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Classification determines methods for organizing the diversity of life on Earth.
Since life first appeared on Earth 3.5 billion years ago, many new types of organisms have evolved.
Many of these organisms have become extinct, while some have developed into the present flora and fauna of the world. It is believed that the extinct species may outnumber the living ones by 50 to 100 times.
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At the same time a large number of species are still unknown and yet to be identified and described. Every year about 15,000 new species are discovered. Thus, extinction and diversification continue nonstop on earth, and some of these we know by the fossils (impressions or dead remains of past organisms) they left behind in the rocks. In order to study the vast diversified world of living and extinct organisms it is essential to classify them into groups (categories) so that they could be named, remembered and studies properly.
Basics in Biological Classification:
Biological Classification involves the techniques of characterization, identification, nomenclature and grouping of organisms. Sometimes, the terms such as biological classification, taxonomy and systematic are used as synonyms. However, G. Simpson (1961) regards them as separate fields.
He explained classification as the ordering of organisms into groups and taxonomy as the study of principles and procedures of classification. He considered systematics as the study of organisms and their diversities and the evolutionary relationships among them. The scope of systematics includes identification, nomenclature and classification.
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Need of Classification:
Classification is the arrangement of organisms into groups and sub-groups on the basis of similarities and dissimilarities and placing them in a hierarchy that brings out their relationships.
The organisms should be classified due to following reasons:
(i) Classification makes identification and the study of a wide variety of organisms easy.
(ii) It reveals inter-relationships among different groups of organisms.
(iii) It gives information about the organisms and fossils of other localities.
Importance of Classification:
The contribution of classification is complementary to all branches of basic and applied biology directly or indirectly.
1. Understanding the diversity:
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Classification helps in knowing about bio-resources, their diversity and evolution.
2. Understanding phylogeny:
Classification based on the studies of fossil reveal evolutionary relationships (phylogeny) among the organisms. For example, the fossil of Cooksonia that appeared about 420 million years ago was the first known vascular land plant from which other vascular plan is have descended. Similarly the Archaefructus and Amborella are the oldest known angiosperms in the fossil record.
3. Inter-relations:
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Classification is essential to understand the inter-relationships among different categories of organisms.
4. Development of other Biological Science:
Classification provides information for the development of all branches of biology, e.g., biogeography, ecology, ethology, forestry etc.
5. In agriculture and forestry:
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Taxonomic knowledge provides vital information about the pests and hosts that is essential for a cultivator before undertaking any control measures.
6. Mineral Prospecting:
Taxonomy gives the correct sequence of geological events in an area. It is basic to any search for fossil fuels and mineral deposits.
7. Biological Control:
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Taxonomic knowledge is essential for accurate identification of pests and its natural enemies. Now a days instead of insecticides (chemical control), natural enemies of pest are introduced for biological control. Thus, correct identification of target species ensures a effective control at minimum costs. It would otherwise confusing, expensive and even chances of introducing wrong control agents.
8. Conservation of wild life:
Due to human greed and ignorance, many species of flora and fauna have become extinct and many others are following suit. Taxonomists help to identify these endangered species and help in initiating the wild life conservation programmes.
9. Quarantine:
The spreading of new pests and diseases from one country to the other through the agency of human is called quarantine. It can be checked by establishing quarantine laboratories at airports, seaports, railway stations etc. Here, taxonomists provide correct and prompt identification of the pest of disease.
Nomenclature (L. Nome = Name; Calere = To Call):
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The term nomenclature means the scientific naming of organisms according to an established system. The naming of plants on a scientific basis is called botanical or plant nomenclature. In earlier days common or vernacular names were in use which generally changes with change of language. Later in the haunt of one common internationally accepted name for a species, scientific names (Technical names) have been introduced in form of polynomial, binomial and trinomial systems of nomenclature.
(a) Polynomial Nomenclature:
Before 1750, taxonomists started using a string of descriptive Latin words to designate a species. For example, in the herbal of Clusius (1583) a species of willow is named Salix pumila angustifolia altera.
However polynomial nomenclature was discarded for two reasons:
(i) It was lengthy and difficult to remember
(ii) It often differs from scholar to scholar based on the characters chosen by them.
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(b) Trinomial Nomenclature:
Mayer (1953) introduced the concept of sub-species which meant the geographically defined aggregates of local populations. The naming of a species, especially the animals, up to sub-species level is called trinomial nomenclature, e.g. Homo sapiens sapiens, Homo sapiens Neanderthals. Botanists don’t consider trinomial nomenclature.
