Biston betularia is a very special moth, the Peppered Moth. http://en.wikipedia.org/wiki/Peppered_moth Textbooks have long cited it as a simple example of natural selection in action. Cryptic against bark where it rests during the day, it comes in light and dark forms.
Dark forms tend to predominate in areas where industrial pollution (for example) has darkened tree bark. The thought is that light forms on dark bark are more easily spotted by birds and eaten. Ditto for dark forms on light bark.
I coaxed the moths onto a convenient tree trunk and photographed them.
The study of melanism in peppered moths is far more complicated, and far more interesting, that what I've outlined. For the whole story, from the 19th century to the present, and illustrating the process by which scientific understanding is tested, retested, and evolves, you ought to read Grant, 1999.