Evolution, 4th Edition

120 CHAPTER 5

debilitating condition called sickle-cell anemia, which drastically decreases sur-

vival. But the S allele has not been eliminated by natural selection. Why?

The β-hemoglobin locus is under balancing selection, which is selection that

maintains genetic variation with a population. Balancing selection is fundamen-

tally different from selection on beneficial and deleterious alleles, which acts to

remove genetic variation. To understand how selection can preserve variation, we

will now explore the biology of the β-hemoglobin locus further.

Overdominance

FIGURE 5.18 shows that the S allele has high frequency in regions where malaria

is common. Malaria is a disease caused by a protozoan parasite (Plasmodium) that

is transmitted by mosquitoes in tropical regions around the world. About 500,000

people die each year from malaria, making it the most deadly infectious disease on

Earth.

Alleles at several loci make people partially resistant to malaria. One of those is

the β-hemoglobin locus. Individuals who are AS heterozygotes survive the malar-

ial parasite better than AA homozygotes [7]. In this situation, it is most convenient

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(A) (B) (C)

Neutral variant Benecial mutation

FIGURE 5.17 Changes in the environment can suddenly give

a fitness advantage to a mutation that is already present in a

population—a situation called selection on standing genetic

variation. (A) Three copies of the mutation (yellow, orange, and

red) have already recombined onto chromosomes with different

neutral variants at other sites (blue). (B) As the copies of the ben-

eficial mutation spread to higher frequency, the neutral variants

linked to each of them hitchhike to higher frequency. (C) When

the beneficial mutation becomes fixed, the region of reduced

variation nearby on the chromosome is much smaller than if the

mutation had a fitness advantage when it first appeared (compare

with Figure 5.15). Patterns of variation along the chromosome

can therefore be used to determine whether adaptation resulted

from selection on standing genetic variation or from a mutation

that was beneficial initially.

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(A) Frequency of S allele (B) Incidence of malaria

High

Moderate

Low

Malaria free

>15%

7–14%

0.6–6%

<0.5%

FIGURE 5.18 An allele that protects

against malaria in humans is most fre-

quent where the disease is common. The

frequency of the hemoglobin S allele in

Africa (A) is highly correlated with the in-

cidence of malaria (B). Other loci provide

protection against malaria in other human

populations. (After [35].)

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