From time to time black or gray mutant butterfly had misfortune to be born to only be quickly eaten. But in 17-18 century humans started burning coal in such amounts that sooth begin to darken birch tree skin, and white butterflies could no longer hide. White color became disadvantage - they almost all were eaten. But gray and dark mutants now got upper hand and survived much better, producing dark offspring. In a few generations only most butterflies were gray or black.
Whites are back.
That is how mutation-selection works and makes more genetically complex organisms at the expense of numerous trials and waste. Of course, this is information producing. Enviroment decides how useful.