In the process of practical human activity, the concept of form was formed. When describing animals, this concept was already used by Aristotle. However, for a rather long period the definition of “species” was not endowed with scientific content and was used as a logical term. The considered concept acquired the status of a classification unit in the development of systematics. John Ray (English naturalist) developed the idea of the species as a component of taxonomy. At the same time, scientists identified three most important characteristics of this unit. Thus, the species, according to Ray, is a multitude of organisms that share a common origin. This systematic unit combines organisms similar in morphological and physiological characteristics. In addition, it is a self-reproducing system.
Rei considered the origin to be the main indicator. So, the naturalist attributed similar plants to one species, reproducing their own kind from their seeds.
A significant expansion of the concept of species, as well as its deepening, was due to the work of Linnaeus, who showed that the species is a real elementary and stable unit of living nature, isolated from other species. This concept began to be applied as the main classification of animals and plants. However, in those days, the species was considered as a consequence of creative action.
Lamarck proclaimed in his writings a provision that in nature there are no immutable systematic units of plants and animals. Species are constantly turning, changing, moving into other species. In this regard, according to Lamarck, the old systematic unit cannot be separated from the new. Thus, the French naturalist came to the conclusion that the reality of the species was denied, while affirming the idea of development.
Darwinism was based on a different position. This position was scientifically substantiated. In accordance with it, a developing real species is determined by historical development under the influence of natural selection. In accordance with Darwinian teachings, a comprehensive study of systematic units was carried out. So, the morphological criterion of the species was studied, as well as an experimental, genetic study of the structure and ways of its formation. These events were crucial in substantiating the population aspect of the systematic unit as the main form of development and existence of the organic world as a whole.
Today it is believed that the organic environment includes a variety of life forms. In this case, the "view" is a universal phenomenon for all wildlife. The systematic unit under consideration is formed in the course of evolutionary transformations caused by natural selection. As a result, it represents a specific stage (link) in the development of living organisms and is the main form of existence on the planet of life.
One species differs from another in a set of common features - criteria. Together, these signs form the reality of systematic units.
The morphological criteria of a species are based on the presence in all individuals of one species of certain inherited traits. Individuals within the framework of one systematic unit, in other words, have a similar external and internal structure. The morphological criterion of the species is considered quite convenient and simple sign. Moreover, this characteristic was used by taxonomists earlier than other signs and for a certain period was the main one. However, it should be noted that the morphological criterion of the species is rather relative. This symptom is necessary, but insufficient. The morphological criterion of the species does not allow us to distinguish between systematic units that have significant similarities in structure, but do not cross each other. For example, systematic twin units. So, the name "malaria mosquito" includes about fifteen species, indistinguishable externally, but previously considered one species. It has been established that about 5% of all systematic units are twins. Thus, the morphological criterion of a species cannot be the only sign of difference.