Evolution by natural selection is an ongoing process that has been shown in numerous examples including the evolution of pesticide resistant insects. Since pesticides were introduced around the same time as World War II, they have been used to control insects that cause crop losses as long as carrying malaria and other diseases that pose major threats to public health. Many chemical control programs are failing, or have failed altogether, because of the evolving resistance in insects.
Many species are now resistant to nearly all available insecticides. Some species that had once been uncommon have become serious pests because insecticides have gotten rid of their natural enemies. The farmers solution to the problem has been to apply higher levels of insecticide to their crops. This results in a large burden on the environment where the chemicals can endanger human health and natural ecosystems.
This insect resistance evolves quickly because natural selection increases the rare mutations that are not advantageous under normal conditions, but happen to provide protection against harmful chemicals. For example, a population of insects which is being wiped out by the introduction into their environment of some kind of poisonous insecticide. If at least some of the insects happen to be genetically resistant to the effects of that insecticide and remain healthy, they will likely live long enough to reproduce and be able to contribute more offspring than individuals not resistant to the pesticide. These offspring who inherit the genetic variation, which makes the insect resistant to the poison, will also contribute more offspring to future generations than any remaining non-pesticide resistant insects. This population evolves eventually from non-pesticide resistant insects into a population made up largely (or maybe even completely) of pesticide resistant insects. The insects then with more favorable features for their environment leave more offspring, thus natural selection by evolution.