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The organisms living on earth are the product of evolution. All organisms have direct or indirect interrelations. They show diversities in their structures, functions and behavior. Organisms are classified into smaller or larger groups on basis of their similarities and dereferences.

Definition :
The scientific method of dividing organisms into smaller and larger is called classification.

Importance of classification :
We can follow the path of evolution of organisms with the help of similarities and differences
amongst them and their interrelationships. Fauna and flora of various geographical regions
can be prepared. Measures can be taken for conservation of endangered extinction prone
organisms. The geographical distribution of organisms can be explained. Chemotaxonomy, cytotaxonomy and
Numerical taxonomy are the new method of classification are developed through coordination with other disciplines
of science.

History of classification :

2 kingdoms
3 kingdoms
2 empires
4 kingdoms
5 kingdoms
Woese et al.
6 kingdoms
Woese et al.
3 domains
(not treated) Protista Prokaryota Monera Monera Eubacteria Bacteria
Archaebacteria Archaea
Eukaryota Protista Protista Protista Eukarya
Vegetabilia Plantae Fungi Fungi
Plantae Plantae Plantae
Animalia Animalia Animalia Animalia Animalia

Principle of Taxonomy :

The nomenclature and classification of organisms is based on definite rules. Some important rules are as under :

Naming and classification of organisms must be done in latin language.
Organisms must be named by two names.
The 1st name must be of the genus.
The 2nd name must be of the species.
The 1st letter of genus must be in capital letter.
Species name must be written in small letter.
The popular name of the organisms must be written last.
After the species name, the name of the scientist who coined the name must be written in short.
The generic name used for naming one organisms should not be used for any other organisms.

Example :(a) Maize – Zea mays. L. maize
               (b) Man – Homo Sapiens. L. man

Categories of classification :

Taxon : The groups of organisms arranged at various level of classification constitute
taxa (singular - taxon). Each taxon indicates a specific level of similarities and differences.

Kingdom : The major groups are given categories of kingdoms.

Then successfully taxa like subkingdom – division or phylum – subdivision or subphylum –super class – class – subclass – series – order – Family – genus – species are organized.

By moving downwards from kingdom towards species, the differences decrease and the ssimilarities increase.


  The details of various taxa are given below :

Name of Taxon
Organism : Man
Organism : Sunflower
 Kingdom Animal Plant
 Division/Phylum Chordata Tracheophyte
 Sub division/Sub Phylum Vertibrata Angiosperms
 Class Mammal Dicotyledons
 Sub class
 Eutheria Gamopetalae
 Series Primate Inferae
 Homonoidea Asterales
 Family Homonidae Asterales
 Genus Homo Helianthus
 Species Sapiens : L
 annus : L
 Common name
 Man Sunflower

 Species :
 Definition :  A group of organisms showing maximum similarities, in the largest number of characters is called                      a species.
  All members of a species are capable of interbreeding and giving rise to fertile offsprings.
 Genus :
 Definition :  A group of species having a common ancestor is called genus.

 Family  : A group of genera which are closely related constitute a family. Each family possessed certain                            common characters.

 Order :    A group of interrelated families constitute an order. Then, a group of orders forms a series, a group                 of series forms a subclass and so on upto kingdom various taxa are constituted.

 Note :   Just of an understanding, classification of sunflower and man are displayed in the following table.

  Systems of classification  :
   There are three types of systems of classification,
   Artificial system :
   (1) Systems based on some limited number of characters are of this kind.
   (2) For E.g., classification of animals into aquatic, terrestrial and aerial forms or classification of plants into             herbs, shrubs and trees.
   (3) This types of methods are useful in primary identification of organisms.

 Natural system :

    (1) This system takes into account, the similarities and differences in the largest number of characters.

    (2) This systems are more dependable.

    (3) E.g. of this system is classification system of plants and animals.

    (4) This system does not provide any information regarding the phylogeny or evolution.

 Phylogenic systems :

    (1) It follows the evolutionary sequences of organisms.
    (2)They reflect genetic relationship of organisms.
    (3) They are based on aspects like origin of organisms, their ancestors, their evolutionary trends, their                 primitiveness or modernity, their life cycles, etc.
    (4) Example: Whittaker′s system of five kingdoms.

 Two kingdom systems of classification :

 This is a natural system of classification. Organisms are classified into two kingdoms,

 Plant kingdom

 Animal kingdom

 Kingdom :  Character of plant :

 (a) Plants are immobile and possess autotrophic mode of nutrition.
 (b) They can carry out photosynthesis because they possess chlorophyll pigments.
 (c) Their cells are surrounded by a nonliving cell wall.

 Kingdom :  Character of animal:

 (a) Animals are mobile.
 (b) They possess heterotrophic mode of nutrition.
 (c) Their cells are not surrounded by dead cell walls.

 This system is simple and widespread. It contains several illogical and contradictory aspects.

( Defects of two kingdom system)
 In this system autotrophic algae and heterotrophic fungi, both are included in plants.
 Unicellular and multicellular organisms are also arranged together.
 Prokaryotic and eukaryotic organisms are also considered as plants.
 Fungi is also included in plants however, they possess different kind of cellwall, and lacking chlorophyll.
 Euglena are included in both kingdom plant as well as animal. 
 There is no reference to viruses in this classification.

