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Definition of disruptive selection

Definition The term 'average' is used to describe the middle ground. A teacher looks for a bell-shaped curve for her class grades, with 'C' being the average grade. At one end there will be students who did poorly, and earned grades less than 'C.' On the other end are the students who did very well and earned grades higher than 'C.' This bell shaped curve also appears in nature. While some individuals in a population may be at one end of the spectrum or the other for any number of traits, most are in the average range. For example, some individuals are very fast runners, and some are very slow. Overall, most individuals are close to average: they are neither very fast nor very slow. When the average is expressed in nature this is referred to as stabilizing selection. Sometimes, however, one or more extreme is actually what is favored and the average is selected against. When only one extreme is selected, it's called directional selection. When both extremes are selected for equally it's called disruptive selection. Disruptive selection is the rarest of these three types of natural selection, but is a major driving force of evolution. This is because by selecting such opposite traits, directional selection can lead to the development of two separate species. Examples One of the best-studied examples of directional selection is the one of peppered moths in England. The moth gets its name from the peppery-looking coloration on its wings and body. The peppered moth may be a light color or a dark color, with very few individuals being a color in between the two extremes. During the day, the moths sleep on tree trunks that are covered with lichen. Originally, the lighter colored moths were more common because they blend in very well with the lichen and were not as easily seen by birds, who like to eat them. However, after the Industrial Revolution began in London in the early 1700's, the dark colored moths started becoming more common in industrialized areas, while the light colored moth remained the dominant color in rural areas. It turns out that the pollution from the factories in London was turning the tree trunks dark, and the dark colored moths were better-able to blend in and escape predation. Because the pollution was less prevalent in rural areas, the light colored moths were still better-camouflaged on those trees.