From Big Medical Encyclopedia

LOOK (Latin. species) — one of the main forms of the organization live (along with a cell, an organism, a biogeocenosis); main category biol, classifications. Among taxonomic categories (see) V. as taxonomical unit (taxon) is below a subgenus, but above subspecies. All individuals belonging to one V. are characterized by a community of the genetic constitution, morphological similarity and unity of an origin. Exists in time century: it arises, extends (during blossoming), vaguely long time in steady, almost invariable can remain, a state or continuously to change. The final destiny of V. is various: one of them disappear over time, without leaving new branches, others give rise to new types. Occupies a certain space century, or area (see), in limits to-rogo can break up to a number of subspecies (polytypic V.) or to be the presented one subspecies (monotypical V.). Borders between subspecies are conditional, and between individuals of different subspecies of the same V. free crossing is usually observed. Close V. in the nature always are reproduktivno isolated. Takes a certain place century in biogeocenoses (see), occupying in them a certain ecological niche.

Fundamental difference of intraspecific systematic categories from higher, than V., consists in genetic integrity of V., i.e. populations and one V.'s subspecies are not isolated from each other reproduktivno. Therefore in case of a favorable mutation the last, passing from population to population, in rather limited number of generations can quickly extend on all area of Century. V. differs from the lowest systematic categories (subspecies, race, a form, etc.) in the genetic isolation arising thanks to reproductive isolation. In simulated conditions is received much interspecies hybrids (see), including and prolific. Interspecies hybrids sometimes meet in the nature; in some groups of plants hybridization (see) is one of ways of speciation whereas at animals interspecies hybrids are extremely rare. When spontaneous trans-species hybridization is observed, it (even in case of formation of prolific hybrids), as a rule, does not lead to merge of populations of different V. Thus, exchange of genes between different V. is carried out hardly (this phenomenon carries the name of introgression). Fundamental difference of V. from the highest systematic categories (a sort, family, group etc.) consists that between one V.'s populations exchange of genes is carried out freely, and between different V.' populations such exchange either is complicated, or is impossible.

Modern V. because of existence of a trans-species gap on a morph., fiziol. and biochemical, to signs usually easily differ. However such gap (hiatus) exists only during the comparison at the same time existing V. Provesti line in time between ancestral V. and modern it is possible only conditionally. That is why the Latin nomenclature of fossil ancestors of the person where under generic names of Pithecanthropus, Sinanthropus, etc. are described, probably, only asynchronously existing races of an ancient type of Homo erectus is so difficult and tangled.

The concept B. was introduced for the first time by the English botanist Ray (J. Ray, 1628 — 1705) at the end of 17 century. It carried forms which in relation to V. are various to different V., keep this specific nature various, any of which does not come from seeds another and vice versa. In 1753. To. Tenches, laid the foundation of a systematics of plants and animals, entered the double (binary) nomenclature for designation B., according to a cut at first specify a sort, V. (noun), and then a specific epithet (adjective) belongs to Krom, the reduced surname of the author who for the first time described this V. E.g., Canis lupus L is put further. (wolf); Canis familiaris L. (domestic dog); L. — an initial letter of a surname of the scientist (Linney) who for the first time described this

V. Linney considered that V. does not undergo essential changes and their number is constant. Idea of V. as a stage of evolutionary process goes from Ch. Darvin who fairly considered a problem of speciation the central problem of theory of evolution (see Darwinism). Darwin and his followers emphasized convention of borders between subspecies and Century. The main criterion of V. and subspecies Darwin and the majority of taxonomists Darwinians of the second half 19 — the first half of 20 century had an existence of a morphological gap between different V. and lack of such gap between subspecies of one look. During this period the so-called narrow interpretation of V. is widely adopted; as independent V. are described quite often separate subspecies, populations and even individuals — carriers of separate mutations. The Dutch botanists of X. de Fris and J. Lotsv suggested to allocate two forms B. — elementary V. («zhordanona») and wide V. («linneons») consisting of several elementary Century.

The Russian zoologists A. P. Semenov-Tyan-Shansky and L. S. Berg in 1910 paid attention that V.'s most is subdivided into the subordinated taxons and developed system of subspecific categories: subspecies, morphs and tribe. The Trinarny nomenclature promoted establishment of uniformity in the evils, to a systematics of the lowest taxons and prepared perception of the polytypic concept of V. in the middle of 20 century. In the first third of 20 century the typological concept of V. dominated, edges allowed, proceeding from idea of inheritance of acquired characters, a possibility of evolution of a separate individual.

