Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Repeated pesticide use on an insect population can result in some of the insects becoming resistant to that pesticide over time. Which process is responsible for the pesticides no longer being effective on the insects?
natural selection
Pesticide resistance development.