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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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an area of knowledge within whose framework the entire aggregate of objects forming a certain sphere of reality is designated and described in a definite, orderly fashion. Systematics is applied by all sciences dealing with complex, internally branched, and differentiated systems of objects, for example, by chemistry, biology, geography, geology, linguistics, and ethnology.
The principles of systematics may be extremely diverse. Objects may be ordered according to purely formal external signs, for example, by assigning ordinal numbers to elements of a system. They may also be ordered by creating a natural system of objects, such as a classification system based on an objective law; the periodic system of elements in chemistry is a classic example of such a natural system. The resolution of the problems of systematics rests on general typological principles, specifically, on distinguishing the stable characteristics (properties, functions, interrelationships) of the objects forming a system. Systematic units, or taxa, must satisfy definite formal requirements. Specifically, each unit must occupy a single place in a system and must be sufficiently delimited from neighboring units. These requirements are most greatly satisfied by systematics based on established theories on the structure and the laws governing the development of a system. However, inasmuch as the creation of the theory of a system is often exceedingly difficult, in practice systematics is usually effected by drawing upon both theoretical and practical considerations.
E. G. IUDIN
Biological systematics. Systematics has been most greatly developed in biology, where it is used to describe and designate all extant and extinct organisms and to establish kinship and the interrelationships between certain species and groups of species. In striving to create a complete system, or classification, of the organic world, systematics utilizes the data and theoretical propositions of all biological disciplines. Systematics is closely associated with the theory of evolution.
A special function of systematics is to provide a practical guide to the vast array of extant and extinct species; a tremendous number of extant species of animals (approximately 1.5 million), plants (approximately 350,000–500,000), and microorganisms have been described. Animal and plant systematics share the same tasks and methods. At the same time, they are also characterized by certain specific features associated with the very nature of organisms. However, these specific differences do not influence the shared theoretical foundations and purposes of animal and plant systematics. In biology, systematics is often divided into taxonomy (the theory of the classification of organisms) and systematics proper (as defined above). The term “taxonomy” is sometimes used as a synonym of systematics.
SPECIES—A CONCRETE ORGANIC FORM AND A BASIC CONCEPT OF SYSTEMATICS. All organisms belong to a species. The concept of species has changed substantially during the history of biology. Even today systematists sometimes disagree on the nature of species, although a substantial degree of unanimity has been achieved on this cardinal question. From the standpoint of modern systematics a species is a genetically defined group of a population. The individuals of a species are characterized by an aggregate of definite characteristics inherent only to them. They are capable of freely crossbreeding and yielding fertile offspring and of occupying a definite geographic area, or home range. According to its morphological and physiological characteristics, each species is distinguished from all other species (even closely related species of the same genus) by a hiatus; in other words, a gradual transition from the characteristics of one species to the characteristics of another does not exist. The most important manifestation of such a hiatus is the fact that under natural conditions the individuals of different species do not crossbreed.
Rare instances of interspecific crossbreeding in nature do not disrupt the independence and individualism of each species. Indeed, reproductive (genetic) isolation for the most part maintains a species’s independence and integrity among coexisting closely related species. Thus, each species is real not only in the sense that it consists of some number of concrete individuals, but most important, because it is delimited (isolated) from all other species. The boundaries between species are vague or difficult to differentiate when a species is in the process of establishing itself and separating itself from the parental species and has not yet attained complete independence and total reproductive autonomy. The home ranges of such forms are adjacent to one another or overlap. Hybrids may occur in the area. Organisms at this stage of speciation are usually united in a semi-species, and, with the parental or sister form, are joined in a superspecies. Interspecific boundaries between sibling species may be difficult to differentiate. Sibling species exhibit complete reproductive isolation but are practically indistinguishable morphologically and usually share certain other characteristics. Essential differences between sibling species often consist in the characteristics of the karyotype (the set and structure of chromosomes), which prevent or hamper the production of fertile offspring during crossbreeding. Sometimes other isolating mechanisms, including such behavior mechanisms as mating, also play a role. Under all conditions, sibling species that live in close association behave in nature as genetically independent species.
Each species results from prolonged evolution. One species may be transformed into another species (phyletic evolution), or a certain population of a species may develop into a new species by means of divergence (cladogenesis). A formed species is relatively stable in time, with the stability extending far beyond the time scale of human history.
