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?