Most of the butterflies have a dull or dark color.
In an environment there are many species of animals. Each animal within a species is different. In this example, the butterflies are not all the same. Some of them have different colors from each other. Some butterflies are darker than others. This is due to genetic variation.
The butterflies are all the same color.
The butterflies that aren't adapted to have dull and dark colors are the ones who are eaten the fastest. This causes the butterfly to not have any fitness and struggle to survive. Therefore, the butterflies that are adapted to have many offspring in order to keep the species alive.
Due to the bright butterflies having a low chance of survival, after only a few generations there are less of bright butterflies and more of the dark and dull butterflies.
The butterflies adapted to the environment. Its gene pool changed over generations so can have good fitness. Now the butterflies have have a dominant gene to have a dark color so it can camouflage with the trees or mimic an owl.