Q.Why does stabilising selection tend to reduce variation in a population over successive generations, while disruptive selection tends to increase it?

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 diso …

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 …