Species

Species, one of the fundamental concepts of natural science classification. From the quite natural need to sort out the entire mass of objects in the surrounding nature, all peoples have created special names for various animals, plants, and minerals. These names facilitated the recognition of certain objects, since they were associated with ideas about the characteristics typical for each object, the sum of which was expressed in a specific name. Naturally, all such names among all peoples are nouns (e.g., birch, betula, Birke, etc.). The more peoples have to encounter various vital objects, the more frequently objects that present a danger of confusion are met, the more such designations there will be and the characterization of features will be finer. Thus, among the inhabitants of the tropics (Malays, Hindus, Chinese, etc.), there are far more names for animals and plants in the language than among northerners, Europeans. Naturally, special designations were received, mainly, by all objects that aroused some interest; indifferent ones were often designated collectively. Europeans for a long time used only folk names, and in science (in medicine), which arose in Greece and Italy, Greek and Latin names were mainly in use. Only in the 15th century, when masses of new animals and plants, not native to Europe, began to be brought to Europe and the need for their systematization appeared, did the need arise for the creation of new names and new systematic units. Already in the 17th century, the necessity of a basic unit became clear for science as well, which received the name species (species) and initially represented the same thing as folk names. Only in the 18th century, when, thanks to Tournefort and Linnaeus, a second systematic unit was established—the genus (see), genus, representing a collection of similar species, did the possibility of easy designation of species arise. Only from this time did serious attempts to define the concept of species itself begin. These definitions, of course, had to reflect general views on nature, and it is understandable that during the dominance of the theory of the immutability of species, the definition of what a species is had to be different than after the strengthening of evolutionary theory and the recognition that species are changeable. At the same time, initially, while our knowledge of organisms was insufficient, the accuracy of the definition of the concept could not be great. The second half of the 19th century and the current century provided a mass of factual material, which forced not only the recognition of the inconstancy of species boundaries but also put forward new grounds for distinguishing species from one another. Initially, for the characterization of species, almost exclusively morphological features were used, of course, of a different order for plants and animals. But later, other features began to be introduced: for plants—details of flower structure, anatomy, geographical distribution; for animals—biological phenomena, the ability to crossbreed, etc. Our acquaintance with lower organisms had a particularly strong effect on the definition of the concept of species, since in many cases among lower organisms it is not possible to find sufficiently sharp morphological features of difference, whereas their physiological properties can be sharply different (biological species). The study of crossbreeding phenomena, the use of experimental cultures (in plants), the study of nuclear structure (karyology, see)—all this extremely complicated the definition of the concept of species, which, however, cannot be artificially simplified, already due to the differences in scientific requirements, especially since at the present time in many cases an extremely precise definition is necessary, whereas previously the desire for broad generalizations prevailed. Thus, based on insufficient material, it was initially believed that only hybrids between sections of a species are fertile, and hybrids between species are sterile. On this basis, a dispute went on for a long time among anthropologists as to how to consider human races—as species or as sections. At the present time, numerous observations have established the fertility of racial hybrids in humans, and in plants and animals—also of interspecific hybrids in a sufficiently large number. In plants, it was even possible to obtain fertile bigeneric and even quadrigeneric hybrids, which makes the differentiation and characterization of species even more difficult. Likewise, the precipitation method (see) cannot be considered absolutely reliable, since in plants it often does not yield definite results. Practically, the needs for dividing species are extremely diverse depending on the tasks of research; thus, for general surveys of large groups, for initial orientation, broader units are sufficient and even more advantageous, whereas for detailed, precise study, more definite, more sharply outlined concepts are necessary. This also causes difficulties in creating such a definition of the concept of species that would satisfy everyone—taxonomists, geneticists, plant geographers, meadow researchers, bacteriologists, mycologists, etc. In order to give an idea of the difference that is already manifesting itself in the definitions of the concept of species given even by various botanists, one can cite two definitions of species given by the Frenchman Naudin and the German Wettstein. Naudin defines a species as follows: “A species is a collection of individuals, even if very different in appearance, which can mutually fertilize and give rise to offspring that are indefinitely fertile and retain in all generations the traits characteristic of each of the two initial ones, provided that new