In Central Africa, the frequency of the sickle-cell allele has remained stable over the years because the allele protects against malaria. In the United States, malaria was eradicated by eliminating the mosquito that spreads the disease. The decreasing frequency of the sickle-cell allele in African Americans since malaria was eliminated in the United States is an example of:

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Directional selection occurs when a particular phenotype is favored, leading to a change in allele frequency over time. In this case, the sickle-cell allele is no longer advantageous in the absence of malaria and is therefore being selected against, resulting in its decreasing frequency in African Americans.

In Central Africa, the Frequency of the Sickle-Cell Allele Has