Chapter-2: Biological Classification

Introduction
Biological classification is the scientific way of orderly arranging organisms into groups on the basis of their similarities and differences in certain easily observable but fundamental characters.

• Aristotle was the earliest one to attempt a more scientific basis for classification. He used simple morphological characters to classify plants into trees, shrubs, and herbs. He also divided animals into two groups, those which had red blood (enaima) and those that did not (anaima).
• Linnaeus gave a two kingdom system of classification: Kingdoms Plantae and Animalia that included all plants and animals respectively. This system was used till very recently. This system did not distinguish between the

• • eukaryotes and prokaryotes
  • unicellular and multicellular organisms
  • photosynthetic (green algae) and non-photosynthetic (fungi) organisms. Hence the two kingdom classification used for a long time was found inadequate.

• Haeckel introduced three kingdom system with Kingdoms Plantae, Protista, and Animalia.
• Copeland gave four kingdom system having Kingdoms Monera, Protista, Plantae, and Animalia.

• R.H. Whittaker (1969) proposed a five kingdom system of classification. The kingdoms defined by him were named Monera, Protista, Fungi, Plantae, and Animalia. It is the most accepted system of classification. The main criteria for classification used by him include cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.

Table: Characteristics of the five kingdoms 

• Viruses have not been included in the five-kingdom system of classification.
• Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms, and angiosperms all under ‘Plants‘. All these organisms have cell wall. It brought together the prokaryotic bacteria and the blue green algae with other groups which were eukaryotic. It also grouped together the unicellular and the multicellular organisms, e.g., Chlamydomonas and Spirogyra were placed together under algae.
• Fungi which are non-photosynthetic had been grouped as plants (autotrophs), though they also showed a characteristic difference in their walls composition – the fungi had chitin in their cell walls while the green plants had a cellulosic cell wall. When these characteristics were considered, the fungi were placed in a separate kingdom – Kingdom Fungi. All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista. Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in algae within plants and both having cell walls) with Paramecium and Amoeba (which were earlier placed in the animal kingdom which lacks cell wall). It has put together organisms that, in earlier classifications, were placed in different kingdoms.
• Over time, an attempt has been made to evolve a classification system which reflects not only the morphological, physiological, and reproductive similarities, but also phylogenetic, i.e., is based on evolutionary relationships.