biological classification


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Biological Classification : 

Biological Classification Dr. R. K Pillai, Dept of Zoology, Hindu College, Moradabad Pre-Medical Presentation Diversity in Living World

Introduction : 

Introduction Systematics (Gr. systema, system; ikos, body of facts) deals with the study of the diversity of organisms. The term systematics coined by Swedish botanist Carolus Linnaeus in 1735. Taxonomy is the study of the principles and procedures of classification. The term taxonomy (Gr. taxis, arrangement; nomos, law) first proposed by the French botanist August de Candolle (1778-1841) for the theory of plant classification. Biological classification is a subtopic of systematics.

History of Classification : 

History of Classification Aristotle (384-322 BC), the Greek philosopher is known as ‘Father of Zoology’. Aristotle made the first recorded attempt to classify the animals in his book Historia Animalium. Aristotle made two main groups: Anaima (animals with no red blood, invertebrates) and Enaima (animals with red blood, vertebrates). Enaima was further divided into Ovipara and Vivipara. Theophrastus (372-287 BC ), a student of Aristotle, often referred to as the ‘Father of Botany’, classified plants on the basis of form and texture. Aristotle

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Theophrastus divided them into trees, shrubs, undershrubs and herbs. Many of the names given to plants by Theophrastus survive in modern botany. Theophrastus gave names and description of 480 plants in his book Historia Plantarum John Ray, the Cambridge clergyman-naturalist (1627-1705)) was first to introduce and define the term ‘Animal Species’. Ray was really the first to apply the species concept to a particular kind of organism. John Ray

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Ray described more than 18,000 plants and animals in his Historia Generalis Plantarum. John Ray also developed the 'key' for identification of animals. The modern system of classification began in 1758 when Carolus Linnaeus (1707-1778), published tenth edition of his book Systema Naturae. Linnaeus's original name was Carl von Linné, which he "latinized" to Carolus Linnaeus. Linnaeus also recognized that different species could by grouped into broader categories based on shared characteristics. Any grouping of organisms that shares a particular set of characteristics forms an assemblage called taxon. Carolus Linnaeus

Nomenclature : 

Nomenclature Nomenclature (L. nõmen, a name; calator, to call) is the assignment of a distinctive name to each species. First act in taxonomy is identification. The main purpose of classification is to establish relationships. Three types of nomenclatures are: Polynomial Nomenclature Binomial Nomenclature Trinomial Nomenclature

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1. Polynomial Nomenclature Upto the 9th edition of Systema Naturae, Linnaeus had used a polynomial (many-name) system of nomenclature using as many as 15 descriptive Latin words for naming a species. These names were lengthy and difficult to remember. 2. Binomial Nomenclature Linnaeus not only laid the foundation of taxonomy but also introduced binomial nomenclature that we use today. Each organism is designated by two names, the first is the name of the genus; the second called the “specific epithet” identifies the particular species within the genus. No two species can have the same name. The names are always Latin (or Latinized) and the genus is capitalized while species name is not.

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Both names are customarily printed in italics (underlined if handwritten or typed). The system of scientific names developed by Linnaeus in the eighteenth century remains in use today and we still use more than 11,000 names proposed by Linnaeus. 3. Trinomial Nomenclature A trinomial nomenclature is employed to describe subspecies, for example, Gorilla gorilla gorilla is a subspecies of gorilla. Other examples are the modern man called Homo sapiens sapiens, and rat called Rattus rattus rattus. Botanists also do not recognise a subspecies category.

Rules of Nomenclatue : 

Rules of Nomenclatue The International Commission on Zoological Nomenclature has served since 1901 to establish rules by which animal species are named. Schenk and McMaster (1958) gave the catalogue of International Rules of Nomenclature. International Code of Biological Nomenclature is applicable to both plants and animals. Priority is the basic principle of zoological nomenclature. Sometimes different authors give different names to one and the same species. According to the law of priority, the oldest name is valid (the name under which the species was first described is a valid name).

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Scientific names are the names given by biologists which are universal and not liable to change with time or language. One commonly refers to a particular organism by using only the binomial nomenclature, as in Canis familiaris (the dog), Rana tigrina (the frog) and Homo sapiens (the man).

Type Specification : 

Type 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 author when there is no holotype

Taxonomic hierarchy : 

Taxonomic hierarchy Various groups used in classification are called taxa. The arrangement of taxa is called hierarchy. A species represents the smallest taxon, although subgroups such as races, are also used. SPECIES. One or more similar species are grouped into a GENUS. One or more similar genera form a FAMILY. One or more similar families form an ORDER. One or more similar orders form a CLASS. One or more similar classes form a PHYLUM. One or more similar phyla form a KINGDOM. Many biologists now assign similar kingdoms to the same DOMAIN. All domains taken together comprise the living world.

