Evolution__3rd_Edition

118 PART 2 / Evolutionary Genetics
Fitness can be measured ...
Table 5.8
The main mechanisms of resistance to insecticides. Reprinted, by permission of the publisher,
from Taylor (1986).
Mechanism Insecticides affected
Behavioral
Increased sensitivity to insecticide DDT
Avoid treated microhabitats Many
Increased detoxification
Dehydrochlorinase DDT
Microsome oxidase Carbamates
Pyrethroids
Phosphorothioates
Glutathione transferase Organophosphates (O-dimethyl)
Hydrolases, esterases Organophosphates
Descreased sensitivity of target site
Acetylcholinesterase Organophosphates
Carbamates
Nerve sensitivity DDT
Pyrethroids
Cyclodiene-resistance genes Cyclodienes (organochlorines)
Decreased cuticular penetration Most insecticides
are a major economic and health problem. The evolution of resistance to pesticides
causes misery to millions of people, whether through disease or reduced food supply.
The fact that insects can rapidly evolve resistance is not the only problem with using
pesticides against pests a the pesticides themselves (as is well known) can cause ecological
side effects that range from the irritating to the dangerous. But however that may
be, pesticides did not exist during the hundreds of millions of years that insects lived
for before they were introduced in the 1940s, and the rapid evolution since then of
resistance to pesticides provides a marvellously clear example of evolution by natural
selection (Section 10.7.3, p. 276, extends the story, and Box 8.1, p. 213, looks at drug
resistance in the malaria organism itself).
5.9 Fitnesses are important numbers in evolutionary
theory and can be estimated by three main methods
The fitness of a genotype, in the theory and examples we have met, is its relative
probability of survival from birth to adulthood. The fitness also determines the change
in gene frequencies between generations. These two properties of fitness allow two
methods of measuring it.
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