The usual form of the peppered moth Biston betularia in northern Europe has a light 'peppered' pattern of coloration. The moth rests on tree branches and its color pattern camouflages it against predatory attack. The camouflage only works against the right background: birds are more likely to eat poorly camouflaged moths, which therefore have a lower fitness.
The light coloration of tree branches is mainly caused by lichens that grow there. Smoke pollution in the industrial revolution in the UK killed these lichens near to industrial areas, leaving tree branches black. At about this time, around 1830, a 'melanic' form of the peppered moth becomes increasingly common in contemporary moth collections. The melanic form is camouflaged on dark tree branches. Through the 19th century, the melanic form increased in frequency until, near industrial regions, it was the normal type of the moth. The increase was almost certainly driven by natural selection.
The moths satisfy all four conditions for natural selection:
• they reproduce;
• their color pattern is inherited;
• there is variation in their color patterns;
• the different forms have different fitnesses.
This image shows the two forms of the peppered moth. In this case, it is the original, lighter form on the left which is well camouflaged on the tree branch. As the number of trees with dark branches increased with pollution, it was the melanic form which was better camouflaged and increased in frequency.
Can we estimate the fitnesses of the peppered moths?