 Whittaker′s classification into five kingdom :

R.H.Whittaker (1969) proposed a five kingdom classification. The kingdoms defined by him were named   Monera, Protista, Fungi, Plantae & Animalia.
The kingdoms from Protista to metazoa are of Eukaryotic nature. The main criteria for classification used by him include complexity of cell structure , evolutionary status, complexity of body organization, mode of nutrition, mutual evolutionary tendencies, reproduction and Phylogenetic relationships.
The process of gradual evolution is reflected through prokaryota to eukaryota; from unicellular to multicellular to tissue possession.
From ecological viewpoint, these kingdoms illustrate groups like decomposers, heterotrophs, produces etc.

Kingdom : Monera

 (1) Nucleus :

   (a) Organisms of this kingdom do not possess a well-developed nucleus.
   (b) Nucleolus and nuclear membrane are absent.
   (c) The genetic material occurs in the form of a mass of nucleoproteins.

 (2) Organelles of cytoplasm :
 Membraneous organelles are also absent in the cell.

 (3) Types of kingdom Monera :
 Bacteria are the sole members of the kingdom Monera.
 This kingdom is divided into two divisions.

 (a) Schizophyta or Bacteria

 (b) Cyanophyta

 (4) Methods of nutrition:
 Bacteria live mainly as transformers, decomposers and also as parasites, e.g. E.coli
 Cyanophyta are also known as blue-green algae or cyanobacteria. They are autotrophic and posses      photosynthetic pigments.e.g. Spirulina, Oscillatoria.

Kingdom : Protista

 (1) Structure :
   Organisms are unicellular or multicellular.

 (2) Nucleus :
   They possess a well developed nucleus.

 (3) Organelles :
   Membranous organelles like mitochodria, golgibody etc. are present. They do not exhibit division of labor.

 (4) Method of nutrition :
   They are autotrophic, heterotrophic and symbiotic methods of nutritions. Accordingly, protista are classified      into autotrophs, phagotrophs or heterotrophs and symbionts.

    e.g. Algae, Protozoa and Fungi.


  (1) Cellwall :
    It is made up of chitin and cellulose or chitin.

 (2) Organelles :
    Mitochondria with flat cristae, organelles like lomasomes which are responsible for chemical constitution or      Cellwall and Scattered ribosomes are representative characters of fungi.

 (3) Nutrition :
 They exhibit saprophytic or parasitic mode of nutrition.

 (4) Body organization :

 Their body is called mycelium and the filament – like structures are called hyphae. (singular : hypha)

 (5) Example : Yeast, mucor, mushroom.

 Kingdom : Metaphyta

 The plant group is also known as embryophyta. It possesses tissue organization.

 (1) Alternation of generation :
 Two phases called the gametophyte and the sporophyte phase occur alternately during their lifecycle This is    called alteration of generations.

 (2) Bryophyta :
  Bryophyta amongst emboyophyta are without conducting tissues.

 (3) Pteridophyta :
    It possesses conducting tissues but are seedless plants.

 (4) Spermatophyta :
    It possess flowers and seeds.

 (5) Gymnosperms :
    Those Spermatophyta in which the seeds are naked are called Gymnosperms.

 (6) Angiosperms :

    Those Spermatophyta which possess seeds covered by pericarp are called Angiosperms.

 Classification of Angiosperms :
The angiosperms in which the seeds possess two cotyledons are called Dicotyledons and in which the seeds possess one cotyledon are called monocotyledons.

  Kingdom : Metazoa

 This is animal kingdom.

 Characters :

 (a) All multicellular and eukaryotic animals are included in this kingdom.

 (b) Cellwall :Their cells are not enveloped by a nonliving Cellwall.

 (c) Method of nutrition : They are heterotrophic organisms.

 (d) Body organization : Well developed organ system occurs for coordination and regulation.

 (e) Classification : Classification into various phyla is organized on basis of various aspects such as,
 Tissue organization
 Number of germinar layers
 Presence or absence of coelom.

 Development of notochord.
 Presence of a vertebral column.
 Development of cranium.
 Development of jaws.
 Development of limbs.

Development of limbs.

  Status of bacteria and virus :

  Status of bacteria :
  Bacteria are placed in a separate kingdom called Monera due to following reasons.

  (1) The main reason is their primitive nucleus.
  (2) Lack of membranous organelles.
  (3) Unorganized genetic material.
  (4) Genetic material occurs as a circular unit of nucleoprotein mass.
  (5) Any types of Membraneous organelles are absent.
  (6) Possess a different kind of ribosomes.
  (7) Constitution of Cellwall is also different.
  (8) Cyclosis (is the circulation or streaming of the cytoplasm within some living cells) does not occur.
  (9) Constant modification in their subclassification occurs as a new discoveries are made.

 Status of virus:
 In the five kingdom classification of Whittaker there is no mention of some acellular organisms like viruses.      Hence,  viruses are considered a link between living and nonliving.

 They have DNA or RNA and a protein wall surrounding them.
 They do not carry out any metabolic activities.
 They enter into other living cells, degrade their genetic material and using these constituents they produce      new viruses using the constituents of the host cell. This process is called reproduction.
 Hence, they are considered a ‘connecting link’ between living and nonliving.

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