There are three forms of natural selection: driving, stabilizing, and disruptive (disruptive).
Driving selection is natural selection, which acts under a directional change in environmental conditions.
Individuals with traits that match the changing environment gain benefits. At the same time, other manifestations of signs are not preserved. As a result, in the population, from generation to generation, the average value of the trait shifts in a certain direction. A new average rate is being formed instead of the existing one, which has ceased to comply with the new conditions.
As a result of motive selection, some traits disappear while others are formed. If selection acts in one direction for a long time, then it can lead to the transformation of the population into a new species.
- discoloration of the birch moth in industrial areas (industrial melanism);
- reduction of eyes in a mole, digestive system in parasitic tapeworms;
- increased root length in sclerophytes;
- the transformation of the forelimbs into burrowing ones in different groups of animals during the development of the soil as a habitat;
- the formation of antibiotic resistance in bacteria;
- decreased sensitivity of parasitic fungi to pesticides;
- changes in the structure of the limbs in the ancestors of the horse in connection with the development of open spaces.
Limb changes in horse ancestors
Stabilizing selection is natural selection that preserves the average phenotype and removes any aberrant phenotypes.
It manifests itself in constant and favorable environmental conditions for the population.
Stabilizing selection preserves the average value of the trait and narrows its reaction rate. Eliminates trait deviations from the norm, preserves the fitness of the species. Thanks to stabilizing selection, many ancient groups of organisms still exist, for example, ginkgo, coelacanth.
Selection in favor of mean values was found for a variety of features.
- death during a strong storm of birds with too short and too long wings;
- more frequent death at birth or in the first weeks of life of newborn mammals with very low and very high weight;
- preservation of individuals with optimal limb length in hare populations;
- preservation in plants pollinated by insects, a certain structure of the flower, corresponding to the size of insects;
- the existence of relict species of organisms that have remained unchanged for millions of years (cross-finned fish coelacanth, ginkgo, tuatara, etc.).
Disruptive (disruptive) selection is natural selection, in which extreme variants of a trait are preserved, and its mean values are removed.
As a result, several new forms may appear from one original one. Disruptive selection contributes to the emergence and maintenance of population diversity. It leads to divergence (divergence of characters) and the formation of several species from one source.
- the formation of two races of the rattle on hay meadows – early flowering and late flowering;
- the existence of spring and autumn forms and species of fungi due to the recurring drought in the middle of summer;
- the emergence of different subspecies and types of tits in connection with food specialization;
- the emergence of different types of clover.