THE HISTORY OF SICKLE- CELL ANEMIA AND MALARIA In the late 1950s,
the American physical anthropologist Frank B. Livingstone (1928–2005) sought to
strengthen the case for natural selection by historically linking sickle- cell anemia
and malaria. Livingstone asked two important questions: Where and when did the
sickle- cell allele first appear in equatorial Africa? and What conditions led to the allele’s
being naturally selected? He hypothesized that the Bantu, a group of peoples who
speak Bantu languages, carried the mutation with them when they migrated south-
ward from the region of Cameroon and Nigeria (Figure 4.17). Prior to the Bantu’s
arrival, the region was a largely unbroken forest. Bantu populations introduced agri-
culture there, clearing large swaths of the forest for cultivation. The peoples’ iron-
working technology made possible the creation of tools for cutting down large trees,
clearing and plowing fields, and planting crops— mostly yams and cassava.
Even under the best conditions, tropical forests are fragile ecosystems. Once
their trees have been cleared and their fields have been planted, their relatively
poor soil, which normally soaks up rainwater, becomes thin or disappears. Geo-
logical evidence shows a dramatic increase in soil erosion in the region after the
arrival of Bantu populations, due in large part to anthropogenic deforestation
and the overall environmental impact of humans on the landscape. As a result
of these erosive processes, pools of water collect and become stagnant, providing
ideal conditions for the breeding of parasite- carrying mosquitoes (Figure 4.18).
This picture became clear to Livingstone as he developed his research: the newly
created ecological circumstances fostered mosquito reproduction and the spread
of malaria, and the growing host of humans made possible by agriculture- fueled
population growth provided the food resources needed by the mosquitoes. The
(b)(a)
Greater
than 14%
12–14%
10–11.9%
8–9.9%
6–7.9%
4–5.9%
2–3.9%
0–1.9%
Areas where
malaria
is present
FIGURE 4.15 Distributions of the
Sickle- Cell Allele and Malaria In equatorial
Africa, (a) the distribution of the sickle- cell
allele coincides with (b) areas of high malarial
parasite concentration.
balanced polymorphism Situation in
which selection maintains two or more
phenotypes for a specific gene in a
population.
anthropogenic Refers to any effect caused
by humans.
88 | CHAPTER 4 Genes and Their Evolution: Population Genetics
infectious disease gave those individuals with a very rare mutation— the sickle- cell
allele— an adaptive advantage and the ability to survive and reproduce in these new
environmental circumstances. Due to the advantage the heterozygous condition
provides, the S allele was maintained and passed from generation to generation.
For this reason, sickle- cell anemia predominantly affects those whose descendants
came from the malarial environments in large parts of equatorial Africa. Outside
of such malarial environments, the S allele never became advantageous.
OTHER HEMOGLOBIN AND ENZYME ABNORMALITIES Sickle- cell anemia
turns out to be just one of a number of hemoglobinopathies and other genetic
abnormalities in Africa, Asia, and Europe that provide a strong selective advantage
in regions of endemic malaria (Figure 4.19). Heterozygous carriers of abnormal
hemoglobins apparently make poor hosts for malarial parasites.
Thalassemia, a genetic anemia found in Europe (especially in Italy and
Greece), Asia, and the Pacific, reduces or eliminates hemoglobin synthesis. In some
homozygous forms of the mutation, hemoglobin becomes clumped inside the red
blood cells. The spleen then destroys the red blood cells, resulting in severe anemia.
In the areas around the Mediterranean where the genetic frequency is highest— as
high as 80%—the presence of malaria makes a strong case for a selective advantage
for heterozygous individuals, for whom the condition and malaria are not lethal.
An association has long been recognized between deficiency of the enzyme
glucose- 6-phosphate dehydrogenase (G6PD) and malaria. A recessive hered-
itary mutation leads more males than females to lack the gene that is coded to
produce this enzyme (see Figure 4.11). Without the G6PD enzyme, a person who
takes sulfa- based antibiotics or eats fava beans risks the destruction of red blood
cells, severe anemia, and occasionally death. Because of the connection with fava
beans, this severe hemolytic disease is called favism. Its 130 genetic variants occur
in high frequencies in some populations, the highest being 70% among Kurdish
Jews. Heterozygous carriers have a strong selective advantage because they pro-
duce some of the enzyme but are protected from malaria (here again, the parasite
cannot live in the abnor
mal red blood cells).
Analysis of genetic data by the anthropologist Sara Tishkoff indicates that
the mutation for the disease arose between about 4,000 and 12,000 yBP, at the
same time as the abnormal hemoglobins. Populations whose descendants did not
encounter malaria do not have the G6pd mutation or abnormal hemoglobins.
Once inside the human, the sporozoites
travel through the bloodstream to the
liver. In the liver, the sporozoites create
thousands of merozoites. A merozoite
is a daughter cell that results from
asexual reproduction.
The newly produced merozoites enter the
bloodstream and infect the red blood cells.
Within these cells, the merozoites continue to
multiply, eventually causing the cells to rupture.
The merozoites then invade other red blood cells
in the bloodstream, and the cycle continues.
a Sporozoite: The goal of
sporozoite vaccines is to
block parasites from
entering or growing
within human liver cells.
b Merozoite: Vaccines based on
merozoite antigens lessen malaria’s
severity by hobbling the invasion of
new generations of red blood cells
or by reducing complications.
c Gametocyte: So-called altruistic gametocyte-
based vaccines do not affect human disease
but are designed to evoke human antibodies
that derail parasite development within
the mosquito.
Over time, some of the merozoites
develop into male and female
gametocytes, which may be transferred
to another mosquito that bites the human
host. Gametocytes are cells that can divide
to produce gametes, or sex cells.
Within the mosquito, sexual reproduction
occurs, as the male gamete fertilizes the female
gamete. Oocysts, or zygotes, develop and even-
tually release sporozoites. These newly formed
sporozoites travel to the salivary glands of
the mosquito, where they can be transmitted
to the next human the mosquito bites.
Within the mosquito, sexual reproduction
occurs, as the male gamete fertilizes the female
gamete. Oocysts, or zygotes, develop and even-
tually release sporozoites. These newly formed
sporozoites travel to the salivary glands of
the mosquito, where they can be transmitted
to the next human the mosquito bites.
Merozoites
Host’s liver
Host’s red
blood cell
INSIDE HUMAN
VACCINE TARGETS
INSIDE MOSQUITO
Female
gametocyte
Female
gamete
Male
gamete
Oocyst
Male
gametocyte
Fertilizationc
Sporozoites
A mosquito bites a human, passing
sporozoites to the new host. A sporozoite
is a motile form of the parasite.
a
b
1
5 2
4
3
FIGURE 4.16 The Spread of Malaria The life cycle of the malarial parasite, Plasmodium falciparum,
takes place in two hosts: mosquito and human. Both hosts are needed if the parasite is to survive.
Natural Selection: Advantageous Characteristics, Survival, and Reproduction | 89
infectious disease gave those individuals with a very rare mutation— the sickle- cell
allele— an adaptive advantage and the ability to survive and reproduce in these new
environmental circumstances. Due to the advantage the heterozygous condition
provides, the S allele was maintained and passed from generation to generation.
For this reason, sickle- cell anemia predominantly affects those whose descendants
came from the malarial environments in large parts of equatorial Africa. Outside
of such malarial environments,
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