Taxonomy and Systematics – Components, List of Systems, Importance

One of the most fascinating Biology Topics is the study of genetics and how traits are passed down through generations.

Taxonomy and Systematics are Key to Biological Information – How is Taxonomy different from Systematics?

Swedish scientist Augustin P. de Candole first used the term Taxonomy in his book Theories Elmentaire de la Botanique in 1813. The term Taxonomy is derived from two Latin words Taxis means arrangement and Nomos means law. Thus the term Taxonomy means rules or principles of arrangement or classfication. Carolus Linnaeus is regarded as the “Father of Taxonomy”.
A taxonomic ranking always reflects the proper evolutionary relationships among the living organisms being classified. The morphological characteristics of the organisms, used for producing classification and for diagnostic purposes are still indispensable to the taxonomist. The terms taxonomy and systematics are held as synonymous forms but technically they have different meanings.

Taxonomy:

• The branch of biology which deals with the scientific study of rules, principles, and procedures of classification, nomenclature, and identification is called Taxonomy.
• It is the theoretical basis of classification including its principles, procedures, and rules. – Simpson, 1961.
• It is the theory and practice of classification. – Mayr, 1969.

Systematics

The branch of biology that deals with classification and nomenclature is known as systematics. It is the scientific study of all kinds and diversity of organisms and all relationships among them. It covers the informative branch of taxonomy mainly concerned with the comparative and evolutionary relationship among organisms. The term systematics was coined by Linnaeus in 1735.

Relation between Taxonomy and Systematics:

H. L. Manson (1950) treated taxonomy as a broad field that includes four main subjects

• Systematics
• Taxonomic System
• Nomenclature
• Documentation

It means that systematics is a part of taxonomy. According to H. J. Lam (1959) and W. B. Turill (1964), these two terms are synonymous.

Differences between Taxonomy and Systematics:

Taxonomy	Systematics
1. It includes identification, nomenclature, and classification.	1. It includes identification, nomenclature, description, and classification.
2. It deals with the rules and principles of classification.	2. It brings out unique properties at every level of classification.
3. A.P. de-Candolle coined the term.	3. Carolus Linnaeus coined the term.

Components of Taxonomy

According to G. H. M. Lawrence (1955), taxonomy is a science that comprises of three main components. These are the identification, nomenclature, and classification of organisms.

• Identification: It is “the determination of a taxon as being identical with or similar to another already known organism; the determination may or may not be arrived at by the aid of literature or by comparison with the organism of known identity.” Identification is carried out with the help of taxonomic keys. It is assigning an organism with its correct name and placing it in its proper taxonomic category with the help of its specific characteristics.
• Nomenclature: It deals with the determination of the correct scientific name for a known plant or animal. Nomenclature should follow international rules. It is the application of distinctive names to each individual or each of the groups recognized in any given classification.
• Classification: The arrangement of organisms into groups on the basis of their affinities or relationships is termed as classification. On the basis of the specific characters, organisms are divided among specific classification units like species, genus, family, etc. It is a sub-topic of taxonomy that deals with arranging organisms into groups or categories according to a systematic plan on the basis of their similarities, differences, and relationships.

Stages of Taxonomy

• α-Taxonomy: When only morphological characters are used for the identification and classification of plants, then it is called α-taxonomy.
• β-Taxonomy: In addition to morphological characters, it involves genetical, anatomical, cytological, paleontological, physiological, and other characters. It also includes the affinities and interrelationships between separate groups of species. It is also called biosystematics. The terms α and β taxonomy were given by Turill (1943).
• γ-Taxonomy: It is concerned with the description, interrelationship, and evolution of one species from the other.
• Omega Taxonomy: Analysis and synthesis of all information and types of data to develop a classification system based on phylogenetic relationship are called omega taxonomy.

Differences between Classical Taxonomy and Modern Taxonomy:

Classical Taxonomy	Modern Taxonomy
1. Classical taxonomy is also called alpha (α) taxonomy.	1. Modern Taxonomy is also called omega taxonomy.
2. It is based on morphological characters only.	2. It is based on information from all aspects of the study.
3. A species is considered as a fixed entity.	3. A species is believed to be a dynamic entity and product of evolution.
4. On the study of a few individuals description of a species is based.	4. On the study of a large number of individuals description of a species is based.

