Q.Distinguish between stabilising selection, directional selection and disruptive selection, giving one example of each and describing how each type shifts the distribution curve of the character in the population.

Sickle cell anaemia is the textbook example of balanced polymorphism in human populations.

Because heterozygous carriers of the sickle-cell allele have some resistance to malaria, both the normal and sickle-cell alleles are maintained in malaria-endemic populations at stable, non-trivial frequencies — a genetic polymorphism sustained by natural selection. Typhoid and yellow fever are infectious diseases, and multiple sclerosis is an autoimmune disorder — none of these are examples of balanced genetic polymorphism.

A.C. Allison is credited with establishing the connection between sickle-cell anaemia and resistance to malaria.

Allison's studies in malaria-endemic parts of Africa showed that carriers of one sickle-cell allele (heterozygotes) suffered less severe malaria than those with normal haemoglobin, explaining why the otherwise harmful sickle-cell allele persists in these populations — a landmark example of natural selection maintaining a balanced polymorphism. Darwin and Lamarck proposed general theories of evolution long before this specific biochemical link was known, and Weismann is known for the germ-plasm theory, not this discovery.