We constantly encounter the concept of species criteria in life - defining the types of flowers in a flower bed or fish in an aquarium. It can be very useful to be able to distinguish the type of edible mushroom from the poisonous one. But, despite all the seeming simplicity of this concept, in biology, the criteria for a species and the very concept of "species" remain the most ambiguous.
Historical digression
The very concept of "kind" has existed in the concept of people since ancient times. For a long time, a species meant a group of homogeneous objects or objects that meet the criteria of a species. Examples: types of kitchen appliances (frying pan, pot, cauldron) and types of ducks (pintail, teal and mallard). This term was introduced into biology by Carl Linnaeus - under the guise he understood unchanging, discrete (different), objectively existing groups of living organisms. At that time, a typological approach prevailed in biology - the selection of a species was carried out on the basis of several external features.
Today this approach has remained in biology as a morphological criterion of a species. With the accumulation of knowledge in biochemistry, genetics, biogeography and ecology, the requirements for the classification and systematics of all life on the planet expanded. In modern biology, a species is understood as a group of organisms (populations) in which individuals can freely interbreed with each other and produce fertile offspring. At the same time, the main criterion of the species is the impossibility of crossing them with representatives of other species.
Where does this approach not apply
But not all living organisms this approach in determining species is applicable. The species criterion based on reproductive isolation does not apply to organisms that reproduce asexually and by parthenogenesis. The former include all prokaryotes (pre-nuclear, bacteria), the latter - only a few eukaryotes (nuclear), such as rotifers. It is incorrect to use the term "species" in relation to extinct animals.
Evolution to change typology
In 1859, an event occurred that changed the worldview of naturalists and biologists. Charles Darwin's On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life saw the light of day. The author considered the concept of "view" artificial and introduced for convenience.
The achievements of genetics and the development of the theory of evolution have led to the fact that the types of organisms are determined not by differences, buttheir similarity or common gene pool. Now a species is a set of populations that have geographical and ecological commonality, are capable of free interbreeding, and have similar morphophysiological characteristics.
Expanding the definition of the concept
Today, many species criteria are used to systematically position an organism. Each deep ocean trawl raises new creatures from the depths, which biologists are trying to invest in the overall system of the organic world on the planet. This uses a lot of view criteria, and the job is not as easy as it might seem. But for widespread use in biology, the basic criteria of the species, which we all studied in school, are used. We will focus on them.
Classification of species criteria in biology
Species criteria in biology are signs that are inherent in only one species. The combination of these features determines the species of the organism. The main view criteria are:
- Morphological - the totality of all similar features in the structure of the body. It includes all material structures: from chromosomes to the structure of organs, systems and appearance.
- Physiological - the similarity of all vital processes of organisms of the same species. It is at this level that the reproductive isolation of representatives of one species in relation to others is usually recorded.
- Biochemical - this criterion includes the specificity of proteins and nucleic acids, as well as the specificity of enzymatic processes.
- Ecological-geographical - sometimes this criterion is divided into twoindividual. It characterizes the territory of residence of a certain species.
- Genetic - based on the unique set of hereditary material of the species, its quality and structure.
Fractional unit of life organization
The main feature of the species is the common gene pool of its representatives. The unity of the species and its historical stability is ensured by free crossing, which maintains a constant flow of genes within the species. At the same time, the gene pool of a species is constantly changing as a result of mutations, recombinations and natural selection, which becomes the source of the emergence of new species in the process of evolution. Therefore, the species exists, figuratively speaking, only at the moment strictly observed by us.
View types
Description of new species is associated with non-compliance with the criteria of already known species - one or more. The description of the species is based primarily on morphological and genetic criteria. The first draws parallels between external signs, and the second focuses on the genotype. In this regard, in biology, the following types of species are distinguished:
- Monotypic species - all signs, including external ones, are characteristic of all representatives of the species.
- Polytypic species - individuals within a species can have different phenotypes (external qualities), which directly depend on the conditions of their habitat. In this case, the category “subspecies” is used in taxonomy.
- Polymorphic view - in this case, there are several morphoforms within the view(groups of individuals with different colors or other characteristics) that interbreed freely.
- Twin species. These are species that are morphologically similar, live in the same territory, but do not interbreed. More on this concept later.
- “Semi-species”, borderline cases – sometimes the process of speciation endows a group of organisms with traits that change the status of the group. This is a rather complicated category in taxonomy, and often the selection of a species as a semi-species meets with a lot of controversy among biologists-specialists.
Intraspecific diversity
Most species of living organisms on the planet belong to the polymorphic type. Many insects (bees, termites, ants) have developed working polymorphism. Within the species, females, males and workers are distinguished. Such categories are called "castes".
Under the influence of various environmental factors, ecological polymorphism arises. In stag beetles, there are males with different lengths of mandibles - their development is directly related to the conditions for the development of the larva. Under the influence of seasonal factors, seasonal polymorphism arises, when different generations of the same species differ from each other. For example, in the variegated butterfly (Araschnia levana), the generation born in early spring has red wings with black spots, and the summer generation has black wings with white spots.
An example of polymorphism in the biological species Homo sapiens is the presence of four blood groups, a variety of hair colors and skin colors. That's whyall racial prejudices have no biological justification, since all people on the planet are just different morphoforms of a single species of Homo sapiens, and all human races are at the same biological level of development. The undeniable proof of this statement is interracial marriages, as well as the presence of talented artists and scientists among representatives of all races and nationalities.
Twins in nature
Not a very common phenomenon in nature - the existence of two species in the same territory, very similar in appearance, morphology and anatomy, but at the same time incapable of crossing with each other. Most often, such species are found among those animals that choose a sexual partner according to a specific criterion, for example, by smell (insects or rodents) or by the acoustic features of singing (birds).
An example of twin species is that we call malarial mosquitoes 6 species of outwardly identical insects that differ in the shape and color of eggs.
