Certain insect species have become more and more resistant to insecticides. What type of natural selection is this, and how does it differ from the other types?

Answer 1

This is just an example of simple positive selection for those insects that have greater endogenous resistance. As time goes on, insects would grow more resistant to the insecticides from the small amounts that survive the insecticide in the first place. These insects spread their genes through breeding which can pass the gene along. Soon, more and more of the insects have the insecticide resistant gene.