Question

The population in South Africa is more prone to sickle cell anemia (genotype “ss”). People with genotype “ss” survive only up to 14-15 years of age. People with “SS” are prone to malaria because the parasite resides inside their RBCs. People with genotype "Ss" are resistant to malaria because the parasite cannot reside inside their RBCs, and they can live a normal life (without sickle cell anemia). What kind of evolutionary force must be operative in South Africa?

Answer 1

Indeed, there are several alleles, providing protection against malaria, including allele of sickle-cell anemia (hemoglobin S), alpha- and beta-thalassemia, hemoglobin C and hemoglobin E. There are visible signs of strong selection pressure. First, abundance alleles suggests that the selection of supports almost any shape adaptation to the disease. Secondly, the pressure is so strong that despite the fact that almost all of these adaptations is very costly to their media, they are still maintained by selection.

Answer 2

stabilizing selection

Step-by-step explanation:

Sickle-cell anaemia is a recessive disorder caused by the presence of two recessive alleles "s", so the genotype is "ss". This disorder is characterized by sickle haemoglobin. In an area with malaria, heterozygous individuals "Ss" (with one dominant allele and one recessive allele) have an advantage. These individuals will have both normal and sickle haemoglobin. But pathogen that causes malaria affects only normal haemoglobin, so heterozygous individuals will have half of the haemoglobin resistant to the pathogen and those individuals are resistant to malaria.

Stabilizing selection favours heterozygotes Ss, disruptive selection favours dominant (SS) and recessive (ss) homozygotes, while directional selection favours dominant (SS) or recessive (ss) homozygotes. Since in this example, people with genotype Ss (heterozygotes) are at an advantage, then this is an example of stabilizing selection.

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