Ed Yong in Not Exactly Rocket Science:
In the early 19th century, coal-fired factories and mills belched a miasma of soot over the English countryside, blackening trees between London and Manchester. The pollution was bad news for the peppered moth. This insect, whose pale speckled body blended perfectly against the barks of normal trees, suddenly became conspicuous—a white beacon against blackened bark, and an easy target for birds.
As the decades ticked by, black peppered moths started appearing. These mutants belonged to the same species, but they had traded in their typical colours for a dark look that once again concealed their bodies against the trees. By the end of the century, almost all the moths in Manchester were black.
As British air became cleaner and trees lighter in colour, the black moths faded back into obscurity. But in their brief reign they became icons of evolution. As geneticist Sewall Wright put it, they are “the clearest case in which a conspicuous evolutionary process has actually been observed.”
The story has endured a fair amount of controversy. Creationists asserted that the blackening of the moth was just a case of shifting gene frequencies rather than an outright change from one organism into another, ignoring that the former is the very definition of evolution.