The living organisms are quite enormous in number and highly diversed in their characters. Mainly they differ in their form, structure, metabolism and life cycle. For better approach they have been classified into different groups by different workers. Linnacus is known as father of classification as he, for the first time, classified the living organisms in a systematic way, though many naturalists attempted to classify the living world prior to him. His classification was a two-kingdom system, which had a number of demerits. Subsequently Whittaker proposed a more systematic and elaborates five kingdom system of classification. These systems of classification of living organisms and their merits and demerits are given below.
Two Kingdom System
This system of classification was given by Linnaeus (1758). He classified all the living organisms under two large kingdoms.
1. Kingdom Plantae (Plant Kingdom):
This kingdom included chlorophyll containing green plants, mosses, ferns, many colourless and coloured unicellular organisms, moulds, fungi, lichens, bacteria and multicellular seaweeds.
2. Kingdom Animalia (Animal kingdom):
This kingdom included many other unicellular protozoans and multicellular organisms without having chlorophyll and photosynthetic ability.
The two kingdom system of classification was not found to be suitable to many scientists due to large diversity among the organisms and many other limitations. The main demerits of the system are –
(i) Many protozoans possess characters of both plants and animals. For example, Euglena has animal characters but it contains chlorophyll.
(ii) Bacteria and Cyanobacteria (blue-green algae) have many similarities between them and are quite different from other organisms. Thus, it is difficult to place them in their plant or animal kingdom.
(iii) Fungi and moulds were placed under Kingdom Plantae but they possess many characters not common to plants.
(iv) This system is based mainly on two modes of nutrition, such as assimilation and ingestion. The third type absorption was recognized subsequently.
Five Kingdom Systems:
R.H. Whittaker (1969) proposed the five kingdom system of classification to overcome these difficulties and to represent the living organisms according to the evolutionary relationship among themselves. His five kingdom system is based on:
(i) Complexity of Cell structure
(ii) Complexity of organism body
(iii) Mode of nutrition.
Accordingly the five kingdoms are:
- Monera: – Prokaryotes, e.g. bacteria and Cyanobacteria.
- Protista: – Unicellular eukaryotes, e.g. Unicellular algae, diatoms and Protozoa.
- Plantae: – Multicellular producers, e.g. plants.
- Fungi: – Multicellular decomposers e.g. fungi and moulds.
- Animalia: – Multicellular consumers, e.g. animals.
The merits of five kingdom system of classification are:
(i) It reflects better relationship among organisms with regard to levels of organization and mode of nutrition.
(ii) It reflects better evolutionary trend indicating gradual evolution of complex organisms from simpler ones.
(iii) Better placement of certain controversial groups like Cyanobacteria, fungi, Euglena etc.
(iv) Separation of kingdom Fungi from plants seems to be justified as fungi bear own type of structural, physiological as well as biochemical characters.
The demerits of five kingdom system of classification are –
(i) The position of virus is not clear.
(ii) Kingdom Protista includes organisms having diverse form, structure and life cycle, hence does not seem to be proper grouping.
(iii) Some organisms included under Protista (e.g. Dinoflagellates) are not eukaryotic rather mesokaryotic.
(iv) Slime moulds placed under Protista differ considerably from the rest of protists.
(v) The three higher kingdoms: Plantae, Fungi and Animalia seem to be polyphyletic.
However, despite of all these demerits Whittaker’s five kingdom system is widely accepted.