In general, biological classification is the scientific process which is adopted by scientist for arranging the organisms in a hierarchical series of different groups and sub-groups. The classification is done on the basis of their similarities and dissimilarities. In other words, organisms are classified by observing how similar or dissimilar they are. Many biologists around the world have contributed to this process of classification, which took years for scientist to decide the most elemental properties for the classification.
There are basically five kingdom classification which is proposed by R.H. Whittaker (1969)
- Monera kingdom
- Protista kingdom
- Fungi kingdom
- Plantae kingdom
- Animalia kingdom.
The only members of the Monera Kingdom are bacteria. Bacteria are the most copious micro-organisms in environment. Based on their shape, they are grouped under four categories. i.e., the rod-shaped Bacillus (pl.: bacilli), the spherical Coccus (pl.: cocci), the spiral Spirillum (pl.: spirilla) and the comma-shaped Vibrium (pl.: vibrio). Bacteria are very complex in behaviour even though their structure is very simple. As a group, bacteria, exhibit the most extensive metabolic diversity as compared to many other organisms. Bacteria can be categorized as autotrophic and heterotrophs. Autotrophic bacteria synthesise their own food from inorganic substrates. It may be chemosynthetic autotrophic or photosynthetic autotrophic. Heterotroph’s bacteria depend on other organisms or on dead organic matter for food. Majority of the bacteria belongs to this category.
Protista kingdom includes all single-celled eukaryotes. However, the boundaries of Protista kingdom are not well defined. According to some biologist it may be ‘a plant’ or for some other it may be ‘a photosynthetic protistan’. Members of this kingdom are mostly aquatic. Protista kingdom build a link with the others dealing with animals, plants, and fungi. As a eukaryote, the protistan cell body comprises a well-defined nucleus and other membrane-bound organelles. Additionally, some of them have cilia or flagella. Member of this kingdom reproduce asexually and sexually by a method involving zygote formation and cell fusion.
The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat. Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes. Those that depend on living plants and animals are called parasites. They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza. Reproduction in fungi can take place by vegetative means – fragmentation, fission and budding.
Plantae involves all the plants that exist on earth. They are multicellular, eukaryotic, and autotrophic organisms. The plant cell comprising of a rigid cell wall. Majority of the plants survives through photosynthesis for which they require chloroplast and chlorophyll pigment.
It can be classified into three types:
- Artificial system: The earliest systems of classification utilized only gross superficial morphological characters such as colour, habit, number and shape of leaves, etc. They were based primarily on the androecium structure (system given by Linnaeus) or on vegetative characters. Such systems are known as artificial system.
- Natural System: Natural affinities among the organisms and consider not only the external features, but also internal features, such as anatomy, ultrastructure, phytochemistry, and embryology. This type of classification for flowering plants was specified by Joseph Dalton Hooker and George Bentham.
- Phlogenetic system: At present phylogenetic classification systems based on evolutionary relationships between the various organisms are acceptable. This system consider that organisms belonging to the same taxa have a common ancestor.
This group involves all the living as well as extinct living species. As there are millions of species available, the classification of them is very important. Besides, it helps in assigning a systematic position to newly discovered species.
Animals can be categorised into two types on the basis of their symmetry.
- Radial symmetry: In this category the body divides the organism into two identical halves when any plane passing through the central axis.
- Bilateral symmetry: In this category where the body can be divided into identical left and right halves in only one plane. Animals such as arthropods, annelids demonstrate bilateral symmetry.
Animalia can be broadly classified into two types
- Cellular level: Porifera
- Tissue/Organ/ Organ system
Tissue/Organ/ Organ system can be classified into two types
Bilateral can be classified into three types
- Without body cavity (acoelomates)
- With false coelom (pseudocoelomates)
- With true coelom (coelomates)
With true coelom (coelomates) can be further classified as