Sickle-cell anemia is caused by an abnormal hemoglobin molecule that changed the shape of red blood cells. This disease can lead to various health complications that are often lethal. However, heterozygous individuals have minor symptoms. Moreover, heterozygous individuals show an advantage with decreased vulnerability to Malaria. The allele for Sickle-cell anemia is most common in tropical areas where malaria is common. Which population genetics phenomenon could be responsible for this?