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in biology, a category of classificationclassification,
in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and
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, the original and still the basic unit in the demarcation of plant and animal types. The species marks the boundary between populations of organisms rather than between individuals. Because related species are not absolutely permanent (see evolutionevolution,
concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
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), a precise definition of the term is difficult. On the basis of genetics, scientists now include in a species all individuals that are potentially or actually capable of interbreeding and that share the same gene pool. The latter term refers to that collection of characteristics whose combination is unique in the species, although each individual of the group may not display every single one of the characteristics (see geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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). In the few cases where members of different species can interbreed, the offspring are usually sterile (e.g., the mule). Groups distinguished by lesser differences than those marking a species are called variously subspecies, varieties, races, or tribes.



the basic structural unit in the system of living organisms, a qualitative stage in their evolution. As a consequence of this, the species is the basic taxonomic subdivision in the classification of animals, plants, and microorganisms. In sexual, cross-fertilizing organisms, which include virtually all animals, a significant portion of plants, and a number of microorganisms, the species is the aggregate of populations of specimens able to crossbreed by generating fertile off-spring. As a result of this, these populations produce transitional hybrid populations between the local forms, which inhabit a definite area of distribution (territory or body of water), possess a number of common morphophysiological features and types of relationships with the abiotic (inert) and biotic (live) environment, and are remote from other similar groups of specimens in their virtually complete inability to interbreed under natural conditions.

The accumulation of information on the diversity of animal and plant forms at the end of the 17th century led to the notion of the species as completely real groups of specimens similar to one another in the same way that the members of a family resemble one another and also distinct from other such groups of specimens. For example, the wolf, fox, crow, jackdaw, oak, birch, wheat, oat, and so forth were considered to be species. The increasing number of described species required the standardization of their names and the construction of a hierarchical system of species and the larger taxonomic units. The pioneering work in this area was done by the Swedish naturalist C. Linnaeus, who established the bases for the present-day taxonomy of animals and plants in his work Systema Naturae (1735). In combining close species into genera and similar genera into orders and classes, Linnaeus introduced a double Latin name (the so-called binary nomenclature) for designating the species. Each species was designated by the name of the genus followed by a specific name.

By the end of the 18th century, the Linnaean system had been accepted by the majority of biologists throughout the world. During the first half of the 19th century, the French scientist G. Cuvier worked out the concept of the types of structure; thereafter, the type, as the highest taxon—that is, the highest systematic category—was introduced into the Linnaean system. At the same time, ideas began to be formed about changes in species occurring in the process of the development of living nature; these ideas culminated in the appearance of C. Darwin’s theory of evolution. This theory showed the necessity, in organizing a natural phylogenetic system, of proceeding from the successive genetic relationship between the forms of living organisms. By the end of the 19th century extensive material had been accumulated on intraspecies geographic variability, and the concept of sub-species had been introduced. The increase in the number of described species and subspecies of animals, plants, and microorganisms (by the middle of the 20th century, the number exceeded 2 million) led, on the one hand, to the “fragmentation” of species and to the description of any local forms as species, and on the other hand, to the “enlargement” of species; any groups or series of geographic races (sub-species) producing an aggregate of forms that were clearly related and usually interconnected by transitional forms were described as species. As a result of this, the concepts of “minor” species known as microspecies, or jordanons (named after the French botanist A. Jordan), and “major” species known as macrospecies, or linneons (from the name Linnaeus), appeared in the taxonomy. The practice of distinguishing the monotypic and polytypic species among the linneons developed (the polytypic species consists of a series of subspecies). The classical period in the development of taxonomy ended with the work of the Russian naturalist A. P. Semenov-Tian-Shanskii, who accepted the linneon as the basis and defined various subspecies categories (subspecies, morphs, and aberrations).

During the 1930’s, when a synthesis of the achievements of modern genetics and the theory of evolution had been attained, the theory of microevolution began to develop as the aggregate of triggering mechanisms in evolution and speciation. This led to a revision of the basic definitions and concepts in the classification of the lower taxons (by the American scientist T. Dobzhansky and the English scientists A. Cain and E. Mayr). Of essential significance in the modern definition of the concept of species is the virtually complete reproductive isolation under natural conditions. (Certain species that are totally isolated in nature can be effectively crossed with other species under artificial conditions.) Under natural conditions, the inability to crossbreed ordinarily is, of necessity, judged by using museum material from different parts of the areas of distribution of the forms that are of interest and by establishing the presence or absence of hybrids, transitional forms, and transitional zones in the areas of contact of these areas of distribution. In terms of territorial relationships, species can be allopatric, that is, occupying different nonoverlapping areas of distribution, or sympatric, where the areas of distribution more or less overlap or even coincide. The allopatric or sympatric nature of species in most instances is related to the conditions under which the species arose, as well as to what form of isolation (territorial or biological) played the main role in the formation of the given species. Under close scrutiny, almost all species are polytypic. The degree of their polytypicism usually rises with an increase in the area of distribution; it also depends upon the diversity of the physicogeographical conditions in its individual parts. Of particular interest are the so-called twin species, which are difficult to distinguish morphologically and are usually encountered in adjacent overlapping areas of distribution. Evidently, such species arose as a result of the primary occurrence of one of the forms of biological isolation.

