Principle #1 Natural Selection Brown Butterflies have an advantage of the environment they live in because they blend in with the brown tree back and are harder to see by predators. The yellow butterflies have a disadvantage because they do not blend in with that environment and because of their bright yellow color t makes them easier to spot by predators.
Principle #2 Overproduction and selective pressures The brown Butterflies blend in better with their environment so they have an advantage and a better fitness but the yellow butteries do not so they are easily spotted by predictors and get eaten and their population of yellow butterflies decreases.
Principle #3 Differential Survival and Reproduction The brown butterflies who survived and have a greater fitness are able to reproduce and have more offspring with the same traits.
Principle #4 Changes in the Gene Pool In the new generation you will now mostly see brown butterflies with their better fitness for survival. The butterflies with Yellow coloration don't have a better fitness and chance or survival because they do not blend in good with that environment.