Kingdom Systems of Classification

The system of dividing organisms into groups or sets on the basis of similarities and differences is called classification. It simplifies the study of a wide variety of organisms in a very logical manner and helps us in studying the evolutionary process.

Several Kingdom Systems of Classification of Organisms have been proposed from time to time which are discussed below.

Two Kingdom Classification

• Proposed by the famous Swedish naturalist, Linnaeus in 1758. He classified all living organisms under two Kingdoms – Kingdom Plantae and Kingdom Animalia.
• The autotrophic organisms were included in the Kingdom Plantae whereas Kingdom Animalia consisted of non-photosynthetic heterotrophic organisms.
• Other basis for classification were presence or absence of locomotion, mode of nutrition, response to external stimuli etc. in the organisms.

Drawbacks

• • • This was the first classification that gave a hint for advance classification of organisms. The problems related with this system of classification were –
      • Two-kingdom system of classification failed to distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms and photosynthetic and non-photosynthetic organisms which made them all come into one single kingdom.
      • There are few microorganisms like Chlamydomonas, Euglena and the slime moulds which exhibit characters of both plants and animals.
      • There are few organisms that are neither plants nor animals such as Bacteria which donot have cellular structure identical to plant or animal, Sponges and Corals are non-motile and sessile animals with plant like branching.
    • This made scientist to propose a new kingdom that will distinguish all such organisms and their characters.

 

Three Kingdom Classification

• This system of classification was proposed by the German botanist, Ernst Haeckel in 1866.
• He introduced the third Kingdom, Protista, which includes all the unicellular microorganisms separating multicellular and unicellular organisms.

Drawbacks

• • • The problems faced by three kingdom classification were:
      • The organisms with both the characters of animals and plants were in same kingdom.
      • The organisms with prokaryotic and eukaryotic nature were in the same kingdom.
      • Heterotrophic bacteria and fungi placed along with autotrophic algae.
      • Nucleated and non-nucleated organisms were kept together in Protista.

 

Four Kingdom Classification

• This system of classification was proposed by Herbert Copeland in 1956. He included Monera as fourth kingdom that included all the prokaryotes.
• In this classification system, bacteria and microbes were included in Kingdom
• This divided the living world into four Kingdoms, namely, Monera, Protista, Plantae and
  • Monera included prokaryotic organisms with incipient nucleus, like bacteria.
  • Protista included unicellular eukaryotes like protozoa.
  • Plantae included photosynthetic multicellular
  • Animalia included non-photosynthetic multicellular

Drawbacks

• • • Problems faced by Four Kingdom Classification were –
      • Fungi which are non-photosynthetic were included in Kingdom Plantae.

Five Kingdom Classification

• This system of classification was proposed by the American taxonomist, Robert H. Whittaker in 1969. Whittaker’s five Kingdoms were: Monera, Protista, Fungi, Plantae and The organisms were divided on the basis of several distinguishing characters.
• The basis of classification for the organisms as per Whittaker are:
  • Complexity and organisation of cell structure – prokaryotic and eukaryotic cells.
  • Complexity of body organisation of the organism – unicellular and multicellular – simple multicellular forms and complex multicellular forms.
  • Mode of nutrition for all the organisms – Autotrophic and heterotrophic.
  • Reproduction type – asexual and sexual.
  • Phylogenetic or evolutionary interrelations between the organisms.

Advantages

• • • It has placed prokaryotes in a separate Kingdom Monera.
    • It has placed all unicellular eukaryotes in a separate Kingdom Protista.
    • It has elevated fungi to the level of a Kingdom.
    • It is based on characters which evolved very early in the organic world, g., cellular organisation and modes of nutrition.
    • It has tried to bring out phylogenetic relationships among organisms.

Drawbacks

• • • This system of classification had overcome all the problems faced by the two, three and four classification systems. But, still some of drawback of the system were –
      • The Kingdoms Monera and Protista are still heterogeneous groups. Both include photosynthetic (autotrophic) and non-­photosynthetic, walled and wall-less organisms.
      • Phylogenetic relationships, particularly of lower organisms, are not fully reflected.
      • Algae was separated into plantae, Protista and Monera kingdoms.
      • In case of algae, distinction between unicellular and multicellular organisms is not possible. Due to this, unicellular green algae such as Chlamydomonas, Volvox , have not been included in the Kingdom Protista.
      • There was no kingdom or even class for viruses in the system.
      • Archaebacteria differ from other bacteria in structure, composition and physiology.

 

Six Kingdom Classification

• Gray and Doolite proposed six kingdom for classification of organisms.
• These six kingdoms included Kingdom Archaebacteria, Kingdom Eubacteria, Kingdom Protista, Kingdom fungi, Kingdom Plantae and kingdom Animalia.
• The classification was based on the gene sequences of the organisms.
• The major change from Whittaker classification to this classification was that archaebacteria was separated from eubacteria.
• The separation of bacteria was on the basis of absence of peptidoglycan in the cell walls of the archaea and the presence of branched chain lipids (a monolayer instead of a phospholipid bilayer) in its membrane.

So, this was all about the Kingdom Systems of Classification of Organisms which includes Two Kingdom Classification, Three Kingdom Classification, Four Kingdom Classification and Most acceptable Five Kingdom Classification. 

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