Science bio question- houseflie?

Easy! The Black flag spray was effective on the majority of the population of houseflies when it was first used. However, lets say out of 100 flies 1 fly was immune to the spray because of a genetic mutation causing, for example (I made this up) extra hairs in its respiratory tract to filter out the poison. That one fly will now go on to reproduce while it's 99 companions died. Therefore, the offspring will have a much higher rate of having that beneficial mutation. Thus, after many years that one ancestral super-fly has now reproduced and many of the population has the same mutation, therefore making the poison less effective.