crossbreedings do not disrupt their transmission.” This definition approaches Lamarck's definition with the only difference that Lamarck calls a species a collection of similar individuals. Wettstein defines a species as “an abstraction of all individuals distributed in a certain area, identical or retaining the same features for a more or less long time in their offspring.” The difference between these definitions lies primarily in the fact that for Naudin, as for very many naturalists, a species is a concrete phenomenon, whereas for Wettstein, a species is an abstraction. To this day, there is a dispute about what a species represents—a reality or an abstraction. Against Wettstein's definition, one can say the following. If we compare two individuals of the same species, we will see that they undoubtedly present both signs of similarity and signs of difference, and only conditionally do we neglect the signs of difference and attach greater importance to the signs of similarity. Sometimes the signs of similarity are so obvious that we can immediately say that this is such-and-such a species; but sometimes, especially having a large number of individuals in hand, we only with difficulty and conditionally assign extreme forms to one and the same species (cf., e.g., human races). On the other hand, the study of species, in many cases appearing to us homogeneous, has shown that when applying more thorough research methods, they turn out to consist of more or less numerous groups that remain constant only under certain conditions of reproduction (mainly in the absence of crossbreeding). Many authors have shown that the number of such complex species is quite large. This led to the establishment, on the one hand, of numerous subdivisions of species into smaller units, and on the other, to the creation of such concepts as collective species (species collectiva), subspecies, varieties, and elementary species. The number of elementary species into which a mixed species can be divided can sometimes be very large (several tens). Karyological study of both animals and especially plants, as well as the work of geneticists and ecologists, has clarified that in a number of cases, using methods of hybridological or genetic analysis, it is possible to isolate from a seemingly homogeneous, but in reality mixed, complex of individuals of one species (so-called populations) certain groups characterized by the constancy of hereditary traits during sexual reproduction. It is believed that such groups, which were distinguished even earlier by practitioners, e.g., from cultivated plants, under the name of races, have the same composition of hereditary traits. Johannsen proposed calling such groups genotypes (see). The study of genotypes, especially plant ones, has shown that one and the same genotype, or pure line, under different conditions of existence can present different traits, external or internal, as a result of which sometimes sharply different individuals, or phenotypes (see), are obtained. The same Johannsen proposed calling the collection of such phenotypes of one and the same genotype a biotype. Thus, the idea that races or elementary species are hereditarily constant must be somewhat changed and limited in connection with the influence of the external environment. We see that complexes of individuals or species of plants and animals can have, in essence, a very different composition, a different significance. Lotsy drew attention to the fact that large species are, for the most part, variable. It was mainly them that evolutionists had in mind when speaking about the variability of species. But on the other hand, there are small elementary species, unusually constant in their traits. Lotsy proposed calling the former species linneons, and the latter—jordanons. A linneon corresponds to a collective species, a jordanon—to a more or less elementary species.

All the above-developed propositions relate, mainly, to higher and lower animals (Metazoa and Protozoa) and plants (Metaphyta and Protophyta) that reproduce sexually. But, as is known, there are many animals and plants that do not have a sexual process and sexual organs and, nevertheless, represent definite species. In many cases, it is extremely difficult to find species-specific morphological distinguishing features in such organisms due to the simplicity of their structure. One has to resort to numerous, even experimental, studies in order to establish any characteristic features. On this basis, for example, bacteriologists do not consider it possible to limit themselves to morphological features alone, but introduce into the characterization of species in bacteria the nature of growth on various media, the ability to produce pigments, features of nutrition and respiration (aerobes and anaerobes), the relationship to parasitism, and various biological reactions. Only the totality of all these and other data makes it possible to establish species identity. Other lower and even more highly organized creatures present difficulties of a different kind. Thus, very many organisms possess a widely developed capacity for polymorphism (see). It is known how complex the life cycle is, for example, in aphids (see); some green algae change their forms little when conditions of existence change, but instead have a complex life cycle; others, on the contrary, respond to every change with the formation of new structural forms and are extremely polymorphic. Excellent examples of such polymorphism are presented by many colonial green algae. Such differences are especially sharply manifested with sufficient definiteness of so-called morphogenic stimuli, in the form of differences in light, quantity and quality of water, etc. In connection with this, as the botanist