Natural selection evidence

Natural selection is the process by which populations change over generations, and it works through four linked ideas. First, individuals within a population naturally vary in their traits. Second, some of that variation is heritable, meaning it can be passed to offspring. Third, in a given environment, some variations make survival and successful reproduction more likely than others. Fourth, individuals with those favorable variations tend to leave more offspring, so those traits become more common in the population over time. It is important to be precise here: natural selection acts on populations across generations, not on a single organism changing during its own lifetime, and the environment does not create new variation -- it "selects" among variation that already exists in the population from mutation and inherited differences.

One of the most thoroughly documented examples is the peppered moth in England. Before the mid-1800s, the light-colored form of the moth was far more common, since it was well camouflaged against light, lichen-covered tree bark, while a darker form existed but was rare, since it stood out to bird predators on that light bark. During the Industrial Revolution, soot from factories darkened tree trunks and killed the lichen in many areas. In those newly sooty areas, the dark moths became better camouflaged and the light moths became easier for birds to spot; over subsequent generations, researchers recorded the proportion of dark moths rising sharply in polluted areas. After pollution-control laws reduced soot in the mid-1900s, recorded surveys showed the light-colored form becoming more common again in those same areas.