HbS is the gene that causes sickle cell anemia, an

HbS is the gene that causes sickle cell anemia, an inherited disease that causes red blood cells to collapse and become crescent-shaped instead of round. People with this type of anemia have problems transporting oxygen. High expression of this gene is concentrated in areas where malaria is reported (for example, tropical areas), an infection caused by a parasite that develops in red blood cells. Heterozygous individuals for the Hb/HbS gene have greater resistance to malaria than non-carriers since the parasite cannot develop. 

How is the mechanism of natural selection represented in this situation? How does the increased occurrence of the HbS gene represent an adaptation? What role do mutations play in this case?