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Basis of Classification

The aim of biological classification is to recognize the vast number of known plants and animals into categories that could be named, remembered and studied.
The science of classification is called systematics or taxonomy. Classification can be based on two different systems, namely natural and artificial.
  • Features of organisms thar are similar in struucture, but may be used for different functions.
  • Biologists group organisms togther which are structurally similar and share common ancestors.
  • Natural classification is based on the ideas of homologous structures1 and evolutionary relationships2.
  • Artificial classification is also used as the basis for dichotomous keys that biologists use to identify organisms. Carolus Linnaeus (1707–1778), a Swedish scientist, classified, described and named plants and animals. He based his classification on sexual characters, i.e., number of stamens in a flower. Thus, Linnaeus’ system was considered to be an artificial system of classification.
Scientific names to each group or taxon, recognized in biological classification, is called nomenclature.

Carolus Linnaeus was the first to use the binomial system of nomenclature. In this system of naming, each animal or plant is given a biological name or scientific name which consists of two parts—the first part is the genus name or generic name and the second is the species name or specific name. The generic and the specific names are in Latin and are printed in italics. For example, the scientific name for humans is Homo sapiens. The genus is Homo and the species is sapiens. The species name always begins with the small letter of the alphabet.
The groups into which the organisms are classified can be arranged from the largest to the smallest group. This sequence of arrangement of taxa in a descending order in the classification is called taxonomic hierarchy. This is shown in Figure 1.2. The classification of organisms under different taxonomic hierarchy is shown in Figure 1.3.

Figure 1.2 Taxonomic Hierarchy

Figure 1.3 Classifi cation of Organisms Under Different Taxonomic Hierarchy

Carolus Linnaeus in 1758 classified the living organisms into two kingdoms, plant kingdom and animal kingdom. Modern biologists now agree on a five-kingdom classification as proposed by R. H. Whittaker, such as Kingdom Monera (Prokaryota), Protista, Mycota (Fungi), Plantae (Metaphyta) and Animalia (Metazoa). Table 1.1 shows the classification of the five kingdoms and their features.
 Table 1.1 Five Kingdoms

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