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Taxon is a classificatory unit of any rank. In Linnean hierarchy, the number of taxa is five namely : classes, order, genus, species and varieties. Georges Leopold Cuvier (1769-1832), the French naturalist, added the ‘Phylum’ in taxonomy. In taxonomy, the correct sequence is: Class-Order-Family-Tribe-Genus-Species . Microbiologists and botanists use the term ‘Division’ instead of ‘Phylum’. All kingdoms have more than one phylum. The kingdom Plantae contains several divisions (= phyla), including flowering plants, conifer trees, mosses, ferns and several other groups.

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A class is a subdivision within a phylum, within the phylum Chordata there are five classes: mammals, birds, reptiles, amphibians and fishes. An order is a category within a class, Carnivora is an order of flesh-eating animals within the class Mammalia and there are several other orders of mammals like cattle, rodents, bats, seals, whales, etc. A family subdivision of an order consists of a group of closely related genera, which in turn are composed of groups of closely related species. The animals such as lion, tiger, leopard from the genus Panthera, and cats from genus Felis are included in the family Felidae.

Species Concept : 

Species Concept Species is the basic unit of taxonomy. Species inhabiting the same geographical area (identical or overlapping) are sympatric. Species inhabiting different geographical areas are allopatric. Related species which are reproductively isolated but morphologically similar are called sibling species. A species restricted to a given area is called endemic species. The traditional concept of species was given by Linnaeus in Systema Naturae, this one based on morphology and is also known as ‘Morphological concept’. Genetic species concept was given by Lotsy (1918) according to which, a species is a group of genetically identical individuals.

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Species that contain two or more subspecies are called polytypic species. Species that are not subdivided into subspecies are called monotypic species. Modern concept of species is biological species concept introduced by Ernst Mayr (1942). Mayr defined species as groups of interbreeding natural populations that are reproductively isolated from each other groups. Two different species are reproductively isolated. A new species is formed when genotypic changes accumulate in a population resulting in reproductive isolation. Ernst Mayr

Systems of Biological Classification : 

Systems of Biological Classification There are four different systems of classification, mainly used by taxonomists: 1. Artificial System 2. Natural System 3. Phylogenetic System 4. Phenetic System

1. Artificial System : 

1. Artificial System A classification based on one or a few superficial similarities. Adopted by Pliny (first century AD) for animals on the basis of habitats, e.g., land, air and water. The classification of plants on the basis of habit into (i) herbs (ii) undershrubs (iii) shrubs and (iv) trees is an artificial classification. The Linnaean system of classification of plants on the basis of number and arrangement of stamens is artificial. Linnaeus used number, union, length and certain other characters of stamens as the basis of classification of plants in his book 'Genera Plantarum' (1737). Linnaeus proposed classes as Monandria ( 1 stamen), Diandria (2 stamens) and Tri- and polyandria ( 3 and more stamens). Flower is chosen as the primary basis of classification, floral features show less variations than do vegetative features in response to environment.

2. Natural System : 

2. Natural System A natural system of classification compares many characteristics. This increases the amount of information available from the grouping. A natural system of classification indicates taxonomic position and relatedness. Natural system is based on morphology and evolution and is used nowadays. The most important criterion used for the present day classification of living organisms is based on anatomical and physiological characteristics. Natural system of classification of the seed plants was proposed by George Bentham (1800-1884) and Joseph Dalton Hooker (1862-1883). Bentham and Hooker published three volumes of "Genera Plantarum" describing 202 families of angiosperms.

3. Phylogenetic System : 

3. Phylogenetic System The classification based on evolutionary sequence and the genetic relationships among the organisms. Proposed by Adolf Engler (1844-1930) and his associate Karl Prantl (1849-1893) in their book Die Natürlichen Pflanzen Familien for plant kingdom. Adopted by the English botanist John Hutchinson (1884-1972) in two volumes of his book "Families of Flowering Plants". Phylogenetic classification is based on common evolutionary descent. “Taxonomy without phylogeny is similar to bones without flesh” is the statement of A.L. Takhtajan (1967) who wrote the book "A System of Phylogeny of Flowering Plants". A modern method of classification called cladistics is based on evolutionary history. To identify groups sharing recent common ancestry, many biologists use a procedure called cladistics.

4. Phenetic System : 

4. Phenetic System Phenetic taxonomy (classification) is based on the overall similarity of organisms evaluated without regard to phylogeny. Phenetic classification is based on observable characteristics of existing organisms . Phenetic classification did not have a strong impact on animal classification and scientific interest in this approach is in decline.

