How would natural selection apply to the ability to become resistant to pesticides and the population of flies

The flies more resistant to the pesticides are more likely to survive and pass this resistance down to the next generation.





This continues until you end up with a fly which is highly resistant to the pesticides.How would natural selection apply to the ability to become resistant to pesticides and the population of flies
The population of flies will have differences in how the pesticide affects them, just as some humans are more prone to different diseases. Treatment will pesticide will kill the flies that are easier to kill with that pesticide. This will leave only the flies that are more resistant to the pesticide to reproduce. The flies that are the most resistant will have a reproductive advantage over other flies in the population.