Basis of Biological Classification: Five Kingdom Classification & Features

The basis of the biological classification concept is similarity of morphology and phylogeny (evolution history). There are numerous living beings in this world. Approximately, there are 8 million species of existing plants and animals. Classifying biological organisms, therefore, is required in order to retain the data, study the diversity and establish phylogenetic relationships among them.

• Living organisms are classified mainly on the basis of similarities and dissimilarities between them. 
• It practically ensures the correct lineage of an organism in the living world. 
• The principles of any modern system of classification are mainly based upon the existing genetic relationship among diverse groups of living organisms. 
• Thus, closely related groups are brought together by biological classification.

Key words: Biological classification, Organisms, Kingdom, Monera, Protista, Fungi, Plantae, Animalia.

---

What is Biological Classification?

[Click Here for Sample Questions]

Biological classification is the categorization of living organisms into different groups or sets. The classification is done on the basis of their relationship i.e similarity or dissimilarity among organisms or both.

Suppose, a viper, a cobra, a parrot, a dog, a sparrow and a wolf are considered. Just by looking at their morphologies, the viper and cobra can be considered as snakes, the parrot and sparrow as birds and the dog and wolf as mammals of the dog family. This is how biological organisms have been classified and in recent classification systems, other different characters are also taken into account along with the morphology.

---

Purpose of Biological Classification

[Click Here for Sample Questions]

The main purposes of classifying biological organisms are as follows.

1. It provides simple practical means of discussion relating to specific organisms.
2. It is a tool for identifying animals and plants.
3. It provides explanations regarding diversity of organisms.
4. Classification provides new data through observation, correlation and analysis of the stored data of living organisms.
5. It provides conclusions regarding phylogeny.

---

Basis of Biological Classification

[Click Here for Sample Questions]

Classification is required because a variety of organisms have risen through evolution. Therefore, in simpler words, classification is based on evolutionary relationships or phylogeny. However, classification is only possible on the basis of features that were produced by evolution and sometimes appeared in phylogeny.

Classification is done taking some characteristic features into account. Organisms with similar characteristics are grouped into one. Rest of them with different features have been put into some different groups. The main characteristic features considered for modern day classification are;

1. Type of cell- prokaryotic or eukaryotic.
2. Number of cells- unicellular or multicellular.
3. Mode of nutrition- autotrophic or heterotrophic.
4. Level of organization.

For example, plants and animals are put under different kingdoms owing to the distinct differences between them regarding mode of nutrition, motility, cell structure, etc. 

In the animal kingdom, for example, distinct differences lie between a pigeon and a dog in terms of limb. In pigeons, limbs have been modified to wings.

Previously, Aristotle had classified plants into herbs, shrubs and trees on the basis of external morphology and animals on the basis of presence or absence of red blood cells. Other important characteristics were not considered. Therefore, this method of classification did not stand strong enough.

Many systems of classification have been put forward which mainly divided the living world into various kingdoms.

---

Two Kingdom Classification

[Click Here for Sample Questions]

This system of classification was introduced by Carolus Linnaeus. He divided living organisms into two kingdoms— Plantae and Animalia. Classification was based on superficial morphological characters.

• Kingdom Plantae includes all plants and Animalia includes all animals. 
• This system of classification did not take into account most organisms like different algae, bacteria, etc. and neither considered cellular features. 
• So, this system of classification stood inadequate.

Read More: Classification of Plants

---

Three Kingdom Classification

[Click Here for Sample Questions]

The three domains of life as proposed by E. Haeckel were Protista, Plantae and Animalia. Haeckel included all the unicellular organisms like fungi, algae, slime molds, protozoans and bacteria under the kingdom Protista. This classification was based on the fact that they exhibit both plant and animal-like characteristics. It was believed that protists evolved from Monerans. Fungi, plants and animals have evolved from the Moneran precursors.

