/ / Example of stabilizing selection and driving selection. Examples of the action of stabilizing selection

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 whodiffer 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.

example of stabilizing selection

Variants in nature

An example of stabilizing natural selection innature - 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, researchers were able to reliably establish examples of the effect of stabilizing selection, confirm adaptability to survival in birds with an average degree of fertility.

The choice in favor of average values

When comparing birds with different number of offspringit turned out that there are several signs that characterize the examples of the stabilizing selection form. 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.

Let us consider one more example of a stabilizingselection, associated with birds. When during the experiment it was decided to analyze the size of the wings of 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 of individuals with medium signs.

example of stabilizing natural selection

Causes of low fitness

Considering this example of actionstabilizing forms of natural selection, let us try to identify the main reasons for the small fitness of individual individuals to 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 the survival conditions is significantly reduced. This phenomenon was called balanced polymorphism.

example of a stabilizing form of selection

Examples of polymorphism

Basic examples of the stabilizing formnatural selection (polymorphism) - 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.

examples of the stabilizing form of natural selection

The mechanism of variability

Examples of stabilizing and driving selection havea 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 stabilizing form of selection that he considered confirmed the constant change in the genetic makeup. Thanks 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.

Examples of the action of stabilizing selection

The purpose of the stabilizing form of natural selection

It is able to protect the formed genotypefrom the negative impact of the mutational process. An example 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 "living fossils" that live in stable environmental conditions:

  1. Hatteria, which has the features of reptiles that existed during the Mesozoic era.
  2. Latimeria, which is a descendant of brushpiper fish, familiar from the Paleozoic era.
  3. The North American opossum, 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 has been formed.

example of the action of the stabilizing form of selection

Influence of ecology on variability

Constant conditions are not necessarilyare 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 youngsters 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 species of animals 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 away alldeviation from the established norm, contributes to 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 of the environment.

example of the stabilizing form of natural selection

The teachings of A. Wallace and Charles Darwin

The theory of natural selection was created asthe main creative force that guides the process of evolution and determines its forms. A natural selection was to consider the process by which only individuals surviving and having offspring possess hereditary traits useful for specific conditions of life. When evaluating 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 rejecting negative combinations and mutations.

Those organisms that have poor-quality genes,they can not survive in certain conditions, they perish. Natural selection is able to "work" on the basis of the 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 the selection and culling of certain negative qualities of a living organism take place, but genotypes that carry 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 naturalThe conditions under which a factor appears that has become mutational. For example, industrial melaninogenesis, characteristic for butterflies of birch moth, is associated with darkening due to industrial soot trunks of birches. 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 to the sidethe existing factor explains the emergence of cold-loving and thermophilic animals and plants. Driving selection led to the adaptation of bacteria, fungi, other pathogens of human diseases, animals to various pesticides and medicines. Motion selection allows one to explain the occurrence of eye reduction in cave dwellers and moles, as well as the loss of wings in some birds. With such a 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.

A tearing selection allows obtaining extreme speciesadaptations, 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.

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