In 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. The sickle cell trait provides an advantage against malaria compared to people with normal hemoglobin. In the United States, the parasite that causes malaria is not present, but African Americans whose ancestors were from equatorial Africa have the sickle-cell β-hemoglobin allele. These differences in traits illustrate a. inclusive fitness because people have evolved molecular differences to adapt to environmental stimuli
b. inclusive fitness because β-hemoglobin increases the proliferation of beneficial traits in the population
c. relative fitness because people have evolved molecular differences to an environmental pathogen
d. relative fitness because the molecular differences in β-hemoglobin are passed to the next generation