As is known from biology, for example, suchrelict plant, like a ginkgo, or such a cross-shaped fish, like the coelacanth, exist almost unchanged for about a million years. How can such a stability of a species be explained in the presence of a constant mutation process in nature? The answer to this question is given by the consideration of the concept of "stabilizing selection" - a teaching that was developed by the well-known evolutionist I.I. Schmalhausen.
This type of selection can only be observed inconstant environmental conditions for quite a long time. That is why typical and well-adapted individuals are mostly in a constant environment, and mutants that are somewhat different from them must die. 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 tophenotypic population homogeneity. With its long-term environmental impact, one may get the impression that the species or population is unchanged. However, this immutability is only apparent and can relate only to appearance. At the same time, the gene pool undergoes certain changes (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, fertilized eggs die. Also, spontaneous abortions are often caused by the death of embryos with abnormalities in the middle chromosomes.
Stabilizing selection for severalHundreds of thousands of generations are responsible for saving species from significant changes, discarding mutated forms. Without it, some stability in living nature would not have been achieved.
Along with the stabilization,there is also individual selection, which is advisable to consider on the example of plants. Its main essence lies in determining the quality of selected plants with the help of an individual assessment of their offspring. In contrast to the mass selection, with individual seeds of the selected plants are not mixed, and sown in families separately in separate areas to get a qualitative assessment of their offspring. Thanks to this approach, you can get a selection of genotypes, but not on the phenotype.
Unconscious selection is more applicable to a personand produced without a specific pre-set task. Thus, Darwin proved that, for example, if a peasant in the farm has two cows, he slaughters exactly the meat that gives less milk, and among hens, hen will use bad layers for meat. Both in one and in the second case, the peasant, seeking to preserve productive animals, makes the selection, without setting himself the goal of breeding new breeds. It is in such a somewhat primitive form that Darwin shows unconscious selection.
Studying the lives of the people of Australia and South Africa,the scientist became increasingly convinced of the correctness of his judgments. Unconscious selection for a long time was made by man at the domestication of wild animals. It was this that served to formulate the important conclusion that in agriculture new species of animals and plants can be obtained only through selection. Thus, Darwin’s theory of the unconscious selection “derives” another, no less important, theory of artificial selection, which is formulated in the form of a law subordinating the entire process of breeding new varieties of plants and animal breeds.
