Discuss how classification systems have undergone several changes over a period of time?

The classification systems have undergone several changes with time. The first attempt of classification was made by Aristotle. He classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.
Therefore, Linnaeus gave a two kingdom system of classification. It consists of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were large numbers of organisms that could not be classified under the two kingdoms.
To solve these problems, a five kingdom system of classification was proposed by R.H Whittaker in 1969. On the basis of characteristics, such as cell structure, mode of nutrition, presence of cell wall, etc., five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia were formed.

Scientific classification of living beings was first done by Aristotle. He used morphological characters as the basis of classification. He classified the living beings into plants and animals. He further classified the plant into trees, shrubs and herbs. He further classified the animals on the basis of presence of absence of red blood.

After that, Linnaeus proposed two kingdoms, i.e. Plant Kingdom and Animal Kingdom.

But there were certain organisms which could be kept in both of the kingdoms or could not be kept in either of the kingdoms. Hence, a need was felt for a better system of classification. At present, the Five Kingdom Classification is the most accepted one. This was proposed by Robert Whittaker in 1969. Whittaker used phylogenetic relationship to classify the living beings. 

Classification was born instinctively out of a need to all organisms for our own use since the dawn of civilization. Aristotle was the earliest to attempt a more scientific basis for classification. He used simple morphological characteristics to classify plants into trees, herbs and shrubs. He also divided animals into Animals with red blood and those who do not have red blood. Linnaeus proposed two-kingdom system of classification with Plantae and Animalia including plants and animals respectively.

The above system did not distinguish eukaryotes and prokaryotes, unicellular and multicellular, photosynthetic and non-photosynthetic organisms. A need was felt for including besides gross morphology, other characteristics like cell structure, nature of cell wall, mode of nutrition, habitat, method of reproduction, evolutionary relationships etc., Recently R.H. Whittaker proposed a five-kingdom classification to answer above the Five kingdoms are

• Monera
• Protista
• Fungi
• Plantae
• Animalia.