An example of natural selection is the frequently observed phenomenon of insecticide resistance in pest insects. Consider what happens when a pest insect population is sprayed for the first time with a chemical insecticide. The population will be composed of individuals that vary in their susceptibility to the insecticide. Assume that susceptibility and resistance are based on a single gene locus with 2 alleles. The frequency of the resistant allele in the population is initially low.
After spraying, there is differential survivorship in the population; resistant and somewhat resistant individuals are more likely to survive. Among the survivors, the frequency of the resistant allele is higher than before. The survivors mate and leave offspring, producing generation 2. Generation 2 will also have the higher frequency of the resistant allele. If this generation is also sprayed, another episode of differential survivorship will occur.
Generation 3 will have an even higher frequency of the resistant allele. Repeated application of the insecticide each generation will result in an increase in the frequency of the resistant allele until most of the population is resistant and the insecticide ceases to be effective.