Stabilizing Selection

Stabilizing Selection Definition

Stabilizing selection refers to any force or forces that push a population toward the average or median trait. Stabilizing selection describes what happens to an individual trait when its extremes are selected against. 

This increases the frequency of the trait in the population, as well as the alleles and genes involved. Stabilizing selection has formed many traits that are common among entire groups of species. Comparing the three types of stabilizing selection.

Stabilizing Selection Examples

Robin eggs

In this case, the number of eggs in a robin nest has been selected through stabilizing selection. There is no evidence that robins are capable of raising more than four chicks successfully. The reason for this is probably due to the size of the birds and their ability to provide food for two adults at the same time. 

Due to the size and amount of food needed by a chick, most penguins can only raise one chick at a time. Even though they stabilized on different numbers, both forms of stabilizing selection maximize the fitness of the species in their environment.

On the stabilizing selection graph, you can clearly see that the median trait population increases, while the other populations decrease. Five eggs would be too many in this case, and some of them would die. Three is too few, however. Either the eggs are not viable enough to rely on only three eggs or predation and other forces require more than three eggs to survive.

Hypothetical Lemurs

Crazy Island is home to a population of multicolored lemurs. A particular population of lemurs has been observed by scientists, and they have noticed the following changes in the lemur’s color.
There is no doubt that the lemurs have undergone stabilizing selection. Dark and light lemurs have almost disappeared, while middle brown lemurs have increased. The reason for this cannot be determined without more information. 

In order to understand what may have caused the stabilizing selection, ecologists and evolutionary biologists must observe the population, noting peculiar aspects of the various forms. It is not an easy question to answer, and there may be more than one answer.

It is possible that predators had an easier time spotting darker and lighter lemurs in the case of the lemurs. It is easy to test this hypothesis if the lemurs have only one predator. A scientist would simply observe the predator and determine which lemurs it prefers. 

Predation would support the theory that stabilizing selection is caused by predation. The amount of lemurs that predators eat, along with models showing how that level of predation could produce the coloration, would be further evidence.

It is much more common for a species to have multiple selective pressures, and for each pressure to act differently on various traits. For instance, the lighter version may be under attack from predation, while the darker version might be overheating. Both traits could be driven by predation, but not entirely. 

(Dark colors absorb more solar heat.) It is possible that female lemurs prefer brown lemurs because they are more likely to survive. It would be a form of sexual selection, driving a trend of stabilizing selection.

Common Causes of Stabilizing Selection

The direction of individual traits is determined by stabilizing selection, directional selection, and disruptive selection. Stabilizing selection pushed the trait toward the average instead of one or both extremes, but it can be driven by any selection process. Predation, resource allocation, coloration of the environment, food type, and a wide variety of other factors are some of the most common causes of selection.

Through history, many traits that we don’t discuss regularly have been affected by a variety of factors. Let’s take the modest insect as an example. An insect’s exoskeleton is a miraculous structure made of chitin and other structural molecules that surrounds its organs and keeps them hydrated in harsh environments. 

While this shield has been modified into an almost infinite number of forms, it was first selected through stabilizing selection. Insects’ ancestors didn’t have this adaptation, but once it evolved, it became highly favored.

Apart from the fact that the most average individual is selected for, stabilizing selection has no common cause. As with all forms of selection, stabilizing selection is caused by the increased fitness and reproductive success of the median individuals. In one way or another, extreme versions or traits are disadvantageous. Evolutionary terms describe this disadvantage as decreased reproduction. Through reproduction, they can only pass on traits that are encoded in their DNA. 

The increase in median traits represents their success in stabilizing selection. There are other extreme traits that are not as successful, possibly causing their owners to die. Thus, the median animals have access to more resources, which further boosts their success. Consequently, stabilizing selection is responsible for many traits shared by entire species. Synapomophies are referred to as these.

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