Importance of Taxonomy

• Diversity of Living World: Taxonomy indicates the diversity of the living world, it is essential to make a catalog of organisms with their correct identification. It gives information about the specific habitat, habits, characteristics/affinities, etc., of different organisms.
• A universal system: Taxonomy provides a universal system and helps in communication between biologists.
• Evolutionary Relationships: By knowing the similarities and dissimilarities of various categories of organisms, along with the study of fossils, biologists are able to know the evolutionary sequences or phylogeny of different kinds of organisms.
• Other Biological Sciences: It is essential to know organisms being used in the study of other biological sciences like genetics, cytology, physiology, pathology, etc.
• Harmful Organisms: Taxonomy is useful in recognizing harmful organisms, weeds, vectors, pests, and pathogens. Control measures are not possible unless and until we are able to identify the vector, and know its life history, mode of spreading the disease, and vulnerable periods of its life cycle. Similarly, control of pathogens can not be achieved without their identification and knowledge of their life cycle.
• Medicinal Plants: Many medicines are obtained from plants. Similarly, a number of medicines are obtained from bacteria and fungi. Systematic knowledge is essential for their identification.
• New Organisms: Systematics helps us to identify newly discovered organisms.
• Breeding: All breeding programmes require the knowledge of different traits present in different varieties, subspecies, and related species.
• Indicator Organisms: Some organisms function as indicators of habitats, such as the occurrence of clean or polluted water or air, etc., e.g., Colpidium of polluted water, Prosopis of groundwater. The availability of coal and petroleum can be detected by the presence of certain fossils.
• Forestry: Forest management is possible only with the help of sound knowledge of taxonomy.
• New Source of Food: Systematics helps in finding new food resources, such as fishes, arthropods, molluscas, algae, etc.
• Conservation: Taxonomy helps us in the identification of useful wild organisms and thus in the conservation of natural resources.
• Study of Ecology: It can be done only through sound knowledge of plant, animal, and microbial taxonomy.

Species Concept

What is species and the characterization of species are supported by different views. However, recently such a concept on species has been developed that is accepted by almost all biologists and this concept comes under the biological species concept. In the early part of the 18th century, Carolus Linnaeus (1707-1778) proposed a clear concept of species. He considered species as the unit of classification. Buffon (1707-1788) supported the ideas of Linnaeus. But in the subsequent period, some critics opined that species is the imaginary entity of man and there is no real existence of the species. The followers of such a concept are known as nominalists. According to them, for the purpose of recognizing groups of organisms, they are included in a species and named so. However, on the question of the existence of a species, there is no doubt at present. However, in framing the definition of the species, there are different views. Among these, the prominent ones are the morphological species concept, biological species concept, and evolutionary species concept.

Morphological Species Concept

Linnaeus was the proposer of this species concept. According to this species concept, a species represents a group of organisms that are identical by morphological appearance and are distinctly separated from another group residing side by side. Though such a concept on species appears to be true it is faulty for many reasons. There are instances of the existence of such groups of organisms that appear to be morphologically similar but do not normally interbreed. Such groups of organisms are considered as sibling species. Drosophila pseudobscura and Drosophila persimilis are two sibling species of Drosophila. These two species though appear alike but failed to perform mating. Therefore, if species is considered as a unit of classification, the morphological species concept cannot give a justified definition of species.

Biological Species Concept

As the morphological species concept appears faulty the biological species concept has been proposed as an alternative. Principally Mayr (1940) was the proposer of this species concept. According to Mayr Species is a group of organisms that is reproductively isolated from another such group. However, Dobzhansky gave a definition of a species that states that species are the most inclusive Mendelian populations that share a common gene pool. However, the biological species concept takes a species as a dynamic entity and the individuality of a species may be determined by its morphological features. The biological species concept could be able to remove the errors in the morphological species concept in many respects. According to the biological species concept, the members of the same species are capable of inbreeding and contain similar gene pools. Therefore, the members of a species naturally show similarity within them. On this ground, it may be said that the biological species concept is established on the ideas of the morphological species concept and it is a special modification of the morphological species concept.