---

Five Kingdom Classification

[Click Here for Sample Questions]

R.H. Whittaker in the year1969, introduced the Five Kingdom system of classification. The five kingdoms as proposed by him are Monera, Protista, Fungi, Plantae and Animalia. Of these five kingdoms, only the kingdom Monera consists of prokaryotic organisms. The main basis of classification used in this system include cell structure, body organization, reproduction, mode of nutrition and phylogenetic relationships.

1. Kingdom Monera

Salient features of these organisms are;

1. Cell wall is generally composed of chemical substances like peptidoglycan and polysaccharides.
2. Absence of true nucleus— cell without nuclear membrane and nucleoplasm.
3. Absence of membrane bound organelles like plastid, mitochondria, golgi body, endoplasmic reticulum, lysosome, etc.
4. DNA is not associated with histone protein.
5. A 70S type of ribosome is present (subunits of 50S and 30S).
6. Mode of nutrition- members of Monera may be parasitic, saprophytic, symbiotic, photoautotrophic, and chemoautotrophic.
7. Reproduction- asexual mode of reproduction carried out by binary fission or budding.

Examples- Includes all Archaebacteria and Eubacteria.

2. Kingdom Protista

Salient features of protists are as follows.

1. Eukaryotic unicellular organisms are included under this kingdom.
2. Presence of true nucleus.
3. Mode of nutrition may be phototrophic, parasitic or holozoic.
4. Presence of membrane bound cell organelle.

Examples- Algae, Protozoans and some Molds.

3. Kingdom Fungi

Salient features of kingdom fungi are 

1. Fungi are eukaryotic, unicellular or multicellular organisms devoid of chlorophyll.
2. Mode of nutrition may be saprophytic, parasitic or symbiotic.
3. Cell wall in these organisms is made up of chitin and cellulose.

Examples- Agaricus, Rhizopus, Yeast, Penicillium, etc.

4. Kingdom Plantae

Salient features of organisms in this kingdom are as mentioned below.

1. Organisms are multicellular eukaryotes with cell walls mainly composed of cellulose, hemicellulose and pectin.
2. Cells bear chloroplast with chlorophyll pigment.
3. Mode of nutrition is autotrophic.
4. Non-motile organisms.

Examples- Funaria (moss), Pteris (fern), Pinus (gymnosperm), Mangifera (angiosperm), etc.

5. Kingdom Animalia

Salient features of animals that belong to kingdom animalia are as follows.

1. Organisms are multicellular and eukaryotic with cells devoid of cell walls.
2. Cells do not possess plastid and vacuole.
3. Mode of nutrition is heterotrophic (mainly derived from plants).
4. Cells bear centrioles.

Examples- Invertebrates like Hydra, Sponge, Cockroach, Worms, etc. and Vertebrates like FIshes, Amphibians, Reptiles, Birds, Mammals, etc.

Also Read:

---

Things to Remember

• Classification is the grouping of living organisms into different hierarchical positions.
• Basis of biological classification is similarity of morphology and phylogeny. 
• Classification establishes the phylogenetic position of an organism.
• Two kingdom system of classification was introduced by Linnaeus.
• He categorized living organisms into two kingdoms— Plantae (plants) and Animalia (animals).
• R.H. Whittaker introduced the Five Kingdom system of classification of living organisms.
• The five kingdoms proposed by Whittaker are— Monera, Protista, Fungi, Plantae and Animalia.
• Kingdom Monera have the prokaryotic organisms in which DNA does not contain histone proteins. 
• They possess 70S type of ribosome and membrane bound organelles absent.
• Kingdom Protista includes eukaryotic unicellular organisms, DNA containing histone proteins, 80S type of ribosome, and membrane bound organelles.
• Kingdom Fungi involves eukaryotic multicellular organisms with cell walls composed of chitin and cellulose.
• Kingdom Plantae is about eukaryotic multicellular organisms with cell walls composed of cellulose, hemicellulose, pectin, lignin, etc.
• Cell contains plastids and chlorophyll which helps in the phototrophic mode of nutrition.
• Kingdom Animalia includes eukaryotic multicellular organisms. The cell does not contain plastids, vacuoles, cell wall etc.