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Criteria are signs by which individuals of one species differ from individuals of other species.
The main criteria are morphological, physiological, genetic, biochemical, ecological, geographical and historical.

The morphological criterion characterizes the features of the external and internal structure of individuals of a given species. So, different types of plantain differ in the shape, size and arrangement of leaves, in the structure of the stem.

The physiological criterion characterizes the features of vital processes (reproduction, metabolism, irritability, etc.). The most important trait is the ability to interbreed freely and produce fertile offspring. Individuals of different species usually cannot interbreed and give offspring due to the different structure of the reproductive organs, differences in mating behavior, and incompatibility of germ cells.

A genetic criterion is a certain set of chromosomes with a characteristic structure and composition. This is the most important criterion as it determines the reproductive isolation of a species.

The biochemical criterion reflects the similarity of the chemical composition (proteins, nucleic acids, etc.) and the peculiarities of metabolism in individuals of a given species.

The ecological criterion is the features of environmental factors necessary for the vital activity of individuals. Each species can only inhabit an environment where the climate, soil characteristics, topography and food sources are consistent with its endurance limits.

Close species inhabiting the same territory occupy different ecological niches. Thus, the great spotted woodpecker feeds on pine and larch seeds in winter. The black woodpecker extracts the larvae of barbel from under the bark of spruce trees, and the lesser spotted woodpecker extracts insects from the wood of alder or stems of herbaceous plants.

The geographical criterion characterizes the territory (range) in which individuals of a given species live and go through the full cycle of their development.

The historical criterion characterizes the origin of this species. Based on the study of the history of a group of closely related species. This criterion is complex in nature, since it includes a comparative analysis of the modern ranges of species (geographical criterion), a comparative analysis of genomes (genetic criterion), etc.

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