A concrete example: the peppered moth

One moth, two colors

The peppered moth is an English nocturnal moth that exists in two forms: a light form, speckled with gray and white, and a dark form, almost black. This color difference is due to natural genetic variation, present in the population well before the events we're about to describe.

Before the Industrial Revolution

Before the 19th century, the tree trunks where these moths land were covered with light-colored lichens. Against this light background, the light form of the moth was well camouflaged from predatory birds, while the dark form stood out much more and was spotted more easily. As a result: the light form largely dominated the population.

During the Industrial Revolution

Industrial pollution in England blackened tree trunks (with soot) and killed the light-colored lichens. Against this new dark background, it was the light form that became visible and vulnerable to predators, while the dark form found itself better camouflaged. In just a few decades, the proportion of dark moths became a large majority in certain industrial regions.

After pollution was reduced

Starting in the second half of the 20th century, anti-pollution laws allowed tree trunks to regain a lighter color. The proportion of light moths then rose again, confirming that the observed mechanism was indeed reversible and directly linked to the environment.

What this example demonstrates

• The variation (light/dark) already existed before any change in the environment: pollution didn't create it, it changed which form had the advantage.
• It was the predatory birds which, by more easily spotting the less camouflaged form, caused the proportions in the population to shift: this is natural selection in action, observed and measured over just a few decades.

Remember the key point

This example, one of the best documented in ecology, shows natural selection at work on a human time scale, and confirms that it acts on proportions within a population, never on an individual that would "change color."