Limitations of Biological Species Concept:
The biological species concept is not solely error-free. There are some limitations to it.

• For the extinct species, the biological species concept is hardly applicable.
• In the case of asexually reproducing species, this species concept is not applicable because there is no way of exchange genes between two organisms.
• Species residing in isolated geographical regions or species at different time periods of the earth hardly come under the viewpoint of the biological species concept.

Evolutionary Species Concept:

Because of the limitations of the biological species concept efforts were made to generate a species concept that may be applicable to all the cases. As a result of this, the evolutionary & species concept comes under this species concept. The evolutionary species concept was proposed by Simpson in 1961 and supported this view by Grant (1971). The definition of a species as per this concept may be given in the following way. An evolutionary species is a lineage evolving separately from others and with its own evolutionary role and tendencies. In the case of extinct species and asexually reproducing or sexually reproducing species the application of the evolutionary species concept is tenable. According to this species concept, a species is a unit of evolution.

Species and Its Polymorphic Variations:

Whatever the definition of a species there are morphological variations within the members of a species. Primarily a species may be considered as monotypic or polytypic populations. However, as per the monotypic species concept, this species should be considered as a single interbreeding population and there should be known multiple groups within a species. As per the polytypic species concept, there may be more than one separate population and those remain distributed in different geographical zones. Such species groups remaining in different geographical areas may exhibit some morphological dissimilarities but can interbreed and are called polytypic species, e.g., various species of North American sparrows have been united with multiple geographical races. Such species groups come under subspecies or race. In the case of many animals existence of sub-species or races may be noticed.

Sub-divisions of Species

Based on the polymorphism of species, it may be subdivided into subspecies, Cline, and Deme.

Sub-species:
The population groups that are phenotypically dissimilar but not isolated reproductively are called subspecies. Previously subspecies were considered as variety. Subspecies become the principal subdivisions of a species and this may again be the complex of many local populations. As the members of subspecies are reproductively isolated they may easily be identified generally as the subspecies populations reside at different geographical areas. All the geographical areas come under the perimeter of the distribution of a species. Each subspecies may be named following certain rules and in that case, the principle of trinomial nomenclature is adopted. Presently the subspecies in which the human belongs is Homo sapiens sapiens. The subspecies of the lion is Panthera leo persica. As the subspecies reside at different geographical areas they are also considered a geographic race.

• Cline: Sometimes within several local populations, a continuous variation may be noted. Such a variable population capable of interbreeding is known as Cline. The variations as observed in the dines are considered as the effect of geographic and ecological influence. Therefore, dines are the further subdivisions of a species.
• Deme: Any local population capable of interbreeding is known as deme. The members belonging to a deme can exchange their genic property. Therefore, they are share-holders of the same gene pool and they may be considered as the smallest species population. A deme may also be taken as a partly isolated group.

Sibling Species:
Morphologically similar two groups of organisms exhibiting reproductive isolation is known as sibling species. Individuals belonging to sibling species are usually sympatric. According to the morphological species concept, sibling species come under the same species. Therefore, the existence of sibling species is supportive of the biological species concept. In fruit flies, Drosophila persimilis and Drosophila pseudobscura are the two sibling species. Several sibling species of Anopheles mosquitoes are Anopheles maculopennis, A. Sacharovoti, A. Subalpinus, and A. atropurvus.

Taxonomic Hierarchy

Taxonomic hierarchy (Gk, Hierarches = rank) is the sequence of arrangement of taxonomic categories in descending order during the classification of an organism. The main aim of taxonomic study is to assign an organism to an appropriate place in a systematic framework of classification. This framework is called a taxonomic hierarchy. The systematic framework of classification in which taxonomic groups are arranged in a definite order, from higher to lower categories is called Hierarchy.

