What is stabilizing selection in evolution?

Stabilizing selection is one of the five main types of selection that drive the evolution and adaptation of species to their environment. It is one that favors the phenotypic characteristics of average individuals in a population, ruling out extreme phenotypes over time.

It is a class of natural selection and is the most important of the four that also includes directional selection, disruptive selection, and sexual selection. In fact, we can say that stabilizing selection is the opposite of disruptive selection, in which extreme phenotypic traits are favored over average.

What is natural selection?

Natural selection is one of the processes by which the heritable traits of a species change over the generations allowing their natural adaptation to the environment in which they live . It is a selection process through differential reproduction and survival of individuals of a species as a consequence of differences in their phenotypic traits.

Natural selection differs from artificial selection, which is another selection mechanism that drives the development of new species, in that it does not depend on human intervention for it to occur, but is driven by interactions between species and between animals. individuals in the different ecosystems of which they are a part, with the intervention of the environment not made up of living beings (climate, humidity, etc.).

Stabilizing selection is one of four forms of natural selection in which phenotypes stabilize around an average, rather than shift to one extreme as in directional selection, differentiate into markedly different phenotypes as in diversifying selection. , or moving towards sexually attractive characteristics as in sexual selection.

Characteristics of stabilizing selection in evolution

We can see the effect of stabilizing selection either when a population has evolved to exhibit an average phenotypic streak much more frequently than the extremes. We can easily recognize this by drawing a histogram with the distribution of the phenotypic traits.

In cases where stabilizing selection was one of the driving forces in the evolution of a particular trait, we can see that the distribution curve for this trait in the population is shaped like a symmetrical bell centered around the average phenotype.

On the other hand, we can also observe stabilizing selection in action by comparing the distribution of phenotypic traits at different points in time. If this distribution curve is observed to get narrower and taller without shifting sideways, stabilizing selection is at work.

For example, if in a population of animals of a certain species there were originally a high number of individuals with opposite fur colors (black and white, for example) but after several generations we observe that gray fur or fur with black and white spots begins to predominate , then we are in the presence of stabilizing selection.

In other words, the distribution of phenotypic traits during stabilizing selection tends to:

• Acquire bell-shaped centered around the median phenotype.
• Having a constant average over time , that is, the bell does not move towards either of the two extremes, but remains in the same place.
• Show a decrease in variance , causing the bell to become narrower and taller while the frequency of extreme phenotypes becomes less and less.

Why does stabilizing selection occur?

There may be a number of reasons why the average phenotype is favored over the extremes. This is usually a situation where the average phenotype balances the benefits and disadvantages of extreme phenotypes, thus improving their chances of survival and reproduction.

 For example, one of the extreme phenotypes may make it easier for the species to feed more efficiently, but at the same time make it more susceptible to predators. In that case, the average phenotype represents the balance between the probability of feeding well and the probability of being a victim of a predator, thus conforming the optimal combination that prevails over time.

Stabilizing selection and diversity

As its name implies, stabilizing selection tends to stabilize phenotypes around an average. This causes stabilizing selection to decrease phenotypic diversity in a species. In some cases, the genes responsible for extreme traits may disappear. However, this is not necessarily the case, since most of the phenotypic traits associated with stabilizing selection tend to be polygenic traits. That is, they are traits that do not depend on a single gene but on a combination of many genes that fulfill additional functions.

Stabilizing Selection Examples

Weight or size of pups at birth

In the case of mammals, the size of the offspring usually follows a stabilizing selection, favoring an intermediate size between very small and very large individuals. Those that are very small usually have little chance of surviving as they are too weak. On the other hand, too large calves can endanger the life of the mother at the time of giving birth since it makes it difficult for her to exit through the birth canal.

Litter size or number of pups

The number of offspring of many species also tends to favor an average number that is neither too high nor too low. In many species, the young are very susceptible to environmental conditions and predators, so large litters are needed to ensure the survival of some viable young. However, too large a number of pups makes it difficult to feed them all, resulting in malnutrition in the litter that reduces the chances of survival and reproductive success in the future.

Insect color and camouflage

Another very common example of stabilizing selection is related to the color of some insects. For example, in the case of some green beetles, the more common shade of color may arise as a consequence of stabilizing selection. This will happen if the beetles live in an environment where they are constantly surrounded by medium green plants, which makes it easier for them to camouflage themselves better than if they were dark or too light green.

References

Fowler, S. (2013, April 25). Mechanisms of Evolution – Concepts of Biology . Pressbooks. https://opentextbc.ca/conceptsofbiologyopenstax/chapter/mechanisms-of-evolution/

Gelambi, M. (2019, March 4). What is stabilizing selection? (With examples) . lifer. https://www.lifeder.com/seleccion-estabilizadora/

Khan Academy. (nd). Natural selection in populations (article) . https://es.khanacademy.org/science/ap-biology/natural-selection/population-genetics/a/natural-selection-in-populations

LesKanaris. (nd). Characteristics and examples of stabilizing selection – Interesting – 2022 . https://us.leskanaris.com/2970-stabilizing-selection-in-evolution.html

People per project. (2020, May 30). Characteristics and examples of stabilizing selection . https://en.peopleperproject.com/posts/21293-characteristics-and-examples-of-stabilizing-selection