Glück showed, parts of one and the same amphibious plant can have a completely different appearance if grown under or above water. Species rich, for example, in essential oil or alkaloids, develop very little of them under certain living conditions. Individuals of one and the same species grown on soil poor in nutrients can be sharply different from those grown on rich soil (a different phenotype), etc. Experiments by Bonnier and others have shown that even halves of one and the same plant, grown in lowlands and in high-mountain conditions, can produce plants so sharply different from each other already in the first year that any systematist, not knowing the origin of these forms, would recognize them as separate and distinct species. At the same time, new plants grown from the seeds of these halves retain the characteristics of lowland plants in lowland conditions, and mountain plants in the mountains. However, sometimes the differences cannot be reduced to external influences. Thus, among orchids there is the genus Catasetum, which has a species with three forms of flowers: bisexual, female, and male. Sometimes (and more often) all three forms of flowers develop on different individuals, but sometimes all three forms of flowers are encountered on one and the same individual. The different flowers are so different that for a long time individuals of one and the same species were assigned to different genera. Thus, it is impossible at the present time to give a completely satisfactory, i.e., suitable for all cases, definition of the concept of species. From the above, it is clear that in many cases of collective species, they have to be broken down into separate subdivisions. Usually, any deviation from the accepted typical character of a species is designated by the word variety, or varietas. The needs of systematic description have caused a whole series of such designations, which, however, are by no means applied in the same way by everyone. Thus, many accept the concept of variety simply to denote a subdivision of a species, while others link it to the influence of the environment. Likewise, the concept of "form" (forma) shares the fate of the concept of "variety." Therefore, for terrestrial individuals of amphibious plants, some accept forma terrestris, and others var. terrestris. Besides those already mentioned, in descriptive biology, the following are used as subdivisions of a species: modifications, races, mutations (see), clones.

M. Golenkin.

Biological species, species sorores, were first established in zoology by N. A. Kholodkovsky. Species are usually distinguished by external features; however, it happens that animals very similar in appearance differ significantly in their biological (resp. and physiological) properties. Thus, for example, bark beetles, Myelophilus piniperda L. and M. minor Hartig, which are difficult to distinguish by the character of the surface of the elytra, make tunnels on pine trees that are completely different in arrangement. The common field fly (Musca corvina F.) is egg-laying, while the M. larvipara Portsch, indistinguishable from it in appearance, is viviparous and, moreover, has ovaries of a different structure. The Hessian fly (Cecidomyia destructor Say) lives on wheat and cannot develop on oats, whereas the very similar C. avenae Marchal possesses the opposite properties. Some species of Chermes (parasitizing on conifers), barely distinguishable by minor features, have very significant biological peculiarities. Thus, for example, the life cycle of Chermes viridis Ratz is two-year, with the alternation of a generation reproducing virginly (parthenogenetically) with a bisexual generation being accompanied by the migration of the latter from spruce to larch; at the same time, Ch. abietis has a one-year life cycle without migration and with exclusively parthenogenetic reproduction. Such forms of animals are distinguished by Kholodkovsky as biological species. Yu. Filipchenko subjected some of the biological species of Chermes to biometric study and confirmed their distinction using methods of variation statistics. The question boils down to whether there are forms of animals that are absolutely indistinguishable morphologically but have definite biological peculiarities. In any case, the concept of "biological species" is useful, because biological peculiarities can prompt a more subtle study of external morphology and the finding of such distinguishing features that could not have been noticed or evaluated without such guidance.

L. Kursanov. Vicarious species (from vicarius—substituting) are closely related species that replace one another in different geographical localities. There are especially many vicarious species among animals of the Palearctic and Nearctic regions. Such are: the European bison (Bison europaeus) and the North American bison (Bison americanus), the European and American beavers (Castor europaeus and Castor americanus), the common European small tortoiseshell (Vanessa urticae) and Vanessa milberti from North America, etc. The presence of numerous vicarious species speaks in favor of the common origin of both compared faunas. Vicarious species can also exist in contiguous areas of distribution. For example, the longhorn beetle (Monochammus sartor), found in Western Europe and Poland, is replaced in the European part of the USSR and Siberia by the closely related species M. quadrimaculatus. In parasitology, vicarious species and subspecies are forms of parasites that replace one another in the same host in different areas of its distribution: such is the infusorian Epidinium ecaudatum caudatum from the stomach of an ox in the north of the USSR, which is replaced by the form Epidinium ecaudatum hamatum in the south of the USSR.