A species, which is a qualitative stage in the process of evolution, is a basic unit of nature, but nevertheless is not homogeneous. It also comprises intraspecific systematic categories, among which the basic and generally acknowledged category is the subspecies, or geographic race. The formation of a subspecies is associated with the features of the habitat; that is, a subspecies represents a species’s form of adaptation to different conditions in various environments. In most cases, the characteristics of one subspecies gradually change into the characteristics of another; in other words, there is no hiatus between subspecies. The home ranges usually do not overlap, and two subspecies of the same species do not occur together.
The individuals of different subspecies of the same species are, as a rule, capable of freely crossbreeding. Hybridization among subspecies usually occur in the border areas of home ranges, which largely explains the transition between the characteristics of subspecies. The majority of relatively widespread species are polytypic, consisting of from two to several dozen subspecies. Those species that do not form subspecies are said to be monotypic. At the same time, the formation of a subspecies is an initial stage in the divergence of a species; that is, a subspecies, at least potentially, represents an incipient species.
The study of intraspecific (mainly geographic) variations and intraspecific forms, which in the early stages of the development of systematics attracted little attention, started developing intensively in the early 20th century. This new interest led to the complete revision of the previous, principally morphological, concept of species and to the development of the modern concept of the polytypic, or more accurately, synthetic, species. In addition to a species’s morphological properties, its physiological, biochemical, genetic, cytogenetic, populational, geographic, and certain other properties were taken into account. The species was no longer considered a monolithic unit but a complex system independent from other analogous biological systems. The contemporary concept of species is an important biological generalization that has enriched the understanding of the very process of the formation and establishment of different species and that has opened broad possibilities for the study of species.
Modern systematics has taken an important step in revising several Darwinian concepts, which seemed to make sense in Darwin’s time but today have been proved incorrect. These concepts had to do with the conditionality of the boundaries of a species (the unreality of the species) and the absence of definite boundaries between species and of a theoretical difference between a species and a variation.
In the 20th century the concept of a polytypic species has been developed, establishing a broader interpretation of a species. This has resulted, particularly in zoology, in a change in ideas regarding the number of species included in various groups. Many species that had been considered completely independent proved to be only subspecies and members of polytypic species. This led to the inclusion of fewer species in certain well-studied groups than were earlier accepted, despite the discovery of new species. Thus, in 1955 there were approximately 8,600 accepted species of birds as compared with approximately 18,000–20,000 species in 1914. The number of mammalian species was reduced from 6,000 in 1914 to approximately 3,500 in 1953.
There has been a tendency among botanists to interpret very narrowly the term “species”; such interpretations have often resulted in substantial disputes. For this reason, many small species that essentially represent subspecies or other intraspecific forms have been described in plant systematics. Specific subdivisions that are smaller than subspecies are variously interpreted by botanists and are referred to as forms or variations.
TAXONOMIC CATEGORIES AND THE NATURAL SYSTEM. Systematics analyzes all forms of similarity and kinship (primarily morphological) among the many various species and separates the most similar and closely related species into groups called genera. Further expansion of the circle of species and the use of broad general characteristics have resulted in the segregation of ever more generalized groups and the classification of these groups into subordinate groups, forming a hierarchical system of the organic world. The simplest taxonomic categories consist of the following series (ranging from lower to higher categories): genera are joined into families, families into orders, orders into classes, and classes into phyla (in animal systematics) or divisions (in plant systematics). Intermediate links have been established between the above categories as a better understanding of systematic (phylogenetic) relationships has been developed. Thus, animal systematics consists of more than 20 categories, including the subgenus, tribe, subfamily, and suborder.
Ultimately, all phyla are united into kingdoms. Linnaeus believed that there were two kingdoms—the animal and plant kingdoms. Since the mid-20th century the notion of four kingdoms existing in the organic world has gained widespread support.
The term “taxon” was introduced in the 1940’s to designate a real taxonomic group of any systematic rank or scope. Thus, the family Felidae, the genus Luscinia, and the species Passer domesticus are all genuine taxons. Usage of the term in the sense of rank or category is incorrect.
Systematics establishes the similarities between species and groups of species and unites them accordingly not by general appearance or certain particulars but on the basis of the very structure of the organisms. Thus, similarity from the point of view of systematics reflects blood kinship and the degree of this kinship as well as the extent of common origin. For example, although bats and birds are very similar, they are classified according to their structure into two different classes (Mammalia and Aves, respectively). At the same time, if birds and mammals are compared with more remote organisms belonging, for example, to another phylum, the common structure of birds and mammals as vertebrates is more apparent than their differences. Some cactuses and cactus-like Euphorbia, despite their resemblance, belong to different families; however, they are united together in the class of dicotyledons.
