Evolution due to Natural Selection.

http://en.wikipedia.org/wiki/Peppered_moth_evolution

Environmental Changes

Before the Industrial Revolution, the peppered moth was mostly found in a light grey form with little black speckled spots. The light-bodied moths were able to blend in with the light-coloured lichens and tree bark, and the less common black moth was more likely to be eaten by birds. As a result of the common light-coloured lichens and English trees, therefore, the light-coloured moths were much more effective at hiding from predators, and the frequency of the dark allele was about 0.01%.

During the early decades of the Industrial Revolution in England, the countryside between London and Manchester was blanketed with soot from the new coal-burning factories. Many of the light-bodied lichens died from sulphur dioxide emissions, and the trees became covered with soot. This led to an increase in bird predation for light-coloured moths, as they no longer blended in as well in their polluted ecosystem: indeed, their bodies now dramatically contrasted with the colour of the bark. Dark-coloured moths, on the other hand, were camouflaged very well by the blackened trees.

Although a majority of light-coloured moths initially continued to be produced, most of them didn't survive, while the dark-coloured moths flourished. As a result, over the course of many generations of moths, the allele frequency gradually shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce.

Typica and carbonaria morphs resting on the same tree. The light-colored typica (below the bark's scar) is nearly invisible on this pollution-free tree, camouflaging it from predators.