Both sickle cell anemia and malaria are serious conditions which can kill people before they reproduce. Why would selection pressure favor absence of one over the other?
Selection pressure can favor absence of sickle cell anemia over malaria because sickle cell anemia is a genetic disorder caused by a mutation in the hemoglobin gene that can affect the ability of red blood cells to carry oxygen. This mutation is more common in regions where malaria is endemic because people with sickle cell trait (heterozygous for the mutation) have some resistance to malaria. However, people with sickle cell anemia (homozygous for the mutation) suffer from severe health complications and have a reduced lifespan, which can limit their reproductive success. Therefore, over time, individuals with sickle cell trait who are able to maintain resistance to malaria without suffering from the negative effects of sickle cell anemia may have a higher reproductive success and pass on their genes to future generations. This could eventually lead to a decrease in the frequency of the sickle cell allele in the population.