Binomial Nomenclature:
Carolus Linnaeus (Karl Von Linnae), the great Swedish naturalist, devised binomial system of nomenclature in his book Philosophia Botanica (1751). Now the binomial system of scientific naming of organisms becomes a common and established practice.
According to binomial nomenclature, the scientific name of an organism composed of two Latin or Latinized words i.e. the first word is called genus (= generic name or generic epithet) followed by the second word called species (= specific name or specific epithet). For example, the botanical name of sugarcane is Saccharum officinarum. Very rarely the generic and specific names are same. They are called tautonymes, e.g. Gorilla gorilla, Catla catla, Naja naja, Rattus rattus etc. Some organisms (fossils and most microorganisms) are known by their technical names only.
Rules of Binomial nomenclature:
The rules of nomenclature are framed and standardized by five separate codes such as:
(i) International Code of Botanical Nomenclature (ICBN),
(ii) International code of Zoological Nomenclature (ICZN),
(iii) International Code of Bacteriological Nomenclature (ICBacN),
(iv) International Code of Viral Nomenclature (ICVN) and
(v) International Code of Nomenclature for Cultivated Plants (1CNCP).
These codes help in avoiding errors, duplication, confusion and ambiguity in scientific names.
Some rules framed under these codes as well as the rules set by Linnaeus are as follows:
1. The scientific names of plants and animals should be in Latin or Greek because it is officially dead language. The use of Latin for naming also means that no-one can be offended by being forced 10 use someone else’s language.
2. The scientific names prior to the 1.8.1758 for animals of Systema Naturae (10th Edition) and 1.5.1753 for plants of Species Plantarum by Linnaeus are not recognized.
3. These names should be in italics when printed or separately underlined when hand written to indicate their Latin origin.
4. The genus starts with capital letter, while species in small letter.
5. The name of the author, first reporting it should remain in abbreviated form at the end of the scientific name and it is printed in Roman, e.g. Oriza sativa Linn. The scientific name with name of the author at the end is called complete scientific name.
6. Only one valid name for one species is permitted and it is based on the rule of priority that is the author first effectively and validly publishing the name will be considered.
7. In case of changing a scientific name that is double citation, the name of the second author is placed in bracket after the scientific name and the first author’s name in abbreviated form comes after that, the new name is always based on the older name and it is called the basionym.
8. To avoid confusion no two generic names in any kingdom can be same. Specific names can however be repeated as they often qualify the generic name. For example, the specific name of both mango (Mangifera indica) and tamarind (Tamarindus indicus) are the same which mean Indian.
9. The reporting of a new species of plant should be accompanied by a Latin description or Diagnosis.
10. When a plant species is reported, the author should submit a herbarium sheet of the specimen (Dried plant with reproductive part placed on a sheet of paper). This is designated as type specimen (holotype, isotype, paratype, topotype, lectotype etc.). Holotype is the type specimen submitted by the original author at the time of publication. All other specimens of the same species collected at the same time are called isotypes.
The specimen cited with the original description other than holotype or isotype(s) is called paratype. When the same specimen is collected from the same locality from where the holotype was collected is called topotype. In absence of a holotype, the type specimen selected from the original material by a subsequent author is called lectotype. In the absence of original type specimen, a new specimen selected from a new location by a new author is called neotype. This concept of permanent naming to a type specimen is called typification and the type specimens should be preserved in the Herbaria of all international Botanic Gardens.
Hierarchy of Classification:
Classification is not a single step process but solves hierarchy of steps in which each step represents a rank or category. A category is an abstract term which basically represents a rank or taxon (plural: taxa). A taxon represents a group of real biological objects having a set of common features called correlated characters. For example, Monera is a taxon of bacteria belonging to category kingdom.
All the categories arranged in ascending order constitute a taxonomic hierarchy. There are seven obligate categories used in taxonomical studies of all known organisms which are: Kingdom, Division (for plants) or Phylum (for animals), class, order, family, genus and species. These broad categories have been further divided into sub-categories by using prefix super-sub-or infra-to facilitate more sound and scientific placement of various taxa. As we go from the species to kingdom, the number of correlated characters goes on decreasing.
Linnaeus first introduced the hierarchical system of classification and is called Linnaean hierarchy. In his classification two categories i.e. phylum and family were not included. In taxonomic hierarchy, species is the basic unit of classification and is the lowest category while kingdom is the top most category.
In addition an eighth category, called domain (synonymous with super-kingdom) is sometimes used above the kingdom level. Recently a new category “tribe” has been incorporated between genus and sub-family while “paravorder” incorporated between infraorder and super-family. The purpose of the introduction of parvorder and tribe was to allow a steady gradient in time among the level of classification.
