Stabilizing Selection in Evolution

Newborn being weighed
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Stabilizing selection in evolution is a type of natural selection that favors the average individuals in a population. It is one of five types of selection processes used in evolution: The others are directional selection (which decreases the genetic variation), diversifying or disruptive selection (which shifts genetic variation to adjust to environmental changes), sexual selection (which defines and adapts to notions of "attractive" features of the individuals), and artificial selection (which is the deliberate selection by humans, such as that of the processes of animal and plant domestication).

Classic examples of traits that resulted from stabilizing selection include human birth weight, number of offspring, camouflage coat color, and cactus spine density.

Stabilizing Selection

  • Stabilizing selection is one of three main types of natural selection in evolution. The others are directional and diversifying selection. 
  • Stabilizing selection is the most common of those processes. 
  • The result of stabilizing is the over-representation in a specific trait. For example, the coats of a species of mice in a forest will all be the best color to act as camouflage in their environment. 
  • Other examples include human birth weight, the number of eggs a bird lays, and the density of cactus spines.

Stabilizing selection is the most common of these processes, and it's responsible for many of the characteristics of plants, humans and other animals.

Meaning and Causes of Stabilizing Selection

The stabilizing process is one that results statistically in an over-represented norm. In other words, this happens when the selection process—in which certain members of a species survive to reproduce while others do not—winnows out all the behavioral or physical choices down to a single set. In technical terms, stabilizing selection discards the extreme phenotypes and instead favors the majority of the population that is well adapted to their local environment. Stabilizing selection is often shown on a graph as a modified bell curve where the central portion is narrower and taller than the normal bell shape.

Polygenic Traits Bellcurve
Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the extremes and most in the middle. David Remahl/Wikimedia Commons

Diversity in a population is decreased due to stabilizing selection—genotypes which are not selected are reduced and can disappear. However, this does not mean that all individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are actually a bit higher statistically than those in other types of populations. This and other kinds of microevolution keep the "stabilized" population from becoming too homogeneous and allow the population the ability to adapt to future environmental changes.

Stabilizing selection works mostly on traits that are polygenic. This means that more than one gene controls the phenotype and so there is a wide range of possible outcomes. Over time, some of the genes that control the characteristic can be turned off or masked by other genes, depending on where the favorable adaptations are coded. Since stabilizing selection favors the middle of the road, a blend of the genes is often what is seen.

Examples of Stabilizing Selection

There are several classic examples in animals and humans of the results of stabilizing selection process:

  • Human birth weight, especially in underdeveloped countries and in the past of the developed world, is a polygenetic selection which is controlled by environmental factors. Infants with low birth weight will be weak and experience health problems, while large babies will have problems passing through the birth canal. Babies with average birth weight are more likely to survive than a baby that is too small or too large. The intensity of that selection has decreased as medicine has improved—in other words, the definition of "average" has changed. More babies survive even if they might have been too small in the past (a situation resolved by a few weeks in an incubator) or too large (resolved by Caesarian section).
  • Coat coloration in several animals is tied to their ability to hide from predator attacks. Small animals with coats that match their environments more closely are more likely to survive than those with darker or lighter coats: stabilizing selection results in an average coloration that's not too dark or too light.
  • Cactus spine density: Cacti have two sets of predators: peccaries which like to eat cactus fruits with fewer spines and parasitic insects which like cacti that have very dense spines to keep their own predators away. Successful, long-lived cacti have an average number of spines to help ward off both.
  • The number of offspring: Many animals produce multiple offspring at once (known as r-selected species). Stabilizing selection results in an average number of offspring, which is an average between too many (when there is a danger of malnourishment) and too few (when the chance of no survivors is highest).

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