Key Topics

`star` Two Kingdom system of classification
`star` Three and Four Kingdom system of classification
`star` Five Kingdom system of classification
`star` Six Kingdom system of classification

Earliest classification

• The earliest classification of living organisms by humans was `color{green}"instinctive"` rather than scientific. It was borne out of the need to use organisms for food, clothing and shelter

• `color{green}"Aristotle"` was the first to attempt a more scientific basis of classification. He classified plants into herbs, shrubs and trees based on `color{green}"simple morphological characters"`. He classified animals based on the `color{green}"presence or absence of red blood"`.

• Gradually with more and more discoveries and development of better scientific method, new classification systems were developed.

• These are shown on the right hand side image:

`color{purple}♣ color{Violet} "Just for Curiousity"`

• Aristotle called the animals with red blood as `color{green}"enaima"` and animals without red blood as `color{green}"anaima"`.

Two kingdom system of Classification

• Developed by `color{Green}"Carl Linnaeus"` (1758). He is regarded as the `color{Green}"Father of Classification"` .

• Divided organisms into `color{Brown}"Plantae and Animalia kingdoms"` that included all plants and animals respectively.

• This system was used till very recently.

• Eventually, the two kingdom classification used for a long time was found `color{Green}"inadequate"`.

• A need was also felt for including, besides gross morphology, other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc.

`color{purple}♣ color{Violet} "CRITERIA"`

• Based on `color{Green}"gross morphology"`. The classification was carried out based on some functional and structural characteristics such as `color{Brown}"mode of nutrition, response to external stimuli, conductile and contractile system, locomotion and cell wall"`.

• So plants were organisms (which had cell wall, were autotrophs and did not move) and animals were organisms (which did not have a cell wall, were heterotrophs and could move).

`color{purple}♣ color{Violet} "MERITS"`

• Classification of organisms into plants and animals was easily done easily done and was `color{Green}"easy to understand"`.

• The classification was not done abruptly but `color{Green}"scientifically"`, based on but based upon `color{Green}"distinctive characters"`.

• It gave rise to the process of classification using `color{Green}"systematic methods"`. Later, for development of better methods, more and more characters were taken into consideration to classify the living organisms.

`color{purple}♣ color{Violet} "DEMERITS"`

• This system did not distinguish between the `color{Green}"eukaryotes and prokaryotes ,
unicellular and multicellular organisms and
photosynthetic (green algae) and non-photosynthetic (fungi) organisms"`.

• A large number of organisms did not fall into either category like `color{Green}"microorganisms"` such as bacteria, micro algae and micro fungi .The reason could be because the organisms were not properly known by that time.

• This placed together groups which widely `color{Green}"differed"` in other characterstics.

• Organisms like bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and the angiosperms were grouped under ‘Plants’ due to the presence of a cell wall in their cells.

• It brought together the prokaryotic bacteria and the blue green algae with other groups which were eukaryotic.

• It also grouped together the unicellular organisms and the multicellular ones, say, for example, Chlamydomonas and Spirogyra were placed together under algae.

• The classification did not differentiate between the heterotrophic group – fungi, and the autotrophic green plants, though they also showed a characteristic difference in their walls composition – the fungi had `color{Green}"chitin"` in their walls while the green plants had a `color{Green}"cellulosic"` cell wall.

`color{purple}♣ color{Violet} "Just for Curiousity"`

• Around 1800, during the earlist attempts, as more minute organisms were discovered, micro-fungi and multicellular microalgae, which were immotile and in some cases `color{Green}"plant-like"` in form, were naturally positioned in the plant kingdom.

• Other motile, microscopic forms like unicellular protozoa, multicellular invertebrates and bacteria were grouped together as a single group, the `color{Green}"infusoria"`, within the animal kingdom.

• Eventually, the biologists realized that infusoria was a `color{Green}"heterogenous group"` and transferred bacteria from it to plant kingdom. Thus, kingdom plantae included the bryophytes, pteridophytes, angiosperms, gymnosperms, fungi (macro and micro), algae (macro and micro) and bacteria; and all animals including unicellular protozoa, multicellular invertebrates were grouped in kingdom Animalia.

