As is known from biology, for example, a relic plant such as ginkgo, or a cysterae fish such as coelacanth, has been virtually unchanged for about a million years. How can such species stability be explained in the presence of a constant mutational process in nature? The answer to this question is given by considering the concept of “stabilizing selection” - a doctrine that was developed by the famous evolutionist II. Schmalhausen.
The indicated type of selection can only be observed under constant environmental conditions for a sufficiently long time. That is why in an unchanged environment, mainly, there are typical and well-adapted individuals, and mutants that are slightly different from them should die. A stabilizing selection is confirmed by a sufficient number of examples. So, after strong winds and snowfalls in North America, about 136 half-dead stunned sparrows were found, only 72 of them survived. Among them were sparrows with medium wings, thanks to which they had more stamina than those who had wings either very long or very short.
Stabilizing selection can lead to phenotypic homogeneity of the population. With its long-term impact on the environment, the impression of the invariability of the species or population may be created. However, this immutability is only apparent and may relate only to appearance. In this case, the gene pool undergoes certain changes (a mutation occurs).
Stabilizing selection is also characteristic of man. An example is Down syndrome. After all, it is known that violations of small 21-22 pairs of chromosomes lead to such a hereditary disease. In the event of deviations in the number and shape of large chromosomes, the death of fertilized eggs occurs. Also, spontaneous abortions are often caused by the death of embryos with abnormalities in the middle chromosomes.
Stabilizing selection for several hundred thousand generations is responsible for the conservation of species from significant changes, rejecting mutating forms. Without it, some stability in wildlife would not have been achieved.
Along with the stabilization already considered, there is also individual selection, which is advisable to consider with the example of plants. Its main essence is to determine the quality of selected plants using an individual assessment of their offspring. Unlike mass selection, in case of an individual, the seeds of selected plants are not mixed, but are sown separately in separate plots for families in order to obtain a qualitative assessment of their offspring. Thanks to this approach, it is possible to obtain selection by genotype, and not by phenotype.
Unconscious selection is more applicable to a person, and is done without a specific predetermined task. Thus, Darwin proved that, for example, if the peasant has two cows on the farm, he will slaughter the one that produces less milk, and among the hens they will use poor laying hens for meat. In one and in the second case, the peasant, seeking to preserve productive animals, makes a selection without setting itself the goal of breeding new breeds. It is in this somewhat primitive form that Darwin shows unconscious selection.
Studying the life of the peoples of Australia and South Africa, the scientist became more and more convinced of the correctness of his judgments. Unconscious selection from ancient times was carried out by man when taming wild animals. It was this that served as the formulation of an important conclusion that in agriculture new species of animals and plants can be obtained only through selection. Thus, another, no less important, doctrine of artificial selection, which is formulated in the form of a law that subjugates the entire process of breeding new varieties of plants and animal breeds, “follows” from Darwin’s doctrine of unconscious selection.