New Systematics : 

New Systematics Sir Julian Huxley (1940) proposed the term ‘New systematics’ Cytotaxonomy - based on cytological study. Karyotaxonomy - based on nucleus and chromosomes. Morphotaxonomy - based on morphological characters. Biochemical taxonomy - based on biochemical studies. Chemotaxonomy - based on specific chemicals like secondary metabolites. Numerical taxonomy - based on statistical methods. Experimental taxonomy - based on experimental - determination of genetical interrelationshipsand role of environment in theirformation

Classification of Organisms : 

Classification of Organisms 1. Two Kingdom System 2. Three Kingdom System 3. Four Kingdom System 4. Five Kingdom System 5. Six Kingdom System

1. Two Kingdom System : 

1. Two Kingdom System Biologists since the time of Aristotle divided the living world into two kingdoms: Plants and Animals. Carolus Linnaeus also distinguished two kingdoms of living things:Animalia for animals and Vegetabilia for plants. Linnaeus divided each kingdom of living organisms into classes. Two kingdom system was quite reasonable at that period of time since plants and animals could be very clearly distinguished. Aristotle

2. Three Kingdom System : 

2. Three Kingdom System Ernst Haeckel, a German naturalist, in 1866 created a third kingdom Protista to include unicellular protozoa and algae. Microbes had been discovered nearly two centuries before by the Dutch scientist Antony van Leeuwenhoek (1632-1723) “the Father of Microbiology”. Haeckel placed all unicellular protozoa and algae (microscopic) organisms in a new kingdom. In his “Generelle Morphologie der Organismen” Haeckel named this third kingdom and the organisms contained within it the Protista, “the first of all, primordial” Ernst Haeckel

3. Four Kingdom System : 

3. Four Kingdom System In the 1930, French marine biologist, Edouard Chatton noticed two kinds of organisms prokaryotes such as bacteria that lack nuclei, and eukaryotes, more complex organisms that have nuclei. In 1938, American biologist Herbert F. Copeland made a fourth kingdom called Monera (now termed Kingdom Prokaryotae).

4. Five Kingdom System : 

4. Five Kingdom System In 1969, the American ecologist Robert H. Whittaker proposed that fungi differ enough from other microorganisms to justify calling them a separate kingdom . This led to a five kingdom scheme of classification comprising Monera, Protista, Fungi, Plantae and Animalia. Five Kingdom classification is based on (a) complexity of cell, (b) complexity of organism, (c) mode of nutrition and (d) major ecological role. Whittaker's scheme is still widely accepted as a logical classification of living things. Robert H. Whittaker

5. Six Kingdom System : 

5. Six Kingdom System The most recent classification is the six kingdom system proposed by three microbiologists Carl Woese, O. Kandler and M. L. Wheelis in 1990. Woese, Kandler and Wheelis proposed that all organisms be divided into three major groups called domains above kingdom level: Eukarya, Bacteria and Archaea. After comparing the base sequences of 16s rRNA from a variety of organisms, Woese, Kandler and Wheelis proposed that bacteria should be divided into two kingdoms Eubacteria and Archaebacteria. In Woese’s system, the domain Eukarya includes four of the six kingdoms: protists, fungi, plants and animals. The other two domains have just one kingdom each; the domain Bacteria contains the kingdom Eubacteria and the domain Archaea contains thekingdom Archaebacteria. Six kingdom scheme is currently accepted by many biologists.

Kingdoms of Life : 

Kingdoms of Life Kingdom Monera This kingdom, also known as Prokaryote, comprises bacteria, filamentous actinomycetes and photosynthetic blue green algae (Cyanobacteria) which have the same cellular structure. Kingdom Protista Most members of this kingdom are one-celled eukaryotic organisms, although, there are some colonial forms. Protists are usually found in aquatic environment. Kingdom Fungi Kingdom Fungi , also known as Mycetae, comprises mainly fungi which are nonmotile. Fungi have a rigid, thin cell wall which in most species is composed of chitin. All fungi are nonphotosynthetic, eukaryotic organisms

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Kingdom Plantae The members of this kingdom are nonmotile, terrestrial, multicellular that contain chlorophyll and produce their own organic compounds. All plant cells have a cellulosic cell wall. Kingdom Animalia The members of this kingdom are eukaryotic, heterotrophic and multicellular. Most animals are motile and capable of sexual reproduction. Animal cells are without cell wall. Animals are capable of sexual reproduction, but also able to reproduce asexually.

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