Question

Sickle cell anaemia: The selective forces that a population experiences can vary by context, especially by geographic place.

a. Explain how natural selection affects the evolution of haemoglobin in humans and how and why the selective pressures differ among human populations. Use Africa and Europe as examples of human populations, give genotypes and explain selective pressures.

b. Is natural selection on haemoglobin in African populations best described as disruptive, stabilizing or directional selection?

c. Is natural selection on haemoglobin in Australian populations (no malaria) best described as disruptive, stabilizing or directional selection?

d. For human populations living in Australia (no malaria), do you expect the Hs allele to eventually go extinct? Explain why or why not.

Answer 1

a. In Sickle cell anemia, natural selection affects the phenotype by keeping the harmful allele in the population. Sickle cell anemia is caused by the abnormality of Allele A that produces normal Hb to allele S that produces sickle-shaped RBC. This sticking of RBC reduces their affinity for oxygen transport in humans. The presence of HbsHBs is lethal in humans giving rise to autosomal recessive sickle cell anemia. However, in the population of Africa where malaria is quite common, the heterozygous HbAHBS gives an advantage by making the individual resistant to malaria. This is called heterozygous advantage and so natural selection is keeping the recessive allele S to prevent the incidence of malaria in the African Population. But in Europe, the incidence of malaria is much less. In such a case, natural selection will try and remove the recessive allele from the population so the frequency of  HbSHBS and HbAHBS will be much less than the African population. In Europe, the selective pressure favors HbAHbA while in Africa the selective pressure favors HbAHBS.

b. The natural selection of hemoglobin in the African population is best described as stabilizing or balancing selection. In stabilizing selection, the two extremes of the phenotypes are selected against (HbSHBS ) while the intermediate (HbAHBS) is favored as selection tends to eliminate sickle allele owing to its lethal nature while favoring the heterozygous for its advantage against malaria.

c. The natural selection of hemoglobin in Australian populations (no malaria) is best described as directional selection. In directional selection, only one end of the distribution is favored. In Australia, natural selection will try to eliminate S allele from the population and so only the normal form of Hb (HbAHbA) will be favored shifting the curve to one end.

d. For human populations living in Australia with no malaria, there is no need to keep the harmful Hs allele in the population anymore and so it will eventually go extinct. Since HbS in the heterozygote state will not have any advantage in the Australian population, there is no need to keep the allele and selection pressure will act against both HbSHBS  and HbAHBS

P.S. Please rate this answer Thumbs Up if you liked it. Thanks.