Anopheles mosquitoes, which carry the malaria parasite, cannot live above elevations of 1800 metres. In addition, oxygen availability decreases with higher altitude. Consider a hypothetical human population that is adapted to life on the slopes of Mt. Kilimanjaro in Tanzania, a country in equatorial Africa. Mt. Kilimanjaro's base is about 793 metres above sea level and its peak is 5895 metres above sea level. If the incidence of the sickle-cell allele in the population is plotted against altitude (feet above sea level), which of the following distributions is most likely, assuming little migration of people up or down the mountain?

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Quizplus · Accepted Answer

The sickle-cell allele provides protection against malaria, so it is more common in populations where malaria is prevalent. Anopheles mosquitoes, which carry the malaria parasite, cannot live above elevations of 1800 metres. Therefore, there would be little to no selection pressure for the sickle-cell allele at higher altitudes where mosquitoes are not present. Thus, the incidence of the sickle-cell allele is expected to decrease with higher altitude. Choice B is the only option that reflects this trend, with a peak incidence at the base of the mountain and a decrease at higher elevations. The other options show no clear pattern or show an increase in incidence at higher altitudes, which is not supported by the given information.

Anopheles Mosquitoes, Which Carry the Malaria Parasite, Cannot Live Above