Criteria of the species: morphological, physiological, genetic, environmental, geographical, historical, ethnological.
The morphological criterion is the similarity of the external and internal structure of organisms. Karl Linnaeus, for example, defined species as integral groups of organisms that are distinct from other life forms in terms of structure. In other words, the presence of structural features that make a certain group of organisms similar to each other and at the same time different from all other groups is the criterion for classifying them as a given species.
Individuals within a species are sometimes so variable that only by morphological criterion is it not always possible to determine a species. There are species morphologically similar. These are double species that are open in all systematic groups. For example, in black rats, two double species are known – with 38 and 49 chromosomes; the malarial mosquito has 6 double species, and the small pinch fish, which is widespread in fresh water bodies, have 3 such species. Twin species are found among a wide variety of organisms: fish, insects, mammals, plants, but individuals of such twin species do not interbreed.
The genetic criterion is a set of chromosomes characteristic of each species; their strictly defined number, sizes and shapes, DNA composition. The chromosome set is the main species trait. Individuals of different species have different sets of chromosomes; therefore, they cannot interbreed and are reproductively limited from each other in natural conditions.
The physiological criterion is the similarity of the body’s reactions to external influences, the rhythms of development and reproduction. The basis of this criterion is the similarity of all processes of life, and especially reproduction. Representatives of different species, as a rule, do not interbreed or their offspring are sterile. However, there are exceptions. For example, dogs can breed by mating with wolves. Hybrids of some species of birds (canaries, finches), as well as plants (poplar, willow) can be prolific. Therefore, the physiological criterion is also insufficient to determine the species affiliation of individuals.
An ecological criterion is a position characteristic of a species in natural communities, its connection with other species, sets of environmental factors necessary for existence.
A geographical criterion is an area of distribution, a specific range occupied by a species in nature.
The historical criterion is the community of ancestors, a single history of the appearance and development of the species.