Industrial pollution caused a change in the environment and this changed the selection pressure on the moths. The soot from industry coated the bark of tress changing them from light to dark. (This is particularly noticeable on trees with naturally white bark such as white birch, Betula paperifera.)

A scientist named Kettlewell showed that the light forms of the moths were conspicuous on the dark, polluted bark of trees. Melanistic moths, on the other hand, were well hidden on the dark bark. This caused a change in the selection pressure.

Moth predators (mostly birds) can easily spot a light coloured moth on a dark tree so the light coloured moths are more likely to be eaten. A dark moth on a dark tree is so well disguised that it escapes detection and thus escapes predation. By the time the mating season rolls around there will be fewer light coloured moths to contribute their light colour genes to the next generation. On the other hand, moths with black genes will be more successful in hiding from predators so more dark moths will survive to the breeding season. Offspring will inherit their parents' genes for colour. Therefore, the next generation of moths will have a greater number of dark forms than light forms because there were more dark parents than light parents in the breeding population.

Let's do a wee experiment to simulate the effect of selective predation.
To the experiment!