Evolution__3rd_Edition

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Figure 5.9
The global incidence of malaria
coincides with that of the sickle
cell form of hemoglobin. (a) A
map of the frequency of the S
allele of hemoglobin. (b) A map
of malarial incidence. Redrawn,
by permission of the publisher,
from Bodmer & Cavalli-Sforza
(1976).
Sickle cell hemoglobin confers
resistance to malaria
(a)
(b)
CHAPTER 5 / The Theory of Natural Selection 125
Malarial zone
0.000–0.020
0.020–0.040
0.040–0.060
0.060–0.080
0.080–0.100
0.100–0.120
0.120–0.140
>0.140
some advantage in malarial zones. Allison (1954) showed that, although SS is almost
lethal, the heterozygote AS is more resistant to malaria than the homozygote AA.
(Allison’s was the first demonstration of natural selection at work in a human population.)
The full reason was discovered later a AS red blood cells do not normally sickle,
but they do if the oxygen concentration falls. When the malarial parasite Plasmodium
falciparum enters a red blood cell it destroys (probably eats) the hemoglobin, which
causes the oxygen concentration in the cell to go down. The cell sickles and is destroyed,
along with the parasite. The human survives because most of the red blood cells are
uninfected and carry oxygen normally. Therefore, where the malarial parasite is common,
AS humans survive better than AA, who suffer from malaria.
Once the heterozygote had been shown physiologically to be at an advantage, the
adult genotype frequencies can be used to estimate the relative fitnesses of the three
genotypes. The fitnesses are: