Monday, 16 December, 2019

Systematics

Systematics

Taxonomy

Biology is an important branch of science dealing with the study of life. It is difficult to define life. But it is easy to discriminate living things from non-living things. On this basis life is defined as a physicochemical entity exhibiting growth, movement, irritability and reproduction. Biology includes two branches, namely Botany and Zoology. Botany deals with the study of plants and Zoology deals with the study of animals.

The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification. Scientists who study and contribute to the classification of organisms are known as systematists or taxonomists, and their subject is called systematics (Gk. Systema = systems or order or sequence) or taxonomy  (Gk. Taxis = arrangement; nomos = law).

History of classification

References of classification of organisms are available in Upanishads and Vedas. Our Vedic literature recorded about 740 plants and 250 animals. Few other significant contributions in the field of classification are :

Chandyogya upanishad : In this, animals are classified into three categories-viviparous Jivaja, oviparous Andaja and minute Udbhija.

Susruta samhita : It classified all ‘substances’ into sthavara (immobile) e.g., plants and jangama (mobile) e.g., animals.

Parasara : Here, angiosperms were classified into dvimatruka (dicotyledons) and ekamatruka (monocotyledons). He was even able to find that dicotyledons bear jalika parana (reticulate veined leaves) and monocotyledons bear maun laparna (parallel veined leaves).

Hippocrates and Aristotle : They classified animals into four major groups like insects, birds, fishes and whales in his Scala naturae.

Nomenclature

Nomenclature is the assignment of a distinctive name to each species. Almost all plants (and animals too) are known by different common names in different parts of the world. Even within the same country people of different states and regions use different common names.

Ipomoea batatas, for example, is called sweet potato in English, Shakarkandi in Hindi, Meetha alu in Assamies and Bengali, Kundmul in Telagu, Ratalu in Marathi and Jenasu in Kannad. Similarly, the common house sparrow is called “Goraiya” in India, “Sparrow” in England and “Haussperling” in America. The common names are thus quite confusion. This necessiated the need of giving scientific names so that scientists of different parts of the world could understand each other work.

The earliest scientific names were polynomial, i.e., they were composed of many words (which gave the characteristics of plants), e.g., Sida acuta (a member of Malvaceae) was named as Chrysophyllum folis, ovalis superne glabris parallel striatis subtus, tomentosonitidis. Such long names were difficult to remember. Hence, to make it easier binomial system of nomenclature was introduced.

Binomial system of nomenclature : The credit of giving binomial system of nomenclature goes to Swedish naturalist, Carolus Linnaeus. He employed this system in his book Species Plantarum, published in 1753. According to this system the name of a plant or animal is composed of two Latin (or Latinised) words, e.g., potato is Solanum tuberosum and house sparrow is Passer domesticus. The first word (i.e., Solanum) indicates the name of the genus (called generic name) and the second word (i.e., tuberosum) denotes the name of the species (called specific name). The generic name always begins with a capital letter and the specific name with a small letter and printed in italics.

The generic and specific names always have some meaning. They are based on some special characters of the plant and animals, on the name of any scientist or on some legend.

Usually the name of the author, who names a organism, is also written in full or in abbreviated form after the specific name. Thus, in case of Mangifera indica L., the L. stands for Linnaeus and in Lychnis alba Mill., the Mill. stands for Miller.

Sometimes a single species is described under different names by different authors. These name are called synonyms. In such cases, the name under which the species is first described, is considered to be valid.

Trinomial nomenclature : Certain species are divisible into smaller units, called varieties, on the basis of finer differences. The name of the variety is written after the specific name. Thus, the name may become trinomial or three word name. e.g., Homo sapiens europeus is the name of the man of European race. Trinomial nomenclature is simply an extension of the Linnaean system.

Code of biological nomenclature : Anyone can study, describe, identify and give a name to an organism provided certain universal rules are followed. These rules are framed and standardised by International Code of Botanical Nomenclature (ICBN) and International Code of Zoological Nomenclature (ICZN). The codes help in avoiding errors, duplication, confusion and ambiguity in scientific names. The codes are established and improved upon at International Botanical and Zoological Congress held from time to time. The names of bacteria and viruses are decided by International Code of Bacteriological Nomenclature (ICBN) and International Code of Viral Nomenclature (ICVN). Similarly, there is a separate International Code of Nomenclature for Cultivated Plants (ICNCP).

