Classification of Living Organisms

 

One of the most fascinating and attractive aspects of the living world is its extraordinary diversity. Because of the bewildering diversity of living organisms, it is desirable to classify them into groups based on their similarities.

The branch of biology that is concerned with the description, nomenclature and classification of living organisms is called as Taxonomy. Classification is the arrangements of organisms into groups based on similarities or evolutionary relatedness. Nomenclature is the branch of taxonomy concerned with the assignment of names to taxonomic groups in agreement with published rules. Identification is the process of determining that a particular organism belongs to a recognized group or taxon.

The classification of living things began all the way back with Aristotle. His system of classification was very simple, which grouped animals into the two categories – two-legged and four-legged. Different scientists after Aristotle followed different methods for classifying organisms, but the modern classification system was developed only after 1700 AD

Carlos Linnaeus developed modern classification system known as the Linnaeus system. Linnaeus took all the known organisms at the time and grouped them into two kingdoms which is known as the two kingdom classification. This is one of the earliest schemes of Natural system of classification.

Linnaeus placed all living organisms into two major kingdoms, the Plantae and Animalia, on the basis of some structural and functional characters such as Locomotion, Mode of Nutrition, cell wall, response to external stimuli, etc.

The kingdom Plantae included those organisms that obtain energy by photosynthesis, that do not freely move from one place to another, do not respond to external stimuli, and that possessed cell wall. Contrary to it, the organisms characterised by their mobility, power of ingestion, and lack of cell wall were placed in the kingdom Animallia.

After the discovery of microorganisms which possessed both plant-like and animal-like characteristics simultaneously, and many other enjoyed such characteristics which were unique to them and not found in either plants and animals, it was realized that classifying organisms into just two kingdoms Plantae and Animalia was not appropriate.

Therefore in 1866 Ernst Haeckel recongized the third kingdom and proposed Protista as a new kingdom to accommodate organisms exhibiting characters either common to both plants and animals, or unique to their own.

Thus, the three kingdom system of classification consisting of kingdoms Protista, Plantae and Animalia came into being.

Haeckel classified organisms into three kingdoms on the basis of morphological complexities, tissue system and mode of nutrition. Organisms lacking morphological complexities, tissue system and enjoying diversified modes of nutrition were placed under the kingdom Protista (Algae, Fungi, Protozoa and Bacteria), and organisms with autotrophic mode of Nutrition were considered to be plants and were placed under kingdom Plantae, and those organisms having maximum morphological complexities, diverse tissue system and holophagic mode of nutrition were considered to be animals and were placed under the kingdom Animalia 

  Whittaker System of Classification

Robert Harding Whittaker proposed a comprehensive five kingdom system of classification, comprising of kingdoms – Monera, Protista, Fungi, Plantae and Animalia.

He organised his classification system along evolutionary lines. He classified organisms on the basis of:

1. Cell Type – Prokaryotic or Eukaryotic

2. Level of Organisation – Solitary, Colonial Unicellular or Multicellular

3. Mode of Nutrition – Photosynthesis, Absorption or Ingestion

The microorganisms constitute three of the five kingdoms. The kingdom Monera contains the bacteria and cyanobacteria. These are separated from all other organisms based on their prokaryotic cell structure.

The kingdom Protista contains the unicellular algae and protozoa based on the fact that they have eukaryotic cells and are unicellular.

Unicellular and multicellular fungi are kept in kingdom Fungi, based on the absence of specialized tissues.

According to Whittaker, the Monera were the first organisms on Earth and Protista evolved directly from the Monera. Whittaker proposed that fungi, plants and animals evolved from the protists via three separate direction of evolution. These evolutionary lines were based on their nutritional needs.

According to Whittaker’s hypothesis, the fungi evolved as the most complex multicellular organisms that obtained their nutrients by absorption. Animals evolved based on the ability to ingest other organisms. Plants evolved based on their photosynthetic capacity.

 Carl Woese Concept of Three Domain Classification

In 1977, Carl Woese proposed a three-domain system of classification based on modern techniques in Molecular Biology and Biochemistry, which revealed that there are two types of prokaryotic cells.

Woese performed genetic analysis of 16S and 18S ribosomal RNA, of prokaryotes and eukaryotes respectively. RNA is considered as a good genetic indicator of evolution of one organism from the other. The degree of similarity in rRNA sequences between two organisms that indicates their relative evolutionary relatedness.

The studies of Carl Woese based on comparative sequencing of 16S and 18S rRNA have resulted in the Universal . Tree of Life. His studies showed that there are three distinctly different cell groups, the Eukaryotes and two different Prokaryotes (the Eubacteria and the Archaebacteria). These three phylogenetically distinct groups of cells are called Domains.

All living organisms are incorporated within three domains : Archea, Bacteria and Eucarya. Archea are neither bacteria nor eukaryotes.

Eigh Kingdom System of Classification

Cavalier-Smith 1987 classified protists into several well defined kingdoms. In this regard, the Cavalier and smith divided the organisms into eight kingdom classification on the basis of ultrastructure of cell and genetic organisations – mainly based on rRNA sequence. He divided all organisms into two empires – Bacteria and Eukaryota.

The empire Bacteria includes two kingdoms – Eubacteria and Archaebacteria. The Empire Eukaryota contains six kingdoms – Archezoa, Protozoa, Plantae, Chromista, Fungi and Animalia

The kingdom Chromista includes diatoms, brown algae, cryptomonads and oomycetes. The members of chormista are photosynthetic and have their chromoplast within the lumen of endoplasmic reticulum and not in cytoplasm.