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Evolution__3rd_Edition

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..<br />

Figure 5.9<br />

The global incidence of malaria<br />

coincides with that of the sickle<br />

cell form of hemoglobin. (a) A<br />

map of the frequency of the S<br />

allele of hemoglobin. (b) A map<br />

of malarial incidence. Redrawn,<br />

by permission of the publisher,<br />

from Bodmer & Cavalli-Sforza<br />

(1976).<br />

Sickle cell hemoglobin confers<br />

resistance to malaria<br />

(a)<br />

(b)<br />

CHAPTER 5 / The Theory of Natural Selection 125<br />

Malarial zone<br />

0.000–0.020<br />

0.020–0.040<br />

0.040–0.060<br />

0.060–0.080<br />

0.080–0.100<br />

0.100–0.120<br />

0.120–0.140<br />

>0.140<br />

some advantage in malarial zones. Allison (1954) showed that, although SS is almost<br />

lethal, the heterozygote AS is more resistant to malaria than the homozygote AA.<br />

(Allison’s was the first demonstration of natural selection at work in a human population.)<br />

The full reason was discovered later a AS red blood cells do not normally sickle,<br />

but they do if the oxygen concentration falls. When the malarial parasite Plasmodium<br />

falciparum enters a red blood cell it destroys (probably eats) the hemoglobin, which<br />

causes the oxygen concentration in the cell to go down. The cell sickles and is destroyed,<br />

along with the parasite. The human survives because most of the red blood cells are<br />

uninfected and carry oxygen normally. Therefore, where the malarial parasite is common,<br />

AS humans survive better than AA, who suffer from malaria.<br />

Once the heterozygote had been shown physiologically to be at an advantage, the<br />

adult genotype frequencies can be used to estimate the relative fitnesses of the three<br />

genotypes. The fitnesses are:

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