Valid name

(1) It is not a tautonym (e.g., same generic and same specific name).

(2) The name is new and given according to binomial nomenclature with abbreviation of author’s name.

(3) Its type specimen has been described in latin.

(4) The type specimen is kept in a recognised herbarium/collection.

(5) The new name has been published in a widely circulated scientific journal.

(6) All the names previously given to the organism must be indicated.

(7) If the new name is higher than species, the taxonomic category is indicated.

Type of specification

Holotype : The original type specimen from which the description of a new species is established.

Isotype : Duplicate of holotype, as another branch of the same tree.

Lectotype : Specimen selected from original material to serve as nomenclatural type where there is no holotype.

Neotype : New nomenclatural type when the original type is missing.

Paratype : Any other specimen described along with holotype.

Syntype : Any of two or more specimens cited by an author when there is no holotype.

The taxonomic hierarchy

It is the sequence of arrangement of taxonomic categories in a descending order during the classification of an organism. Hierarchy was first given by Linnaeus who used only five categories – class, order, genus, species and variety. The last one was discarded and three added so that now there are seven obligate categories. They are :

 

 

Some categories have been added to this list. They are called intermediate categories, e.g., Subkingdom, superphylum or superdivision, subdivision, superclass, subclass, superorder, suborder, superfamily, subfamily, tribe, subspecies, variety etc.

Taxon : The term taxon is used to represent any unit of classification. The unit (i.e., taxon) may be large (e.g., Plant Kingdom) or small (e.g., Algae, Fungi, or a single species).

Category : Various sub-divisions of plants kingdom such as division, class, order, family, etc., are referred to as categories. In the hierarchy of categories kingdom is the highest and species is the lowest category. The following is hierarchical series :

(1) Kingdom : It is the highest category in biological classification. e.g., all plants are include in plant kingdom.

(2) Division : It is a major group in the Linnaean hierarchy used in the classification of plants (equivalent to phylum in animal classification). It is a taxonomic category between kingdom and class. The subcategory of division is subdivision. The suffix of division is – ophyta.

(3) Class : A division is divided into classes. It is a taxonomic category between the division and order. Its suffix is – ae. The subcategories of class are subclass and series. In the class contain organism least similar to one another.

(4) Order : A class includes one or more orders. It is a taxonomic category between the class and family. Its suffix is – ales. The subcategory of order is suborder.

(5) Family : An order is divided into one or more families. It is the taxonomic category between the order and the genus. Its suffix is – aceae. The subcategories of family are subfamily, tribe and subtribe.

(6) Genus : The plural of genus is genera. A family includes one or more genera. The generic name is important and printed in Italics (If hand written, it is underlined). The subcategories of genus are subgenus, section and subsection.

(7) Species : It is the smallest rank of taxonomic classification. The first letter of the species is denoted with small letter. The species is printed in Italics (It is underlined if hand written). A genus may include one or more species. The subcategories of species are subspecies or varieties, subvarieties, form and subform.

Species concept

(1) Species is the basic unit of taxonomy.

(2) Modern concept of species is biological species concept introduced by Ernst Mayr (1942).

(3) Mayr defined species as groups of interbreeding natural populations that are reproductively isolated from each other group.

(4) The traditional concept of species was given by Linnaeus in Systema naturae; this is based on morphology and is also known as ‘Morphological concept’.

(5) Species inhabiting the same geographical area (identical or overlapping) are sympatric.

(6) Species inhabiting different geographical areas are allopatric.

(7) Related species which are reproductively isolated but morphologically similar are called sibling species.

(8) Classical systematic is based on the ‘Typological concept’ by Plato and Aristotle.

(9) Genetic species concept was given by Lotsy (1918), according to which, a species is a group of genetically identical individuals.

(10) Species that contain two or more subspecies are called polytypic species.

(11) Species that are not sub-division into subspecies are called monotypic species.

(12) Normally breeding is possible only within a species.

