One Perspective: Directional, Stabilizing and Disruptive Selection
By Branden Holmes (Surroundx) on November 28, 2010
One way to look at the whole natural selection mechanism is to separate natural selection into directional selection, stabilizing selection, and disruptive selection. Imagine a trait for a particular species. It doesn't matter what the trait is; all that matters is that the value of the trait varies between individuals to a large extent. Some individuals will possess a below average value for that trait, others the average, and others again above average. The "value" of a trait simply means the physical size of that trait.
DIRECTIONAL SELECTION.
Directional selection, the selection that fuels evolution, occurs when individuals with an average trait value as well as one of the extremes of trait value, including the value spectrum in between, are at a significant disadvantage when it comes to survival. Imagine, for example, a species which varies in size. And along comes a selective event which favours larger size amongst the individuals of that species. Therefore this fulfils the criteria to be called directional selection because individuals of a below average size, right through to individuals of a slightly above average size are at a disadvantage and have a much higher mortality rate amongst those individuals than do larger than average individuals. When directional selection persists over a few or even many generations, it is then known as orthoselection.
STABILIZING SELECTION.
Stabilizing selection is when individuals with an average trait value for a particular trait, or the phenotype as a whole, are much more likely to survive than either of the two extremes of trait value. This mainly occurs when the environment is relatively static, and natural selection has nothing to selective for; only against (viability selection).
DISRUTIVE SELECTION.
Disruptive selection, in specific contrast to stabilizing selection, is when individuals with an average trait value are specifically at a disadvantage. Thus, individuals at either end of the spectrum are at a specific advantage. And if positive assortative mating exists as part of the criteria of mate choice in sexually reproducing animals, this can lead to speciation. Although such instances are extremely rare, if existent at all.
