How do new species develop? One evolutionary mechanism that may be at play is disruptive selection. Though rare in nature, this process favors extreme traits over those that are more intermediate.
What Is Disruptive Selection?
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 extremes 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.