(13) Two different species are reproductively isolated.

(14) A new species is formed when genotypic changes accumulate in a population resulting in reproductive isolation.

System of Biological classification

Different systems of classification proposed from time to time have been divided into three basic categories viz., artificial systems, natural systems and phylogenetic system (However, Redford, (1986), included mechanical systems as a fourth category).

(1) Artificial system of classifications : These systems are more or less arbitrary as the plants are classified merely on the basis of gross morphology, habit and their importance to man. The main advocates of artificial system of classifications were :

Aristotle : He also classified animals on the basis of habitat into water, land and air.

Theophrastus : Theophrastus was a disciple of Plato and later Aristotle. In his book De Historia plantarum, he classified about 500 kinds of plants into four major group; trees, shrubs, subshrubs and herbs.

John Ray : He was a British botanist who published three volumes of his work Historia Generalish Plantarum consisting of improved classification originally proposed by him in Methodus Plantarum Noven. He was the first to divided the groups herbs, shrubs and trees into Dicots and Monocots on the basis of the presence of two or one cotyledons respecitvely. He coined the term species.

Carolus Linnaeus : Father of taxonomy. A Swedish botanist, who published an artificial system of classification based exclusively on floral characters. Linnaeus published several manuscripts including Hortus cliffortianus and Genera plantarum (1737). In his Genera plantarum he listed all the plant genera known to him. He published his best known Species plantarum in 1753. In this book he listed and described all species of plants known to him. He established binomial nomenclature.

(2) Natural system of classifications : These systems of classification are based not only on the characters of reproductive organs and structural morphology but used as many taxonomic characters or traits as possible to classify the organisms. The advocates of natural systems of classification are listed below :

Carolus Linnaeus : The first natural system of animal classification was developed by Linnaeus in his book “Systema Naturae” (10th edition 1758). Improvements were subsequently made by Haeckel (1864) and Lankester (1874).

George Bentham and Joseph Dalton Hooker : These two English botanists classified plants based on original studies of specimens. They published their well known scheme of classification in Genera plantarum (1862–83). This system of classification is still regarded as the best classification, especially from the practical point of view.

(3) Phylogenetic system of classifications : These systems of classifications are mainly the rearrangements of natural systems using as many taxonomic characters as possible in addition to the phylogenetic (evolutionary) informations. Some important phylogenetic systems of classifications were proposed by :

A.W. Eichler : A German botanist who proposed phylogenetic system of classification and published in the third edition of Syllabus der vorlesungen (1883).

Adolph Engler and Karl Prantl : These two German botanists classified plant kingdom on the basis of their evolutionary sequences. They started with simplest flowering plants and ended with plants of complex floral structures.

C.E. Bessey : He classified flowering plants on the basis of their evolutionary relationships.

John Hutchinson : A British botanist published his phylogenetic system of classification in ‘The Families of Flowering Plants‘.

Armen Takhtajan : A Russian botanist who published his system of classification in Botanical Review.

Arthur Cronquist : Published his classification in ‘An Integrated System of Classification of Flowering Plants‘.

Phylogenetic systems of classification for animals given by Dobzhansky and Mayr.

New systematics or Biosystematics

The term new systematics was proposed by Sir Julian Huxley in 1940. In the new systematics, the species are considered related to one another, mutable and the work of gradual modification. This is in confirmity with the facts of evolution.

Forms of new systematics : There are several forms of new systematics :

(1) Morphotaxonomy : It is based on the structural features of the organisms.

(2) Cytotaxonomy : It is based on cytological information of cell, chromosome number, structure and behaviour of chromosome during meiosis. Karyotaxonomy is a branch of cytotaxonomy which is based on banding pattern of chromosme.

(3) Biochemical taxonomy or Chemotaxonomy : It is based on the protein and serum analyses and on the chemical constituents of the organisms.

(4) Numerical taxonomy : It involves quantitative assessment of similarities and differences in order to make objective assessments. Characters of organisms are given equal weight and the relationships of the organisms are numerically determined, usually with the aid of a computer.

(5) Experimental taxonomy : It is based on the genetic relationship determined with the help of experiments.

