An example of stabilizing selection and driving selection. Examples of the action of stabilizing selection

A typical example of stabilizing selection is a natural selection associated with individuals having a deviation of the main features, in comparison with the average norm.

Features of selection

Each generation gets rid of individuals that differ from the optimal average parameter by certain characteristics. An example of stabilizing selection in living nature is related to the preservation of the state of the population. For a full-fledged existence, its representatives try to select the maximum conditions for fitness for certain conditions.

Variants in nature

An example of stabilizing natural selection in nature is the maximum contribution to the gene pool of new generations from the most prolific individuals. But scientists managed to prove by carrying out numerous observations of the natural populations of mammals and birds, which in fact the situation is somewhat different. If there is a large number of chicks in one nest, it is rather difficult to feed them, so they are much smaller and weaker than those that grow in the average number. Thus, the researchers succeeded in establishing reliably the examples of the action of stabilizing selection, confirming adaptability to survival in birds with an average degree of fertility.

The choice in favor of average values

When comparing birds with different numbers of offspring, it turned out that there are several signs that characterize examples of the stabilizing form of selection. Newborn mammals, having a negligible weight, as well as too much body weight, mostly died at the 1-2 nd week of life. As for the cubs with average parameters, they easily tolerated the first weeks of their existence, developed, died in minimum quantities.

Consider another example of stabilizing selection associated with birds. When during the experiment it was decided to analyze the size of the wings in birds that died after a violent storm, it turned out that most of them had either too short or, on the contrary, very long wings. This example of stabilizing selection also indicates a better survival rate of individuals with moderate characteristics.

The reasons for the appearance of small fitness

Considering this example of the action of the stabilizing form of natural selection, let us try to identify the main reasons for the small fitness of individual individuals to the permanent conditions of existence. Why, with the help of natural selection, is it impossible to clear a certain population from evading undesirable forms? The reason is not only that, as new offspring are born, various mutations take place, but also because heterozygous genotypes are often adapted individuals. In the process of crossing, they give a splitting in the offspring, and new homozygous generations arise, in which adaptation to survival conditions is significantly reduced. This phenomenon was called balanced polymorphism.

Examples of polymorphism

The main examples of the stabilizing form of natural selection (polymorphism) are sickle cell anemia. This severe blood disease is observed in people homozygous for hemoglobin with mutant alleles (HbS), leading to death at a young age. Most of the human populations have a low frequency of this allele, it is associated with certain mutations. But scientists managed to establish the relationship between the presence in the human body of the gene and the presence of malaria in the area. The results of the studies showed that HbS heterozygotes have a higher resistance to a disease such as malaria than homozygotes with a normal allele.

The mechanism of variability

Examples of stabilizing and driving selection have a certain mechanism of accumulation of signs of variability in natural populations. For the first time such a distinctive feature of the stabilizing selection was noted by the outstanding scientist II Shmalhausen. He managed to prove that even under stable conditions of existence, natural selection does not stop for a minute, evolution continues. Even with the same phenotype, the population continues to evolve. The example of the action of the stabilizing selection form considered by him confirmed the constant change in the genetic composition. Due to the stabilizing selection, genetic schemes are created, with the help of which it is possible to create optimal phenotypes from a variety of genotypes.

The purpose of the stabilizing form of natural selection

It is able to protect the formed genotype from the negative impact of the mutational process. An example of the action of the stabilizing form of selection is the existence of such ancient species as ginkgo, hatteria. It is the stabilizing selection that has preserved to our time the "living fossils" that live in stable environmental conditions:

1. Hatteria, which has the features of reptiles that existed during the Mesozoic era.
2. The coelacanth, which is a descendant of brushpiper fish, familiar to the Paleozoic era.
3. The North American possum, which is a marsupial animal existing from the Cretaceous period.
4. Plant gymnospermous species of ginkgo, similar to tree forms, extinct during the Jurassic Mesozoic era.

There is such a stabilizing form of natural selection until such time as there are conditions under which a certain attribute or property was formed.

Influence of ecology on variability

Constant conditions are not necessarily unchanged over a long period of time. In connection with the constant changes in ecological conditions, adaptation takes place with the help of stabilizing selection of certain individuals to them. There is a change in breeding cycles, so that the young ones that have appeared develop during the time period when there are enough food resources to sustain life. If descendants are born earlier or later than the expected dates, they are eliminated by stabilizing selection. How do plants and animals "know" about the onset of winter? Short-term temperature drops are very deceptive. In addition, the boundaries of summer and winter are shifting yearly. Animals that hastily respond to signals can remain without offspring. Therefore, many birds and mammals are guided by the duration of daylight hours. It is this signal for many animal species that stimulates the launch of important functions: molting, migration, reproduction. II Shmalhausen succeeded in proving the connection between universal adaptation and stabilizing selection.

Variations of the deviation from the norm

Stabilizing selection completely sweeps all deviations from the established norm, promotes the formation of genetic mechanisms that ensure the full development and formation of ideal phenotypes on the basis of different genotypes. The result will be a full functioning of organisms even with fluctuations in the external environment.

The teachings of A. Wallace and Charles Darwin

The theory of natural selection was created as the main creative force that directs the evolution process and determines its forms. The natural selection was to consider the process by which only individuals surviving and having offspring possess hereditary traits useful for specific habitat conditions. When assessing natural selection from the point of view of genetics, it can be concluded that it is important for the selection of positive mutations and genetic combinations. They can manifest themselves through sexual reproduction, and as the population grows, they are improved by culling out negative combinations and mutations.

Those organisms that have poor-quality genes are not able to survive in certain conditions, perish. Natural selection is able to "work" on the basis of reproduction of living organisms, if weakened individuals are not ready for a full-fledged offspring or do not leave offspring at all. In this case, not only does selection and culling of certain negative qualities of a living organism take place, but genotypes that bear similar signs are completely destroyed.

On the forms of natural selection

At the moment, it is customary to select the following forms of such selection, it is they who are referred to in the textbooks of biology in schools.

1. Stabilizing natural selection.
2. Motion selection.
3. Tearing off selection.

Motion selection is typical for changing natural conditions, under which a factor that has become mutational appears. For example, industrial melaninogenesis, characteristic of butterflies of birch moth, is associated with darkening due to industrial soot of birch trunks. Since the insects were visible against the background of "new" trees, they were quickly destroyed by birds. Dark butterfly mutants survived, gave offspring, and therefore gradually dark butterfly mutants became the dominant form for this population.

Due to the shift of the average value towards the existing factor, the occurrence of cold-loving and thermophilic animals and plants is explained. Driving selection led to the adaptation of bacteria, fungi, other pathogens of human diseases, animals to various pesticides and medicines. Motion selection can explain the occurrence of eye reduction in cave dwellers and moles, as well as the loss in some birds of wings. With a similar selection, no branching of the characteristics occurs, as a result of which the carrier genotypes are gradually replaced by others, without forming evasive and transitional forms.

The tearing selection allows to obtain extreme types of adaptations, with all intermediate forms dying out. Due to the disseveral selection, two or more forms of variability are formed, which lead to polymorphism. It is the struggle for existence that is the important factor that is the main mechanism of any natural selection. Competition, predation, amensalism are considered to be the three main types of struggle for existence.