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There are many documented cases of natural selection occurring in wild populations. One of the most famous is the story of the peppered moth (Biston betularia ). Within a population of peppered moths there are two different color forms: one mottled (light-colored) and one dark. Under the conditions in which the moth evolved, the mottled phenotype is more common because it mimics the lichen found on trees in a pristine forest. The moths’ predators (birds) can more easily see and prey on the black moth in this environment. 

During the Industrial Revolution, soot from factories darkened the trees and poor air quality destroyed the lichen. This resulted in a change in the population – the dark colored moths became more common. This is because now the mottled color is more visible to predators and the black moths are less visible in the environment.

Since the 1970s, air quality has improved in many places due to clean air legislation. As a result, the trunks of trees are returning to their lighter color due to the presence of more lichen on the trunks. In the image below, on the left you can see the light and dark morphs on the same tree. The light-colored moth (below the bark's scar) is nearly invisible on this tree living in a clean environment, camouflaging it from predators. On the right is a graphic of data collected over a six-year predation experiment. Over 4000 moths were released to determine if birds preyed on one color morph over another. The results of the experiment showed that there was strong differential bird predation against melanic peppered moths and that selection against the dark morph was sufficient in magnitude and direction to explain the recent rapid decline of melanism in post- industrial Britain. 

The first studies on populations of peppered moths were done before we had the ability to look at an organism's DNA. More recent studies on peppered moths have allowed us to learn that it is a mutation in a gene called cortex that is responsible for controlling color. 

In this example, we see the three conditions for natural selection to occur: 

• First, there is a struggle for existence as moths try to avoid being eaten by birds (there are limited hiding places so there is competition among individuals). 
• Second, there is variation in the population – some are mottled colored, and some are dark colored. 
• Third, the variation is heritable – the cortex gene plays a role in the moths color​.

This video (from MooMoo Science) discusses examples of natural selection and focuses on the example of the peppered moth:

To watch this video on YouTube (and see closed captioning) - press the arrow icon in the bottom right corner of the video player.