The development of resistance to insecticides can be thought of as an example of very rapid evolution, in response to a change in the environment. There is strong evidence that the development of resistance is preadaptive rather than postadaptive; that is, the insecticide acts as a selective agent that favors the survival of resistance genes already in the population, rather than inducing the resistance. Evidence comes from: (1) the failure of non-killing doses to induce inherited resistance, (2) relative ineffectiveness of selection within inbred lines, and (3) the development of resistant strains by selection of sibs of resistant flies, so that the direct ancestors of the resistant flies were never exposed to the insecticide.

The question of whether the process of selection utilizes mainly resistance genes that are already present in low frequency in the population or whether mutants arise during the selection process is, in Drosophila at least, resolved in favor of the former.

The precision of genetic analysis depends on the kind of insect that is being studied. It is clear that most, if not all, of the resistance is chromosomal rather than non-Mendelian. The manner of inheritance in different insects is widely different. In some cases there is strong evidence that a single dominant or recessive gene is responsible; in others it is more complex, and in some strains of Drosophila DDT resistance has been shown to depend on genes on all the major chromosomes. It is as if selection has utilized whatever kinds of resistance genes happened to be present.

The rapid response of formerly resistant strains to renewed selection for resistance is readily understood by the fact that the resistance genes have not returned to their initial extremely low frequencies and therefore respond much more readily to selection.