N. I. Vavilov's works and his schools in botany, and also creators of the «synthetic» theory of evolution in zoology became the following stage in development of the theory of V. As opposed to typological approach N. I. Vavilov (1931) put forward idea of wide polymorphic macrospecies as to system, having shown that macrospecies in our understanding — the isolated complex mobile morpho-physiological system connected in the genesis with a certain environment and an area. He emphasized the large volume and V.'s complexity, indicated the need of complex systematic, genetic and biogeographical studying of Century. On the basis of studying of morphogenesis at cultivated plants and their wild-growing relatives N. I. Vavilov and his followers came to a conclusion about polytypic character of macrospecies. Intraspecific taxons to which the macrospecies break up are not separate, and are closely connected, are crossed among themselves, giving at the same time posterity prolific (with rare exception). It was shown that a community of an origin of related V., existence of the general genes, ability to emergence of similar mutations (see), on condition of similarity of the occupied ecological niches (see. Ecology ), in which will work the directed selection pressure is similar, lead to parallelism in morphogenesis. The law of homologous series in hereditary variability (see. Theory of evolution ) allows to predict on the basis of studying of formoobrazovatelny process at well studied V. the main directions of morphogenesis at relatives, but poorly studied V. Ponyatiye of linnean V. in botany, the put-forward H. I. Vavilov, it is adequate to polytypic V.'s concept in zoology; «elementary» V. («zhordanona») of botanists correspond to subspecies (Subspecies) at supporters of the wide concept of V. in zoology.

In the first third of 20 century of a taxonomist, the geneticist, the biogeography and paleontology developed substantially independently from each other and therefore views of V. and speciation among representatives of these disciplines substantially differed. After N. I. Vavilov's works in botany and S. S. Chetverikova in zoology which combined genetic approaches with systematic in the 30th 20 century of the concept of V. underwent critical review. It led to creation of the so-called synthetic theory of evolution and to acceptance wide, or «biological», concepts of Century.

According to modern representations, close V. are reproduktivno separated from each other by a complex system of barriers of the isolating mechanisms of evolution (distinction in terms of reproduction, in a consort behavior, in chemism of gonads, in a structure spermiyev, in chromosomal complements, the lowered survival of hybrid germs in comparison with normal, partial or full of hybrid incapacity, etc.). It is possible to allocate three groups B.: 1) «good» V. which not only are reproduktivno isolated but also morphologically differ from each other, napr, a gray rat of Rattus norvegicus and different forms of black rats of R. rattus; 2) morphologically differentiated V. living in different territories (allopatric V.) which did not get reproductive isolation yet; between such V. in an experiment receiving prolific hybrids is possible; 3) types - the «doubles» who do not have noticeable morfol. distinctions, but reproduktivno isolated. Vyyasnelo, e.g., that under the name «malarial mosquito» disappears actually a little (to 15) morphologically indiscernible, but the types reproduktivno isolated - «doubles» (Anopheles atroparvus, A. labranchiae, A. sacharovi, A. messeae, etc.). It is established that under the name Rattus rattus two look - «double» disappear: 38-chromosomal rats connected by the distribution with expansion of Indo-European civilizations (all Europe, Africa, America, Australia, New Zealand, Asia to the West from India), and 42-chromosomal rats connected by the distribution with Mongoloid settled civilizations (Asia to the East from Burma).

It is necessary to distinguish reproductive isolation, edges interferes with exchange of gene pools between various V., and not combining ability as a private form of reproductive isolation. V.'s Not combining ability among themselves has relative character. E.g., in such families as a rose family (Rosaceae) and rutic (Rutaceae), interspecific crossings are successful rather easily; even reproductive mutually compatibility of some childbirth is observed. In family of a nightshade family (Solanaceae) interspecific crossings are successful at V. with identical number and similar morphology of chromosomes. Concept biol. Century for the organisms not capable to a syngenesis, it is not developed yet. V.'s definition for viruses and bacteria has purely typological character on existence of sharp distinctions — morfol., biochemical or fiziol. Thus, the concept B. for agamic forms is not adequate to a concept B. for the higher organisms breeding sexually. Considerable difficulties rise also in attempts of the appendix biol, V.'s concepts to fossil forms as because of impossibility to use criterion of reproductive isolation to draw line between types in time very difficult.

