Malarial selective pressure might make the sickle cell mutation favorable. This occurs in African population where malaria is endemic and this happens because the people who are heterozygotes for sickle cell anemia (HbA/HbS) are resistant to malaria so they survive the malarial parasite more successfully than either normal or sickle cell homozygotes.
Explanation:
Sickle cell anemia can be seen as an example of Balancing selection in areas where malaria is endemic. The people who are heterozygous for sickle cell anemia have only one copy of sickle cell allele (HbA/HbS). These people have better chances of surviving malaria because the malarial parasite spends a part of the life cycle in the RBC, if they enter into the RBC which are sickle shaped, they will die. The women who are heterozygous have higher fertility rates, that's why natural selection has not eliminated the allele. The loss of deleterious recessive genes through the death of people who are homozygous is being balanced by the high reproductive rates of women who are heterozygous in malaria prone areas. For this reason the selection is called balancing selection.
Sickle cell mutation is most common in Africa and southern half of Europe. The reason for this is because, natural selection made areas with high concentration and history of diseases such as cholera, tuberculosis, yellow fever, and much more.