V. Dogel. Species in microorganisms. In relation to bacteria, the concept of species does not always lend itself to precise definition due to their tendency toward variability and the absence, or more accurately, the lack of proof, of a sexual process in them. Thus, instead of species in the sense that this word is applied to sexual creatures, some authors propose grouping bacteria into clones, as is done in the case of certain flowering plants that reproduce asexually, for example, for the potato, and among protozoa—for ciliates. In relation to protozoa (Protozoa) as sexual creatures, the definition of species has the same character as in the case of so-called higher organisms, with the peculiarity that, in addition to morphological and cytological data, it is necessary here to take into account the entire cycle of their development, and in the case of parasitic forms—also ecological data. Regarding the method of defining species in bacteria, several proposals have been put forward in recent years. Enderlein (1925) considers it necessary to use the comparative-morphological method for determining the species of bacteria, which takes into account all those forms of the microorganism that it has during the cycle of its development. Microorganisms that differ in morphology even during one of the moments of cyclogeny must be considered separate species. Enderlein fully accepts those objections against the use of the physiological method for defining species that were put forward by Leuckart (1850). With the help of the comparative-morphological method, Enderlein made an attempt at a taxonomy of microorganisms, which must be recognized as unsuccessful, since Bact. pestis turned out in this system to be in the same genus as yeast, which is obviously incorrect. Winslow (1914) proposed not to draw a line between the morphological and physiological properties of bacteria when defining a species, since both have at their core the chemical properties of the germ plasm. Such a point of view was adopted by the Society of American Bacteriologists and formed the basis for the definition of bacterial species in the manual by Bergey (1925). Those commissions that compiled this key did not take into account the variability and cycle of development of bacteria. This defect was pointed out by Breed (1928), who believes that it is necessary to base the definition of bacterial species on the morphological and physiological properties of bacteria, including the peculiarities of the chemical structure of their protoplast, which determine the serological properties of bacteria. An example of two microorganisms that are identical in morphology and culture properties can be the scarlet fever streptococcus and the pyogenic streptococcus, which differ only in biological properties. The difficulties created by the existence of a cycle of development (Enderlein), dissociation (Breed), or mutation (Baerthlein) in bacteria can be overcome when defining bacterial species. Variability in bacteria has boundaries, which in the group of intestinal bacteria consist, on the one hand, of smooth forms of colonies, and on the other—rough ones. The shape of the colony and the degree of its transparency depend on the morphology and character of the protoplast of the bacteria: bacteria with a homogeneous protoplast produce transparent colonies, those with a vacuolated one—turbid ones. All other forms of colonies should be considered transitional or mixed types. The cycle of development of bacteria fits within these boundaries. The presence of a sexual process in bacteria is argued for by Almquist, Löhnis, and Enderlein, but precise proofs have not yet been provided by them. Therefore, when defining species in bacteria, it is not yet possible to take sexual forms into account. In connection with the dissociation of bacteria, some of their physiological or serological properties change. The most indicative example of such variability in the biology of bacteria can be the variants of Proteus X₁₉, which are definitely different in the shape of colonies and physiological properties. The difficulties created by the existence of a process of cyclic development or dissociation of bacteria are in no way greater than those that a zoologist or botanist has (Breed), since bacteriology does not yet know of such variability as that which occurs, for example, in insects. For the names of bacterial species, Buchanan proposed that bacteriologists use the International Rules of Botanical Nomenclature (1905–10), adopted with corresponding changes by the Society of American Bacteriologists (1917). The need to be guided by the Bacteriological Code is especially great for bacteriologists, since the currently existing names of species often do not take into account the rules existing in taxonomy, which complicates the use of literature. An example is the species Bact. paratyphi N, described under no less than six different names. On the basis of the considerations presented, in bacteriology, a species should be considered the totality of all individuals that are identical to each other in morphology and biochemical properties, stably preserving their properties within the limits of the cycle of development and transmitting them to their offspring. Such a definition was made by Lehmann and Neumann (1899). It has been supplemented at the present time by Stutzer only with an indication of the need to take into account the cycle of development of the microorganism. M. Stutzer.