Final answer:
An insect population becoming resistant to insecticide is an example of natural selection, where the resistant individuals survive and pass on their advantageous genetic traits, becoming more common over time.
Step-by-step explanation:
An insect population that becomes resistant to a commonly used insecticide is an example of natural selection. In the context of natural selection, individual insects with a genetic variation that confers resistance to a particular insecticide are more likely to survive and reproduce. Over time, these resistant traits become more common within the gene pool, and the population as a whole becomes less affected by the insecticide. This is because the trait for resistance has been naturally selected for, meaning that the insects with this trait have a higher rate of survival and are able to pass on their resistant genes to subsequent generations.