Can be young and ancient century (relic V.), eurysynusic and with a limited area (endemic V.), arisen and living in this territory (autochthonic V.), arisen in one area, but then expanded or changed the area. So, mamontovo a tree and Sequoia sempervirens — the relic V. which arose in the Cretaceous; they were eurysynusic on the northern hemisphere during a paleogenovy era of the tertiary period, and nowadays are endemics of the Californian coast of North America. Almost all wood endemic V. which remained on Earth since the tertiary period (magnolias, a welwitschia, etc.), belong to polyploids of a high level. The wild horse of Przhevalsky — relic V., constantly lives in semi-deserts and steppes of Central Asia (in the recent past and Kazakhstan). The area and number of the dying-away V. can be very small: so, endemic V. and a genus of a hare of Romerolagus diazi lives only on slopes of one volcano Popocatepetl in Mexico and contains only hundreds of individuals.

On distribution distinguish V.: allopatric (i.e. with incoincident areas) and sympatric (i.e. V. jointly living on one space). Sympatric V. usually are more ancient but to comparison with allopatric Century.

Process of speciation is made, as a rule, for hundreds, thousands and even tens of thousands of generations (see. Natural selection ). Plants which undergo natural (in an experiment artificial) an autopoliploidiya and an allopoliploidiya by multiple doubling of chromosome number in comparison with initial V. or initial parent types when chromosome doubling occurs at a hybrid of firstgeneration can have a speciation, Bystry on time. By this way it was experimentally recreated (resintezirovan) a number of cultural species of plants (naira., plum) a row new (e.g., by Raphanobrassica — a hybrid of cabbage and a radish is also synthesized).

For well studied groups of animals (e.g., birds, bugs, butterflies) usually adhere to polytypic V.'s concept, according to V.'s cut consists of a set of the subunits which are not isolated from each other reproduktivno. For poorly studied groups owing to limitation of material taxonomists adhere to the narrow concept of a look. In some groups (rodents, drosophilas, malarial mosquitoes, etc.) at careful studying of new criteria of V. responsible for maintenance of reproductive isolation (ethological and ecological isolation, a structure of generative organs, chromosomal complement etc.), often it is possible to allocate as a part of polytypic V. several types - "doubles".

Century, taxonomical breaking up to subspecies and lower classification groups of individuals, are structurally subdivided into groups of populations, and each of populations — into groups of closely related individuals (Dyoma). Population (but not an individual, on interpretation of lamarkist) is the smallest unit of evolution. Without possessing considerable morfol, distinctions, the next populations can physiologically differ markedly. So, populations of a rodent of a midday sandwort (Meriones meridianus) from the right and left coast of Volga differ on resistance to plague in several thousands of times.

Ecologically far V. irrespective of degree of their relationship seldom enter among themselves sharply competitive relations. The interfaced V.'s evolution a predator and V. of the victim, V. of a parasite and V. of the owner can become a result of relationship of ecologically far V.; according to the rule offered G. F. Gauza in one ecological niche the long time can exist only one Century. Types ecologically relatives (e.g., black and gray rats) enter sharply competitive relations, leaders or to accurate distinction in geographical distribution (allopatriya), or to discrepancy in different ecological niches.

Rules of the description of new V. or audit of existing are regulated by the international codes bacterial. (1948), botanical (1959) it is also evil. (1966) nomenclatures. In these codes rules of the description of new V. are stipulated, cases when new V.'s description is considered invalid are specified. The international commissions on the nomenclature have the right in case of disturbance of codes to cancel any name, either other nomenclature act, or any published work. Exact definition of V. — the most important condition of correctness of any researches in microbiology, virology, medical parasitology, studying of sources of medicinal raw materials, etc.

See also Systematics .

Bibliography: Berman 3. And. and d river. Modern problems of theory of evolution, L., 1967; Vavilov N. I. The law of homologous series in hereditary variability, L., 1967; Darwin H. Compositions, the lane with English, t. 3, M. — L., 1939; At-to about in with to and y P. M. Priroda and the volume of a look at cultivated plants, the Nerd. zhurn., No. 10, page 1530, 1967; Kane A. D. A look and its evolution, the lane with English, M., 1958; M and y r E. A zoological look and evolution, the lane with English, M., 1968, bibliogr.; it, Populations, types and evolution, the lane with English, M., 1974; Timofeev-Resovsky N. V., N. N. and Yablokov's Baneberries. B. Short sketch of the theory of evolution, M., 1969, bibliogr.; Shmalgauzen I. I. Factors of evolution, M., 1968, bibliogr.; about N e, Problems of Darwinism, M., 1969, bibliogr.

H. N. Vorontsov.