Each category is considered a taxonomic unit and represents a taxon (PI. taxa). The taxonomic hierarchy was first established by Linnaeus (1758) in the animal kingdom in the 10th edition of the book “Systema Naturae”, and is therefore, also called Linnaean hierarchy. The categories used in the classification of animals are Kingdom, Phylum, Class, Order, Family, Genus and Species. However in the case of plants, division is used as a category in place of phylum. These seven categories are called obligate categories. These are as follows:

These seven obligate categories have further been split into super or sub-categories to indicate the taxonomic position of a species with more precision. So the present categories are

Taxonomist Simpson (1945) mentioned 21 taxa in his classification. Taxonomic hierarchy is useful as it provides information about relationship of an organism with others, quick identification of a taxon, all major traits and non-repetition of correlated traits of various categories. All the major traits present in an organism can be studied by nothing the traits of various categories in which the organism is classified.

Merits of Linnean Hierarchy:

• Linnaean hierarchy is not strictly rigid but flexible.
• It can accommodate new taxa if needed.
• It provides options in comparative discussion among the different organisms to place them in appropriate categories.

Categories of Linnean Hierarchy

• Species: A group of similar character individuals that can interbreed among themselves but are reproductively isolated from others is known as a species, e.g., all the potato plants of the world are included under Solanum tuberosum. Here the species is tuberosum.
• Genus: A group of related but different species that bear some similar characteristics are included under a genus, e.g., Ficus benghalensis (Banyan), Ficus elastica (Rubber), Ficus religiosa (Pipul), etc., are included under a genus Ficus.
• Family: This taxonomic category of one or more related genera has some common features that are different from those of genera of another family. Solatium, Petunia, and Atropa belong to the family Solanaceae, Examples from animals Genera Panthera (Tiger, Lion, Leopard) and Felis are included under the family Felidae.
• Order: This taxonomic category includes one or more related families with some common features, e.g., Solanaceae and Convolvulaceae families belong to the order Polymoniales. In animals, families like Felidae and Canidae are included under the order Carnivora.
• Class: This taxonomic category contains one of more related orders, e.g., class Dicotyledoneae contains orders like Rosales, Malvales, Ranales, etc. In animals, Class Mammalia includes orders Primata (monkey, gorilla, and gibbon) and Carnivora (tiger, cat, dog, etc.).
• Division: This category contains some common features of the class that are included to a higher category which is called Division in plants and Phylum in animals, e.g., the Division Spermatophyta in plants has seven classes of Gymnosperms and two classes of Angiosperms. In animal classes Pisces, Amphibians, Reptiles, Birds, and mammals are included under the Phylum Chordata.
• Kingdom: It is the highest taxonomic category. All the divisions of plants are included under Plant Kingdom or Plantae. All the phyla of animals are included under the Kingdom Animalia.

The taxonomic categories of mango plant and guineapig is mentioned below:
Mango Plant

• Kingdom: Plant kingdom/Plantae
• Division: Angiospermae
• Class: Dicotyledons
• Order: Sapindales
• Family: Anacardiaceae
• Genus: Mangifera
• Species: Indica

Guineapig

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Class: Mammalia
• Order: Rodentia
• Family: Canidae
• Genus: Cavia
• Species: Porcellus

Wheat

• Kingdom: Plant kingdom/Plantae
• Division: Angiospermae
• Class: Monocotyledonae
• Order: Poales
• Family: Poaceae
• Genus: Triticum
• Species: Aestivum

Man

• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Primata
• Family: Hominidae
• Genus: Homo
• Species: Sapiens

Taxonomic Key

A taxonomic key is a method used to classify and identify objects and organisms. A taxonomic key gives users a systematic and reproducible way to identify items and organisms. It is used for a variety of purposes, such as identifying unknown organisms to a certain taxonomic level (e.g., family, genus, and species). In taxonomic key

• Constant characteristics rather than variable ones should be used.
• Proper measurements rather than terms like ‘large’ and ‘small’ should be used.
• Characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field should be used.
• A positive choice should be made. The term ‘is’ instead of ‘is not’ should be used.

Types of Taxonomic Keys:

1. Dichotomous keys: Keys in which the choices allow only two alternative couplets are known as dichotomous keys. It is of two types:
   • Indented keys: Indents are the choice of the couplet an equal distance from the left margin.
   • Bracketed keys: The choices of the couplet must be numbered.
2. Polyclave keys: It is another type of key that is very easy to computerize.