Attempts to systematize the organic world (or only animals or plants) were undertaken in antiquity, in the Middle Ages, and during later periods, but these attempts were not very scientific. The foundations of modern systematics as a science were established by the British scientist J. Ray and the famed Swedish naturalist C. Linnaeus. One hundred years after Linnaeus, Darwin’s evolutionary theory was incorporated into systematics. In the following decades a major trend was the striving to establish and reflect as completely and accurately as possible the genealogical relationships existing in an evolutionary (phylogenetic) system. At the same time, mistakes were made for various reasons, although mainly for lack of knowledge; for example, kinship relationships between various groups were incorrectly evaluated, and certain groups were inappropriately joined. Such mistakes impart a character of artificiality to a system or its parts.
As knowledge accumulates, errors are gradually discovered and corrected and a system approximates a phylogenetic one, that is, it adequately reflects the kinship relationships between organisms. The increasing complexity of a system and the differences between systems more or less generally accepted in various periods of the development of the science are not accidental but are a logical consequence of the general progress of biological knowledge. Thus, to the extent that the formulation of a system is based on the sum of information from all branches of biology, systematics is in essence a synthesis of that information.
A system of supraspecific groups is usually called a macro-system; the corresponding branch of systematics is called macrosystematics. Information for the construction of macrosystems is primarily obtained from embryology and the morphology of extant and extinct groups.
METHODS AND SIGNIFICANCE. The oldest systematic method used in the study of any group, the comparative-morphological method, is still the principal method used today. This method, from which general biological systematic conclusions have been drawn, will probably always be the chief method used in the study of fossil animals. At the same time, modern scientific methods are gaining widespread acceptance in morphological systematics; for example, electron and. scanning microscopes have opened new possibilities in the study of cell structures.
The introduction into systematics of the study of karyotypes and, in many cases, fine chromosomal structures has led to the development of karyosystematics. As a result, the existence of sibling species has been demonstrated, and forms that had been considered to be subspecies according to the level of their phenetic differences were recognized as independent species. For example, whereas originally one species of gray voles (Microtus arvalis) was thought to exist in the USSR, at least three species have been distinguished. Experimental methods, including natural and artificial hybridization and breeding, have been introduced into systematics. They are used mainly in studying the specific taxons of mammals and other groups.
Biochemical data was introduced into systematics in the mid-20th century (chemosystematics, or chemotaxonomy). The comparative study of the most important proteins (for example, hemoglobins and cytochromes), the nucleotide composition of deoxyribonucleic acid (DNA), and molecular hybridization (genosystematics) makes a better systematic characterization possible and elucidates the interrelationships between groups. Also significant in systematics are ethological indexes, that is, the characteristics of specific behavioral patterns, particularly mating behavior (bird, amphibian, and orthopteron sound signalization). Sometimes ethological indexes are more characteristic of species than are morphological indexes.
The intensive study of the population structure of a species has developed in association with biosystematy. The rapid accumulation of information in systematics and related sciences has resulted in the need for computers to gather, store, and process information. Repeated attempts have been undertaken, especially between the 1940’s and 1960’s, to introduce certain mathematical methods for the purpose of obtaining more objective indexes (numerical systematics). However, although they are often necessary tools in the study of specific and interspecific relationships, mathematical methods arouse skepticism in many systematists when they are applied to superspecific groupings. In such cases they show similarities but do not reveal kinship.
The evaluation of the correlative ranks of superspecific taxons, that is, the creation of a macrosystem, requires an extensive knowledge of various scientific fields and a good sense of proportion and correlation. This ability has long been regarded as a skill acquired through extensive experience and schooling. Although researchers can objectively evaluate species, they almost invariably introduce some measure of subjectivity into the creation of a macrosystem because of differences regarding the role and meaning of a system. Nevertheless, a greater consensus to opinion is gradually being achieved, and consequently, a real possibility exists of constructing a truly natural and generally accepted system of the organic world.
Before the 20th century there was a widespread belief, even among biologists, that systematics was a science that studied external and sometimes accidental and insignificant animal and plant characteristics. The task of systematics was, in their opinion, only to describe, assign names, and classify the diversity and abundance of organic forms. This notion has long been abandoned, and the role of systematics as a general biological science has been recognized.
