Mutation, population fluctuation, isolation create genetic heterogeneity within a species. But their effect is undirected. Evolution is a directed process, associated with the development of adjustments, progressive complexity of the structure and functions of animals and plants. There is only one directional evolutionary factor – natural selection.
Natural selection can affect either certain individuals or entire groups. As a result of group selection, there are often features and properties, unfavorable for a single individual, but useful for the whole population and species (after attack a bee dies, but by attacking the enemy, it keeps the family safe). In any case, natural selection leaves the most adapted organisms to this environment, and acts within the population. Thus, it is the population, which is the field of selection.
Under natural selection, you need to understand the selective (differential) reproduction of genotypes (or gene complexes). What matters in the process of natural selection are not the survival or death of individuals as their differential reproduction. Success in breeding different species can serve as an objective criterion for the genetic-evolutionary natural selection. The biological significance of an individual, which gave the offspring is determined by the contribution of its genotype in the gene pool of the population. Natural selection from one generation to another by the phenotype leads to the selection of genotypes, as the descendants inherit not the traits, but gene complexes. For evolution not only genotypes and phenotypes, which are important, but phenotypic variation.
In the process of expression, gene can influences many traits. Therefore, the scope of the selection may include not only the properties that increase the likelihood of offspring, but also features that are not directly related to reproduction. They are selected indirectly as a result of correlations.
Like other basic evolutionary factors, natural selection causes changes in the ratio of alleles in the gene pools of populations. In evolution, natural selection keeps the creative role.
Excluding from reproduction genotypes with low adaptive value, natural selection keeps favorable gene combinations of different denominations and converts the pattern of genotypic variability, emerging initially under the influence of random factors going in biologically expedient direction.
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