People with one sickle cell allele are not likely to get malaria. What is this an example of?
This is an example of an overdominancy and natural selection. One sickle cell allele provides an organism with an "immunity" to malaria*. Heterozygotes have an advantage of both types of homozygotes (overdominancy). Homozygotes with two sickle cell alleles have a sickle cell anemia (severe disease, which decreases both quality of life and life expectancy). Homozygotes with zero sickle cell alleles do not have any symptoms of sickle cell anemia. On the other hand, homozygotes with zero sickle cell alleles are susceptible to malaria, which also lowers life expectancy. At the same time, heterozygotes have lower concentration of a deficient hemoglobin (meaning lower physiological effects) and a partial resistance to malaria. This gives a possibility to heterozygotes to be naturally selected.
*The mechanism is not clear yet, but in 2011 scientists from Instituto Gulbenkian de Ciência, Oeiras, Portugal have published an article (doi: 10.1016/j.cell.2011.03.049.) explaining possible mechanism of the resistance.