As we know from biology, for example, a relict plant as ginkgo, or Crossopterygii fish such as the coelacanth, there are virtually no changes about a million years.How can this be explained by the stability of species in a permanent mutation processes in nature?The answer to this question is given by the consideration of the concept of "stabilizing selection" - the doctrine, which was designed by renowned evolutionist IISchmalhausen.
This type of selection can only be observed at constant ambient conditions for a sufficiently long time.That is why the same medium, generally are typical and well adapted to its individual, but differ somewhat from these mutants should die.Stabilizer selection is confirmed by a sufficient number of examples.So, after strong winds and snowfall in North America was found about 136 half-dead sparrows stunned, survived only 72 of these individuals.Among them were sparrows having average wings, due to which they had more endurance than those who have had the wings of a very long or very short.
stabilizing selection process results in phenotypic homogeneity of the population.With its long-term effects on the environment may give the impression of the immutability of species or populations.However, the immutability is only apparent and may concern only appearance.At the same gene pool is undergoing some changes (a mutation).
stabilizing selection characteristic of the man.An example is Down syndrome.It is known that for such violations result in hereditary disease on small 21-22 pairs of chromosomes.In case of deviations in the amount and form of large chromosomal death it occurs fertilized eggs.Also, spontaneous abortions often cause the death of embryos with chromosomal abnormalities in the middle.
stabilizing selection for hundreds of thousands of generations is responsible for the conservation of species of significant changes by omitting the promutirovavschie form.Without him there would have been some resistance in nature.
Along with the already discussed stabilization, there are still individual selection, which is useful to consider the example of the plant.Its essence is to determine the quality of the selected plants with the help of an individual assessment of their offspring.In contrast to the mass selection, with individual seeds of selected plants are not mixed and sown separately for families in certain areas to obtain a qualitative assessment of their offspring.Through this approach, it is possible to receive the selection of genotype and phenotype does not.
unconscious selection is more suitable to man, and made no specific pre-set goals.Thus, Darwin proved that, for example, if the peasant economy in two cows for meat slaughter he is the one that gives less milk and chickens for meat among them will be used poor hen.And in one, and in the second case the farmer in order to maintain productive animals, making the selection, do not set ourselves the goal of developing new breeds.It is in this more primitive form of Darwin and shows an unconscious selection.
studying the life of the peoples of Australia and South Africa, scientists are increasingly convinced of the correctness of their judgments.Unconscious selection for a long time produced a man with the domestication of wild animals.That this was the formulation of the important conclusion that in agriculture new species of animals and plants can be obtained only by means of sampling.Thus, from the doctrine of the unconscious selection of Darwin "follows" the other, no less important, the teaching about artificial selection, which is formulated in the form of law, to subjugate the whole process of breeding new plant varieties and animal breeds.