Fundamental difficulties arise in establishing the concept of a species in the obligately agamic (asexual), parthenogenic, and autogamous forms. In these instances, the name species can be given conditionally to groups of similar clones or lines, which possess great morphophysiological similarity, occupy a definite area of distribution, and are in similar relationships with the environment. It is particularly difficult to compare and homologize modern species with fossil ones. In paleontology the prime concern is the succession of forms and the change in the species over time in successive deposits. But a comparision of forms that existed simultaneously in space, as is done for presently extant organisms, that is, in neontology, presents problems in paleontology because of the incompleteness of the fossil material. In regard to this problem, the Soviet zoologist V. G. Geptner (1958) proposed the term “phratry” to designate a concept equivalent in paleontology to species.


Semenov-Tian-Shanskii, A. P. “Taksonomicheskie granitsy vida i ego podrazdelenii.” Zap. Imperatorskoi AN, 1910, vol. 25, no. 1.
Darwin, C. “Proiskhozhdenie vidov putem estestvennogo otbora … .” Soch., vol. 3. Moscow-Leningrad, 1939.
Komarov, V. L. Uchenie o vide u rastenii. Moscow-Leningrad, 1944.
Cain, A. Vid i ego evoliutsiia. Moscow, 1958. (Translated from English.)
Vavilov, N. I. “Linneevskii vid kak sistema.” Izbr. proizv., vol. 1. Leningrad, 1967.
Mayr, E. Zoologicheskii vid i evoliutsiia. Moscow, 1968. (Translated from English.)
Zavadskii, K. M. Vid i vidoobrazovanie. Leningrad, 1968.
Timofeev-Resovskii, N. V., N. N. Vorontsov, and A. V. lablokov. Kratkii ocherk teorii evoliutsii. Moscow, 1969.
The New Systematics. Edited by J. S. Huxley. [Oxford, 1940.]
Linnaeus, C. Systema Naturae. Leiden, 1735.




a type of medicine consisting of a mixture of several kinds of ground (more rarely whole) medicinal vegetable materials. Sometimes, salts, essential oils, and other substances are added to the vegetable material. Species are usually prescribed for internal use, although they are occasionally used externally, for example, with fumigation, therapeutic baths, and gargling.


A chemical entity or molecular particle, such as a radical, ion, molecule, or atom. Also known as chemical species.
(nuclear physics)
A taxonomic category ranking immediately below a genus and including closely related, morphologically similar individuals which actually or potentially interbreed.


1. Biology
a. any of the taxonomic groups into which a genus is divided, the members of which are capable of interbreeding: often containing subspecies, varieties, or races. A species is designated in italics by the genus name followed by the specific name, for example Felis domesticus (the domestic cat)
b. the animals of such a group
c. any group of related animals or plants not necessarily of this taxonomic rank
2. denoting a plant that is a natural member of a species rather than a hybrid or cultivar
3. Logic a group of objects or individuals, all sharing at least one common attribute, that forms a subdivision of a genus
4. Chiefly RC Church the outward form of the bread and wine in the Eucharist
References in periodicals archive ?
Inspection of each animal's tissue under high magnification showed clear species differences in the abundance of VIP fibers approaching ORX neurons.
Thus, they are reported as a percentage of half the species difference for autosomal QTL and a percentage of the whole species difference for X-linked QTL.
Species differences in response to diethylhexylphthalate: suppression of apoptosis, induction of DNA synthesis and peroxisome proliferator activated receptor alpha-mediated gene expression.
The observed species differences were attributed to differences in first pass metabolism rather than species differences in absorption.
Our findings support species differences in habitat tolerance, specifically with respect to amount of cover, as a major influence on habitat use by meadow and prairie voles, with agonistic interactions reinforcing these tolerances.
It would he interesting to compare crawling and burrowing within the same species to determine whether these different strategies to increase velocity depend on locomotory mode rather than simply reflecting species differences.
The dissimilarity in DLC results may be due to treatment level, supplementation forms, species differences, environmental and mangamental factors.
On the other side of the spectrum, the mitochondrial sequence data can help clarify whether the observed variation among samples, such as the color morphs in Harmathoe imbricata mentioned above, reflects species differences or intraspecific variation.
The authors conclude that blood vessel development is a target for environmental chemicals acting as putative vascular disruptor compounds and suggest potential species differences in sensitive vascular developmental pathways.
The newer metric NODF is based on two matrix properties, decreasing fill and paired overlap, and enables to differentiate between a part of the overall nestedness introduced by species differences (NODFspp) corresponding to species occupancy and site (months) differences (NODFm) in species composition (Almeida-Neto et al., 2008).
Intact platelets do not accumulate arachidonate following DHLA administration, and species differences occur in the capacity of animals to metabolize DHLA to arachidonic acid (AA).