None of the species criteria are absolute and universal. Each has a relative character and cannot be used as the only one to determine the belonging of an individual to a particular species.
The morphological criterion is the most convenient for use, but it has a number of limitations.
There are morphologically similar sibling species (for example, six independent outwardly indistinguishable species of mosquitoes of the genus Anopheles, two species of the black rat, several species of barbel, leaf beetle, etc.).
Individuals of the same species can differ significantly in structure. For example, the common viper is represented by many color forms (black, gray, bluish, greenish, reddish and other shades). Differences between organisms of the same species can result from mutations.
For example, locusts are found in two main forms: solitary and gregarious, which are well distinguishable morphologically.
Differences between organisms of the same species appear under the influence of different living conditions. For example, freestanding pines differ in crown shape from pines grown in dense plantings.
This means that on the basis of one morphological criterion, it is impossible to draw a conclusion about the belonging of an individual to a certain species.
On the basis of the physiological criterion, it is also not always possible to draw an unambiguous conclusion about the belonging of an individual to a certain species, since the vital processes in different species can proceed in the same way.
One of the most important features in the study of a physiological criterion is the ability of individuals to interbreed and produce fertile offspring. But individuals of the same species sometimes cannot interbreed. For example, dandelion plants in open areas bloom earlier than similar plants growing in the shade. Pollination between them may not be possible.
On the contrary, there are cases when different species interbreed and give viable offspring. For example, a horse and a donkey, representatives of some species of willows, poplars, hares, canaries, etc. Many interspecific and intergeneric hybrids of cultivated plants are known.
Consequently, to determine the species belonging of individuals, it is not enough to compare them only by a physiological criterion.
The genetic criterion is the most reliable of the species criteria. But it is not absolute either. The number, shape and size of chromosomes in individuals of the same species may differ, this happens as a result of mutations. In different species, the number of chromosomes may be the same, and their shape is similar. So, many species from the legume family have 22 chromosomes.
This means that the similarity according to this criterion is also not enough to classify individuals to the same species.
The biochemical criterion is the most unreliable. There are no substances or biochemical processes that are characteristic of only one species. Individuals of the same species can differ significantly in these indicators, and the mechanism of synthesis of proteins and nucleic acids in individuals of different species is identical. Many biologically active substances perform the same function in the metabolism of different species. For example, chlorophyll in all green plants participates in photosynthesis.
This means that the definition of species on the basis of one biochemical criterion is also impossible.
The ecological criterion also cannot be the only one in determining the species of an individual, since organisms of the same species can adapt to different conditions and live in very different places. For example, different populations of Scots pine inhabit swamps and sand dunes.
Organisms of different species are adapted to the same environmental conditions; therefore, any biotope in nature is inhabited by a community of a wide variety of organisms.
The geographic criterion also has a number of limitations.
The species range is changing. It can expand or, conversely, decrease.
The ranges of many species overlap or overlap. There are many species that do not have clear boundaries of dispersal, but live everywhere (bacteria, some species of lichens). Some species do not have their own range, since they live next to humans (housefly, house mouse, etc.).
Many species have a multi-part range. For example, linden grows in Europe, it is found in the Kuznetsk Alatau and Krasnoyarsk Territory. The ruptured area is in the white stork.
Due to these circumstances, the geographical criterion, like others, is not absolute.
None of the considered species criteria is the main or the most important. For a clear separation of species, it is necessary to carefully study them according to all criteria.