Systems of classification

Source:  Systems of classification    Tag:  kingdom protista phylum

The early classification systems were all artificial systems. Mostly based on human uses.
Aristotle was the first to classify organisms scientifically. He divided organisms into plants and
animals. Animals were further classified according to criteria such as mode of locomotion,
reproduction and presence or absence of red blood cells. Aristotle’s pupil Theophrastu
classified plants according to habit e.g. Trees, Shrubs and herbs, and according to lifespan e.g
annuals, biennials and perennials.Up to the time of Linnaeus scientists used many different methods.
 Carolus Linnaeus(1753), Swedish botanist, introduced binomial nomenclature and classified
about 6000 plants into a hierarchical order of taxa.;  Species,  genus, order, and class. His
 classification of flowering plants was based on the number of stamens and styles of flower. He
identified two kingdoms of organisms; plants and animals.With the discovery of microorganisms the scientists understood that there were organisms

which could not be assigned into either plants or animals. To get over this difficulty Ernest
Haeckel (1866) introduced a third kingdom: Protista. He also introduced the taxon Phylum and
classified many organisms.

With the discovery of electron microscope biologists identified prokaryotic and eukaryotic
cellular organization . Robert H Whittaker (1969) introduced five kingdom system of
biological classification; Monera, Protista, Fungi, Plantae and Animalia. His classification was based on nature of cellular organization, unicellular or multicellular nature and mode of

With the acceptance of Darwin’s theory on evolution and unitary origin of life, taxonomists
began to use natural systems. With recent advancement of molecular biology and the use of
molecular methods in studying evolutionary relationships it became apparent that in the very
early evolution, organisms had separated into three stocks which are now called Domains. Carl
Woese (1977) classified organisms into three Domains Archaea, Bacteria and Eukarya at a
 higher level over Kingdom

Hierarchy of  Taxa from Domains to Species.

                 In taxonomy each level of taxonomical hierarchy is called a taxon (plural-taxa) and  each taxon
                 has a rank and a name.
                 E.g.:  Class: Reptilia
                 Under the hierarchical system there are levels of taxa. Each Domain is divided into kingdoms.
                 Kingdom is divided into phyla (singular phylum), phylum into classes .etc. Many of these
                 categories may also be subdivided.
                  E.g.: Sub-family, Subspecies etc.

                  From domain to species, the number of shared characters among the members in the taxa
                 decreases. From species to domain, the number of individuals in the taxon increases.

Present System of Classification and its basis

                 With the rapid advance of molecular biology new information on the evolutionary
                   relationships of organisms are being collected rapidly. The sequence of bases of DNA of
                   important genes, DNA of mitochondria, the base sequence of  ribosomal RNA, the sequence
                   of amino acids in common proteins, molecular structure of cellular components are used as
                   important taxonomic criteria in modern systematics. As a result classification systems are
                   changing rapidly.

                   We adopt here the three domain classification suggested by Woese for convenience. It is very
                   clear however that the kingdom Protista is not a natural group. It is an artificial group
                   including organisms which have different evolutionary origins. Viruses do not have cellular
                   organization, therefore does not belong to any of the kingdoms. It is also an artificial group
                   considered separately.
Domain – Bacteria
Domain -  Archaea
Domain -  Eukarya
         Kingdom –  Protista
                            P la n ta e


            Domain : Bacteria

                      •    Cellular organization prokaryotic
                      •    Cell wall component  peptidoglycan.
                      •    Lipids in cell membrane are not branched.
                      •    Sensitive to antibiotics.
                      •    Protein synthesis begins with formyl methionine.
                      •    One kind of RNA polymerase enzyme.
                      •    Lives in many habitats.
                      •    Eg: Cyanobacteria, Purple bacteria, Green sulphur bacteria

            Domain : Archaea

                      •    Cellular organization prokaryotic
                      •    Cell wall component lacks peptidoglycan, contains proteins and polysaccharides.
                      •    Lipids in cell membrane have branched and chained structure.
                      •    Not sensitive to antibiotics like streptomycin and chloramphenicol(like eukaryotes)
                      •    Protein synthesis begins with methionine (like in eukaryotes)
                      •    Several kinds of RNA polymerase enzymes (like in eukaryotes)
                      •    Lives in extreme environmental conditions such as volcanic pits, hot springs, salt
                           marshes, deep-sea  etc.
                      •   Eg:  Methanococcus, Thermococcus
                             Methanobacterium, Halobacterium

            Domain Eukarya

                      •    Cellular organization eukaryotic
                      •    Cell wall component lacks peptidoglycan, contains polysaccharides.
                      •    Lipids in cell membrane are not branched.
                      •    Not sensitive to antibiotics.
                      •    Protein synthesis begins with methionine.
                      •    Several kinds of RNA polymerase enzymes.
                      •    Lives in many different environmental conditions.
                      •    E.g. protists, fungi, plants and animals.

                   •    Viruses   -   Discuss the common characteristics of viruses