For organisms that rely on camouflage to help them avoid predators, the environment plays a critical role. After all, a pattern and coloration that hides an organism on one background may cause that organism to stand out brilliantly against another backdrop. Fortunately, for most organisms, dramatic environmental change typically occurs gradually, over long periods of time. But exceptions do occur, providing scientists with extraordinary examples of evolutionary change through natural selection.

Species become well adapted to their environments not by chance or choice, but as a result of natural selection. Predators, competitors, and other environmental challenges put pressure on individuals in a population, making it less than certain that each will survive to reproduce. Genetic variation causes individuals in a given species to have a range of different traits. Individuals with traits that provide an advantage in a particular environment stand a greater chance of reproducing, and thus, passing their genes—and traits—to future generations. Over time, the most suitable characteristics for a given set of conditions are retained in a population, while disadvantageous traits become marginalized or lost altogether.

Environmental changes can alter existing selective pressures or introduce new pressures onto current populations. For example, the introduction of invasive species or climatic shifts in precipitation or temperature can influence survival and/or reproduction in a population—a change that, over time, can alter the prevalence of various traits in that population. A characteristic that was once a disadvantage may become advantageous in a new set of circumstances, and vice versa.

One stunning example of this process occurred over two hundred years ago, near Manchester, England. Prior to industrialization, the trees and rocks in this area were covered with a layer of light-colored lichen. Consequently, the majority of peppered moths living in the woods were also very light in coloration. Those that happened to be darker—the result of a genetic mutation—were more likely to be seen and eaten by birds. Then, pollution from coal-fired power plants and factories began killing off the lichen and blackening the surfaces of trees and rocks. Soon, the light coloration of the peppered moths became a liability rather than an asset: birds ate light-colored moths in much higher numbers than the darker moths of the same species. In fact, dark peppered moths fared so well in this soot-covered environment that, by the end of the 1800s, they made up more than 95 percent of the population. This trend reversed itself when pollution levels in the area fell, tree trunks and branches became lighter in color as lichen returned, and light moths prevailed once again.

Subsequent research of England's peppered moths points to additional explanations for the changes in moth population. For example, migration of moths into or out of the study area could have contributed to some of the changes in the characteristics of the moth population. However, scientists still agree that the ability of predators to locate moths on a particular background had the strongest influence on these population trends.

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