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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The graph would show a lower incidence of the sickle-cell allele at lower altitudes, where the risk of malaria transmission is high due to the presence of Anopheles mosquitoes, and a higher incidence of the sickle-cell allele at higher altitudes, where the risk of malaria transmission is low or absent due to the absence of Anopheles mosquitoes. The distribution would not be a simple linear relationship because sickle-cell trait carriers do not have the same fitness advantage as individuals with sickle-cell disease at high altitudes. Thus, the curve would reach a plateau or even decrease at very high altitudes where the lower oxygen availability may become a greater disadvantage than the advantage of sickle-cell trait against malaria. Choice B represents this distribution pattern.

Anopheles Mosquitoes,which Carry the Malaria Parasite,cannot Live Above Elevations of 1800