  • In regions where malaria is endemic, particularly in sub-Saharan Africa, individuals who are heterozygous for the sickle cell trait (HbAS) have a survival advantage over those who are homozygous normal (HbAA).
  • The malaria parasite Plasmodium infects red blood cells, but sickle-shaped cells are less hospitable to the parasite. As a result, heterozygous individuals are less likely to suffer severe malaria compared to individuals with normal red blood cells.
  • Homozygous sickle cell individuals (HbSS) have a higher risk of severe health complications due to sickle cell disease, but their resistance to malaria can increase the frequency of the sickle cell allele in the population.
• Balancing Selection:
  
  • Natural selection results in a balance between the benefits of malaria resistance and the drawbacks of sickle cell anaemia.
  • The HbAS genotype offers a survival advantage because it protects against malaria without causing the severe symptoms of sickle cell anaemia.
  • This phenomenon is an example of heterozygote advantage or balanced polymorphism, where the heterozygous condition (HbAS) is favored by natural selection, maintaining both the normal and sickle cell alleles in the population.
• Geographical Distribution:
  
  • The frequency of the sickle cell allele is highest in areas where malaria is or was historically prevalent. For example, regions in Africa, parts of the Middle East, and India show high frequencies of the sickle cell allele due to the selective advantage it provides against malaria.

Conclusion

The examples of the peppered moth and sickle cell anaemia illustrate the power of natural selection in shaping the characteristics of populations over time. By favoring individuals with traits that provide a survival advantage in their environment, natural selection drives the process of evolution and leads to the adaptation of species to their surroundings.