Biological classification of organisms

Classification is the arrangement of organisms in specific groups or categories based on certain characters.
System of classification
1) Artificial system: In this system, morphological characters like habit, habitat, number of stamens etc. are considered to classify organisms e.g.: Linnaean system
2) Natural system: In this system, natural relationship are considered to classify organisms Ex: George Bentham and Joseph Dalton Hooker's system
3) Phylogenetic system: In this system, evolutionary relationships are taken into account to classify the organisms. e.g.: Engler and Prantl's system.
Aristotle was the earliest to attempt a more scientific bases for classification. He used simple characters like morphology to classify the plants into herbs, shrubs and trees. He also divided animals into two groups, those which had red blood and those did not.

Two kingdom system of classification
Proposed by C. Linnaeus
He divided organisms into two kingdoms-Plantae and Animalia.
Demerits: A large number of organisms did not fall into either plant kingdom or animal kingdom.
Three kingdom system of classification
Proposed by E. Haeckel.
He introduced a separate kingdom for all unicellular eukaryotes, Protista. He included fungi, protozoans, algae, bacteria and slime molds in Protista.
Demerits: Prokaryotes and eukaryotes were placed together in Protista as well as Heterotrophic bacteria and fungi placed along with autotrophic algae.
Four kingdom system of classification
Proposed by Copeland.
He introduced a separate kingdom, Monera for all unicellular prokaryotes. The term Monera was given by Daugherty and Allen.
Demerits: No proper place in this system of fungi. Other characteristics such as cell structure, mode of nutrition, habitat, methods of reproduction, evolutionary relationships etc. were not included.
Five kingdom system of classification
It is the most advanced and most accepted popular system of classification. It was proposed by R.H. Whittaker(1969). The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia. It is phylogenetic system but mainly based on mode nutrition. Other criteria for five kingdom classification
a) Type of cell structure-Prokaryotic or Eukaryotic,
b) Complexity of organism's body-Unicellular or Multicellular,
c) Mode of nutrition- Autotrophs or Heterotrophs,
d) Major ecological role- Producer, Consumer or Decomposer,
e) Phylogenetic relationship-Evolutionary relationship, simple to complex.
Demerits: Both the kingdoms, Monera and Protista include autotrophic as well as heterotrophic organisms. The three kingdoms(Plantae, Animalia and Fungi) do not have a single ancestor. Members of algae are placed in different kingdoms. No proper place in this system of classification for viruses.