The stabilizing selection, also known as purifying, it is one of the three main ways by which natural selection acts on a certain quantitative and heritable character.
Generally, this type of selection occurs on a particular trait and maintains its size over the generations. In constant environments it is presumably the most common selection model in nature.
This type of selection is responsible for preserving the average characteristics of a population, favoring the reproduction of these individuals..
Natural selection is capable of modifying the parameters - average and variance - of a character in the population. This continuous character is plotted in a normal distribution curve or bell graph (see graph in the image above).
The way in which the selection modifies this normal curve will allow us to conclude whether the selection is diversifying, directional or stabilizing..
In the stabilizing selection model, the population mean does not change over the generations, while the variance decreases (since this type of selection eliminates the extreme values, and the character begins to be more homogeneous).
Although we could think that the stability of the mean in a population could indicate that there are no evolutionary forces acting on it, the phenomenon could be explained by the presence of a strong stabilizing selection.
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Before talking about the types of selection, it is necessary to understand what natural selection is. Although it is a very popular concept, it is surrounded by misunderstandings.
Natural selection is a mechanism that generates changes in populations over time - that is, evolution. This admirable idea was proposed by Charles Darwin in 1859 and it revolutionized all fields of biology. Today, it remains the mainstay of modern evolutionary biology.
Natural selection is differential reproductive success and occurs in the population as long as three conditions occur: 1. there is variation, 2. these variations are heritable (that is, they pass from parents to children) and 3. some variations are associated with an advantage in reproduction (in more exact terms, certain variations have greater biological suitability).
In this way, natural selection is directly related to the reproduction of the individual and not to the "survival of the fittest" and other viral phrases with which we usually associate the concept.
The stabilizing selection acts as follows: in the frequency distribution of the phenotypic characters, the individuals that are in the center of the curve are selected, that is, the most frequent individuals in the population..
This phenomenon occurs because average individuals have greater fitness or biological efficacy. In other words, this average trait gives individuals who carry it some advantage in reproduction - over their peers who do not have the average value of that trait..
This pattern is common in nature, particularly in environments where conditions are stable for long periods of time..
To determine the type of selection that a particular population is undergoing, biologists quantify a trait in the population over generations and observe the change in the parameters of the trait..
As a measure of central tendency, the arithmetic average of the character is usually calculated: the mean. For example, we can evaluate in a human population the weight of a number of its members and calculate the average, say 62 kilos.
However, knowing the mean is not enough and it is also necessary to determine a value that indicates the homogeneity or heterogeneity of the data..
The variance, on the other hand, allows us to know how the values of the sample are dispersed around that average..
In the stabilizing selection model, we expect to find that the mean remains constant as generations pass..
Let's imagine that we are evaluating the evolution of weight in human populations and we calculate the average over several generations. In our results, we see that the average remains constant. We could mistakenly think that the forces of selection are not acting in this population..
Therefore, it is important to also calculate the variance. In this selection model, we would expect a reduction in the variance over time.
In its simplest form, stabilizing selection would tend to reduce variation within populations. However, the decrease in variation occurs at the level of trait variability and does not have to lead to a decrease in genetic variability..
Remember that there are natural mechanisms that generate variability. Furthermore, in many cases, the optimum of a trait is not the same for all phenotypes in a population..
The example that best illustrates the selection model is the weight of human babies at birth. This phenomenon was reported in different countries, including the United Kingdom, the United States, Italy, Japan, among others between 1930 and 1940..
Heavier or lighter babies did not have as high survival rates - compared to average individuals.
We observe the same phenomenon of size stabilization in newborns in the births of other animals and in the laying of their eggs..
It is likely that the stabilizing selection has acted with greater intensity until the arrival of the cesarean section and the effective prenatal care that we see today.
In fact, some studies conducted in the mid-1950s concluded that the selective pressures that led to the birth of average-size babies have been relaxed excessively. By the 1980s and 1990s, the pattern had almost completely disappeared in developed countries.
Larger babies that were previously a complication of delivery can now be delivered using cesarean section techniques. The other extreme, the smallest babies, manage to survive thanks to extensive medical care.
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