Classification of Living Organisms

Summary

Organisms differ in their form, structure and mode of living.  Hence, based on their similarities they should be grouped.  The grouping of related organisms helps us in studying their evolutionary relationships. Classification is the division of organisms on the basis of characteristics into groups and sub-groups. A characteristic may be a particular form or function.

History of classification
    •  Aristotle, the Greek philosopher, classified animals based on whether they live on land, in water or in the air.
    •  Charles Darwin put forward the idea of evolution in 1859, in his book, The Origin of Species.
    •  Ernst Haeckel, Robert Whittaker and Carl Woese have tried to classify living organisms into broad categories, called kingdoms.
    •  Carolus Linnaeus classified all the living organisms into two kingdoms namely, ‘Plantae’ and ‘Animalia’. 
    •  Robert Whittaker, in 1969 proposed ‘Five kingdom classification’ of living organisms. 

Hierarchical classification
The hierarchy can be represented as Kingdom subgrouping into Phylum for animals or Division for plants, Class, Order, Family, Genus and Species. Hence, the basic unit of classification is species. Species includes all the organisms that are similar to breed and produce fertile offspring.

Nomenclature
The scientific naming of an organism is called as nomenclature. Binomial nomenclature, introduced by Carolus Linnaeus is the method of naming an organism with the genus name first and species name later.

Conventions followed while writing scientific names
    •  Name of the genus begins with a capital letter. 
    •  Name of the species should begin with a small letter. 
    •  Scientific name should be in Italics when printed. 
    •  Genus name and the species name should be underlined separately while handwritten.

Five kingdom classification
The five kingdom classification was proposed by R.H. Whittaker in 1969. The five kingdoms were formed on the basis of characteristics such as cell structure, mode of nutrition, source of nutrition and body organisation. It includes Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia.

Kingdom Monera:
It includes prokaryotic cells lacking organised nucleus and membrane bound cell organelles. Some of the Monerans are autotrophic and some of them are heterotrophic forms.Bacteria, cyanobacteria, blue-green algae, mycoplasma are some of the examples of Kingdom Monera.

Kingdom Protista:
It includes algae, diatoms and protozoans. These are unicellular and the simplest form of eukaryotes exhibiting both autotrophic and heterotrophic mode of nutrition. Locomotion and movement are possible by whip-like flagella and hair-like cilia or finger-like pseudopodia.

Kingdom Fungi:
These are multicellular, eukaryotic saprophytes. The cell wall of fungi is made up of chitin.
They feed on dead and decaying matter. They include mushrooms, rhizopus and mucor. Some fungi are symbiotic forming an association with algal cells. These symbionts are termed to be lichens.

Kingdom Plantae:
It includes all the plants that are non-motile, multicellular and eukaryotic organisms with their cell walls made up of cellulose.

Kingdom Animalia:
It includes all the animals that are motile, multicellular, eukaryotic organisms with their cells possessing no cell walls. It exhibits species diversity.

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