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

Classification systems changed as newer, more scientific criteria replaced older ones. Aristotle made the earliest scientific attempt using simple morphological characters (plants as trees, shrubs, herbs; animals with or without red blood). In Linnaeus' time a Two Kingdom system (Plantae and Animalia) was used, but it could not separate eukaryotes from prokaryotes, unicellular from multicellular, or photosynthetic (green algae) from non-photosynthetic (fungi) organisms. Because many organisms did not fit either kingdom, R.H. Whittaker (1969) proposed the Five Kingdom Classification (Monera, Protista, Fungi, Plantae, Animalia), using cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

Since the dawn of civilisation people have tried to classify organisms, and the systems used have undergone several changes over time as our understanding improved.

Aristotle made the earliest scientific attempt. He used simple morphological characters, classifying plants into trees, shrubs and herbs, and dividing animals into two groups — those that had red blood and those that did not.

In Linnaeus' time, a Two Kingdom system of classification was developed with kingdoms Plantae and Animalia, including all plants and animals respectively. This system was easy to use but was later found inadequate because it:

• did not distinguish between eukaryotes and prokaryotes,
• did not separate unicellular and multicellular organisms,
• grouped together photosynthetic (green algae) and non-photosynthetic (fungi) organisms,
• and left a large number of organisms that fitted neither category.

A need was felt to include characters beyond gross morphology — cell structure, nature of the cell wall, mode of nutrition, habitat, methods of reproduction and evolutionary (phylogenetic) relationships.

R.H. Whittaker (1969) proposed a Five Kingdom Classification — Monera, Protista, Fungi, Plantae and Animalia — using cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships as the main criteria. Later, a three-domain system was also proposed, dividing Kingdom Monera into two domains and leaving the remaining eukaryotic kingdoms in the third, giving a six-kingdom arrangement. Such changes continue as our knowledge of characteristics and evolutionary relationships improves.