Three and Four kingdom systems of classification

`color{blue} ul(mathtt ("Three Kingdom Classification system of Classification"))`

• In 1866, `color{Green}"Ernst Haeckel"` purposed three kingdom system of classification for living organisms. He divided them into `color{Brown}"Kingdom Animalia, Kingdom Plantae and Kingdom Protista"`.

• All the plants were placed in kingdom Plantae and animals in the kingdom Animalia. Those organisms which had `color{Green}"neither plant like nor animal like characteristics"` were placed in kingdom Protista.

• This classification was based on `color{Brown}"mode of nutrition ,morphological complexities, division of labour and tissue system"`

• So all the unicellular and colonial eukaryotes such as bacteria, algae, fungi and protozoans were placed under Protists.

• All the multicellular phototrophic plants were placed under kingdom plantae.

• All the multicellular animals were included under kingdom animalia.

`color{blue} ul(mathtt ("Four Kingdom Classification system of Classification"))`

• The four kingdom classification system included Monera in addition to Protista, Plantae and Animalia and was developed by`color{Green}"Copeland"` in 1966.

• With the discovery of `color{Green}"electron microscope"` ( by Knoll and Ruska, 1932),It was recognized that the cells of less complex protists `color{Green}"lacked"` a membrane-bound nucleus and membrane-bound organelles; whereas the more complex protists `color{Green}"possessed"` true nucleus and membrane-bound organelles like those of plants and animals.

• On this account, Copeland created a new kingdom, which he called `color{Green}"Monera"` including the lower prokaryotic protists (bacteria and blue green algae), and the higher eukaryotic protists (other algae, fungi and protozoa) under the kingdom protists.

`color{purple}♣ color{Violet} "Just for Curiousity"`

• Copeland called the kingdom Protista as `color{Green}"Protoctista"` . The terms ‘Plantae’ and ‘Animalia’ were replaced by `color{Green}"Metaphyta"` and `color{Green}"Metazoa"` respectively though their members remained as such. Therefore, his four kingdoms were: Monera, Protoctista, Metaphyta, and Metazoa.

Five kingdom classification system

• `color{Green}"R.H.Whittaker"` in 1969 proposed a Five Kingdom Classification.

• The kingdoms defined by him were named `color{Brown}"Monera, Protista, Fungi, Plantae and Animalia"`.

• The main criteria for classification used by him included
`color{Brown}"cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships"`

`color{purple}♣ color{Violet} "Merits"`

`star` Euglena and other transition types which previously included in both amongst the plants and animals were given a proper place under the kingdom—Protista.
`star` Fungi with their own structural organization, biochemistry and physiology were separately placed under Kingdom Fungi.
`star` A separate kingdom `color{Green}"Monera"` for prokaryotes was created.
`star` It pointed at the `color{Green}"gradual evolution"` of early organisms into plants and animals.
`star` The plant as well as the animal kingdoms were more `color{Green}"homogenous"` in the kinds of their members than in the previous systems of classification.

`color{purple}♣ color{Violet} "Demerits"`

`star` Animal protozoans, showing different modes of nutrition than unicellular plants were included in kingdom—Protista.
`star` `color{Green}"Viruses viroids and prions"` did not find a proper place in this system

6 kingdom system of classification

• It was introduced by `color{Green}"Carl Woese"` in 1990. This system is also called as `color{Green}"three domain system"` as organisms in it are grouped into three domains, i.e., Archaea, Bacteria and Eukarya.

• It basically used the main principles of the five kingdom classification system but divided the Monera into the two domains `color{Green}"Archaebacteria"` and the Eubacteria whereas the other eukaryotes in the third kingdom Eukarya.

• Archaebacteria includes the earliest prokaryotes living in `color{Green}"extreme environments"`. These broadly include the
Halophiles (salt loving),
Methanogens (methane producing) and
Thermoacidophiles.(heat and low pH loving)