Complex sentence: He noted that mutations expressed in red blood cells, such as sickle-cell anemia and various thalassemias, were prevalent only in tropical regions where malaria has been endemic and were favourable traits for natural selection that protected individuals from receiving malarial infection.
Split into simpler sentences: He noted that mutations expressed in red blood cells, such as sickle-cell anemia and various thalassemias, were prevalent only in tropical regions where malaria has been endemic. He further observed that these were favourable traits for natural selection which protected individuals from receiving malarial infection.