Systematics is both independently significant and the basis for many biological sciences. The study of any object from the standpoint of its structure and development (anatomy, histology, cytology, embryology) first requires an understanding of the object’s place among other objects and its phylogenetic relationship to them. Genetics is based on the study of these interrelationships. An awareness of the systematic relationships of species and groups is also necessary in biochemistry.
Systematics is especially important in biogeographic and ecological research, which involves contact with many different species. A true understanding of a biocenosis (ecosystem) is impossible without an accurate knowledge of all component species. Stratigraphy and geologic chronology are based first and foremost on the systematics of fossil animals and plants.
The purely utilitarian role of systematics is also important. Practical work with the animal and plant kingdoms is impossible without an accurate knowledge of species differentiation. For example, systematics is important in the study of agricultural, forest, and livestock pests, as well as of parasites that are injurious to domestic and wild animals, plants, and humans and that pose a threat to the inland fishing industry, marine fishing, and hunting. A thorough understanding of species is also important in conservation practices and the determination of the causative agents, carriers, and reservoirs of diseases affecting humans and domestic and wild animals. The same principles apply to plants, including forest plants, agricultural plants, weeds, and medicinal plants. There has been a growing interest in systematics since the mid-20th century owing to its vital importance. Practical and theoretical systematics has been particularly developed in the 1960’s and 1970’s.
SCIENTIFIC CENTERS, SOCIETIES, AND PUBLICATIONS. Progress in systematics has been a result of intensified field research and the gathering of collections. Expeditions to study the organic world were conducted by systematists as early as the 18th century. Stationary biological regional organizations operate in various parts of the world, and many amateurs collect specimens. Systematic research is impossible without zoological museums and herbaria, which house thousands or even millions of specimens used in the study of the animal and plant kingdoms. There are excellent collections in museums in the USA (Washington, New York, Chicago) and in the major museums of Europe (the British Museum in London and the Museum of Natural History in Paris).
In the USSR, the major collections are at the Zoological Institute of the Academy of Sciences of the USSR, the Zoological Museum at Moscow State University, and the herbaria of the Botanical Institute of the Academy of Sciences of the USSR in Leningrad and of Moscow State University. Since the 1950’s biochemical laboratories and centers of the Academy of Sciences have participated in the study of the general problems of systematics (primarily macrosystematics). In the USSR, for example, research in chemosystematics that was initiated by A. N. Belozerskii is conducted by the biology department of Moscow State University.
The first society of systematic zoology was founded in 1948 in the USA; the society has published the special theoretical journal Systematic Zoology in Washington since 1952. The botanical journal Taxon has been published in Utrecht since 1951. Many articles dealing with general and specific questions on systematics are published in zoological and botanical journals throughout the world; Soviet publications include Zoologicheskii zhurnal (Zoological Journal), Botanicheskii zhurnal (Botanical Journal), and general biology publications, for example, Zhurnal obshchei biologii (Journal of General Biology) of the Academy of Sciences of the USSR. The First International Congress on Systematic and Evolutionary Biology was held in Boulder, Colorado in 1973.
REFERENCESMayr, E. Sistematika i proiskhozhdenie vidov s tochki zreniia zoologa. Moscow. 1947. (Translated from English.)
Mayr, E. Zoologicheskii vid i evoliutsiia. Moscow, 1968, (Translated from English.)
Takhtadzhian, A. L. “Biosistematika: proshloe, nastoiashchee, budushchee.” Botanicheskii zhurnal, 1970, no. 3.
Takhtadzhian, A. L. “Nauka o mnogoobrazii zhivoi prirody.” Priroda, 1973, no. 6.
Takhtadzhian, A. L. “Razvitie sistematiki v SSSR.” Vestnik AN SSSR, 1972, no. 6.
Stroenie DNK i polozhenie organizmov v sisteme. [Collection of articles.] Moscow, 1972.
Mayr, E. “The Role of Systematics in Biology.” Science, 1968, vol. 159, no. 3,815.
Mayr, E. “The Challenge of Diversity.” Taxon, 1974, vol. 23, no. 1.
Chemotaxonomy and Serotaxonomy: Proceedings of a Symposium Held at the Botany Department. New York- London, 1968.
Hennig, W. Phylogenetic Systematics. Chicago, 1966.
Turner, B. L. “Chemosystematics: Recent Developments.” Taxon, 1969, vol. 18, no. 2.
Systematic Biology. [Washington] 1969.
Crowson, R. A. Classification and Biology. London, 1970.
“Computers in Biological Systematics: A New University Course.” Taxon, 1971, vol. 20.
V. G. GEPTNER