Evolution__3rd_Edition

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Pests, such as mosquitoes, evolve
resistance to pesticides, such as
DDT
The fitnesses can be estimated, ...
Percentage mortality on 4% DDT for 1 hour
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CHAPTER 5 / The Theory of Natural Selection 115
5.8 Pesticide resistance in insects is an example of
natural selection
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Malaria is caused by a protozoan blood parasite (Section 5.12.2), and humans are
infected with it by mosquitoes (family Culicidae a genera include Aedes, Anopheles,
Culex). It can therefore be prevented by killing the local mosquito population, and
health workers have recurrently responded to malarial outbreaks by spraying insecticides
such as DDT in affected areas. DDT, sprayed on a normal insect, is a lethal nerve
poison. When it is first sprayed on a local mosquito population, the population goes
into abrupt decline. What happens then depends on whether DDT has been sprayed
before.
On its first use, DDT is effective for several years; in India, for example, it remained
effective for 10–11 years after its first widespread use in the late 1940s. DDT, on a global
scale, was one reason why the number of cases of malaria reduced to 75 million or so
per year by the early 1960s. But by then, DDT-resistant mosquitoes had already begun
to appear. DDT-resistant mosquitoes were first detected in India in 1959, and they have
increased so rapidly that when a local spray program is begun now, most mosquitoes
become resistant in a matter of months rather than years (Figure 5.7). The malarial
statistics reveal the consequence. The global incidence of the disease almost exploded,
up to somewhere between 300 and 500 million people at present. Malaria currently kills
over 1 million people per year, mainly children aged 1–4 years. Pesticide resistance was
not the only reason for the increase, but it was important.
DDT becomes ineffective so quickly now because DDT-resistant mosquitoes exist at
a low frequency in the global mosquito population and, when a local population is
sprayed, a strong force of selection in favor of the resistant mosquitoes is immediately
created. It is only a matter of time before the resistant mosquitoes take over. A graph
such as Figure 5.7 allows a rough estimate of the strength of selection. As for the
peppered moth, we need to understand the genetics of the character, and to measure
the genotype frequencies at two or more times. We can then use the formula for gene
frequency change to estimate the fitness.
Observations
Means of the clusters
Figure 5.7
Increase in frequency of pesticide resistance in mosquitoes
(Anopheles culicifacies) after spraying with DDT. A sample of
mosquitoes was captured at each time indicated and the number
that were killed by a standard dose of DDT (4% DDT for 1 hour)
in the laboratory was measured. Redrawn, by permission of the
publisher, from Curtis et al. (1978).