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When a pesticide is applied to a population, it affects those individuals that are sensitive to its mode of action
But unless the target population is genetically uniform, individuals are likely to differ in their susceptibility to toxins
If a few individuals have the ability to break down the pesticide, they will be at an enormous selective advantage
These individuals will have a superior ability to survive and reproduce after being exposed to the pesticide, and they will therefore leave more offspring than susceptible individuals will leave
In the next generation, a higher proportion of individuals will be resistant to the pesticide
Thus, the application of a pesticide creates a strong selective pressure for resistance to it
When the process is repeated over several generations, a population in which all or most individuals are resistant to the pesticide can be created
This typically leads to a search for newer, even more toxic products