Modern trends in systematics

Two kingdom system of classification : This system of classification is the oldest it was suggested by Carolus Linnaeus in 1758. He divided the living word (organism) in to two kingdoms, Plantae (for all plants like tree, shrubs, climbers, creepers, moss and floating green algae) and Animalia (for animals).

Three kingdom system of classification : Ernst Haeckel, a German biologist and philosopher, suggested a third kingdom protista in 1866 for :

(1) Unicellular organisms such as bacteria, protozoans and acellular algae.

(2) Multicellular organisms without tissue such as algae and fungi.

Four kingdom system of classification : It was proposed by Copeland in 1956. The two additional kingdoms were Monera for the bacteria and blue green algae and Protista for protozoans, algae and fungi.

Five kingdom system of classification : R.H. Whittaker, an American taxonomist. He proposed five kingdom system of classification in 1969.

This system replaced the old, two-kingdom grouping of living organisms. As already discussed, a division of living world merely into plant and animal kingdoms is too simple. It does not take into account the gradual evolution of distinct plant and animal groups and it allows no place for those primitive organisms that even now are neither plants nor animals nor that are both. In this classification eukaryotes were assigned to only four of the five kingdom.

Five-kingdom classification is based on the following five criteria :

(1) Complexity of cell structure.

(2) Complexity of organism’s body.

(3) Mode of obtaining nutrition.

(4) Phylogenetic relationship.

(5) Ecological life style

The five kingdom are : Monera, Protista, Fungi, Plantae and Animalia.

Monera : It includes all prokaryotic organisms, which may have autotrophic or heterotrophic type of nutrition and are surrounded by a rigid cell wall. They are usually unicellular ranging from one to a few micron size. They are important decomposers and mineralizers. It include Bacteria, Actinomycetes, Cyanobacteria, etc.

Protista : They are mostly aquatic unicellular organisms, having eukaryotic organization. They generally have cilia or flagella and have diversified mode of life. Most of them are photosynthetic autotroph but some are holozoic, parasitic or even symbiotic.

Fungi : They are diversified eukaryotic, heterotrophic organisms, body of which is made up of filamentous mycelium. They generally grow as saprophytes or parasites. They include moulds, mushrooms, puff balls etc.

Plantae : It includes all multicellular photosynthetic plants, which may have red, brown or green pigmentations except a few parasitic, saprophytic and insectivorous types. Most of the members are primary producers of land and water (Phaeophyta, chlorophyta, bryophyta, tracheophyta etc.).

Animalia : They are multicellular eukaryotic organisms having holozoic type of nutrition. They show muscular contractility and have a nervous system.

 

a-taxonomy (Turril, 1938) based on collections identification, compilation of fauna and flora. w (omega) taxonomy (Turril, 1938) : It brings out the correct relationships as in new systematics. Cladogram : Tree like graphic representation of evolutionary history of organism. Dendrogram : When cladogram is base on numerical taxonomy it is called dendrogram. Tribe : An intermediate category which is used in between genus and sub family. Taxonomic key : It is a set of alternate characters of different types arranged sequence wise in such manner that by selection and elimination one can quickly find out the same organism. It is of two type : (a) Bracketed key (b) Indented or yolked key.? Floral characters are used as basis of classification and for identifying new species because floral characters are conservative when compared with vegetative characters.? In Bentham and Hooker’s classification Dicotyledons have been kept before Monocotyledons. Seeds plants have been divided into Dicots, Gymnospermae and Monocots. Among the vegetative characters, venation in leaf in one of highly acceptable characters for classification of angiosperms. For declaration of new species, floral characters of new species should be used. The correct sequence of taxa in Linnaean hierarchy is species ® genus ® family ® order® class. Bauhin (1623) proposed the binary system of nomenclature which was elaborated by Linnaeus (1753) into binomial system. Monotypic genus : A genus having only one species, e.g., Homo. Polytypic genus : A genus containing more than one species, e.g., Panthera, Solanum. J.K. Maheswari described the plants of India in ‘Flora of Delhi’. In Bentham and Hooker’s system of classification, evolutionary criteria have not been followed hence not phylogenetic. Hooker compiled first complete flora of India and wrote the book ‘Flora of British India’.

 

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