That living organisms change eventually, adapting to the environment, and even already Ancient Greek thinkers change, guessed. So, for example, Anaksimandr, the representative of miletsky school, considered that all live left water. However long time in biology dominated the provision on the invariance of types. In the 19th century the theory of evolution by means of natural selection was formulated by the English scientist Charles Darwin.
Natural selection
Natural selection is the main evolutionary tool. In the course of existence of a look each its next posterity is exposed to certain mutations. The nature kind of looks for new forms and receptions for the best adaptability of an organism to constantly changing environment. For this purpose all living beings make posterities more, than it "is necessary", than can survive. In population of organisms the hereditary variability which is expressed by a set of any given genetic lines is put. As a result between organisms the competition in survival, and then in an opportunity and the right for reproduction is created. Thus, organisms with heredity, there is more to answering idea of fitness to this environment, have advantages in transfer of the genetic features to the subsequent generation.
As a result of sudden change of environmental conditions it can turn out that demanded are "harmful" alleles (gene forms). At the same time evolution not necessarily leads to increase of complexity of an organism.
Natural selection acts on all levels of the organization – at the level of genes, a cage, an organism, group of organisms and, at last, a look. Selection can act and at the same time on different levels. It must be kept in mind still and the trans-species competition in fight for food resources, for a living space. In that case evolution can lead to extinction of badly adapted look, an example of dinosaurs. The changed conditions promote also emergence of new types on again developed conditions.
Artificial selection
Artificial selection, or selection, is carried out by the person for the purpose of receiving more fruitful agricultural plant or more productive breed of pets. Initially this selection had unconscious, spontaneous character. Over time he received a methodological basis, and selection of couples began to be made for crossing under a definite purpose.
As a result of artificial selection by the person, decorative breeds of pets and plants which in the natural environment would be discarded at once are also removed and inevitably died.
Today artificial selection is carried out already at the genetic level and has fantastic prospects. Against the background of rapid increase in the population of the planet and reduction of land resources under arable lands and pastures, this direction gains invaluably important character.