Use the following information to answer the questions below.
In those parts of equatorial Africa where the malaria parasite is most common,the sickle-cell allele constitutes 20% of the β hemoglobin alleles in the human gene pool.
-Considering the overall human population of the U.S.mainland at the time when the slave trade brought large numbers of people from equatorial Africa,what was primarily acting to change the frequency of the sickle-cell allele in the overall U.S.population?