Example of natural selection in action

Industrial melanism in the peppered moth

In the peppered moth population of Great Britain there is variation in the colour of the moth’s bodies. Some moths have a light body; some have a dark, or melanic body. Body colour is an inherited characteristic.

Three moths camouflaged against different background colours and texturesThree moths camouflaged against different background colours and textures

Before the industrial revolution, light moths had a selective advantage. Because they were well camouflaged against the lichen-covered trees, they were better able to avoid predation.

During the nineteenth century pollution killed off some of the lichens and soot deposits caused the tree bark to appear darker. Light coloured moths were no longer camouflaged and were more likely to be eaten by birds. The dark moths were now better camouflaged.

As a result, dark moths had a greater chance of reproducing and passing on the alleles that made them dark. This led to a gradual increase in the proportion of dark moths until light moths became very rare in industrial areas. The moth populations in these areas had evolved.

This change from predominantly light to dark forms in industrial areas is called industrial melanism.

Note that this change was not due to pollution making the moths darker. The dark variety had always existed in small numbers, but only had a selective advantage when the selection pressure changed.