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: A farmer sprayed his orange trees with a pesticide to eliminate the insects damaging the trees. Some of the insects survived and produced offspring that were also resistant to the insecticide. Which process is illustrated by the pesticide resistance of the offspring?
natural selection
Natural selection.