Diversity in Living Organisms

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03 The Hierarchy of Classification – Group

  • A wide variety of species has evolved on earth over thousands and millions of years.
  • Life forms around us range from small ants to huge trees, colourless worms/insects to brightly coloured flowers and birds.
  • The idea of biological classification provided way to ease the study and identify organisms.
  • The method of arranging organisms into groups or sets based on their similarities and differences is called classification.
  • Attempts at classification have been made since time immemorial.
  • Things were initially classified into living and non-living.
  • Later, when the diversity of living things began to grow, it called for the necessity of biological classification.
  • Many biologists worked on it and each of them classified organisms into different groups according to different criteria.
  • Researchers could only decide the most fundamental characteristics for the classification over a period of years.

Importance of Classification:

  • Classification makes the study of a wide variety of organisms easy.
  • Classification projects before us a picture of all life forms at one glance.
  • Classification helps to understand the interrelationships among different groups of organisms.
  • Classification forms a base for the development of other biological sciences. For ex. Biogeography which is the study of geographical distribution of plants and animals totally dependent on the information supplied by classification.

Basis of Classification:

  • Aristotle, the Greek thinker was the first to classify animals based on their habitat which include air, water and land. But animals in each group have nothing in common, other than their habitat (except few). For example, a fish and turtle could not be grouped together. Hence, habitat could not be a criterion for classification.
  • So, scientists started to classify living organisms based on their characteristics which can be explained in many ways. We need to define more specific characteristics because a group of organisms can be similar enough to be thought of together by certain characteristics. Characteristics like the appearance/form and behaviour/function are some points that decide which organism will be in which group. For example, a dog has limbs but a snake doesn’t. A dog and a snake can move but a plant cannot. These are characteristics of different organisms that classify them into different groups.
  • Fundamental characteristics like the shapes and sizes of stones decide the broadest divisions or main groups among living organisms like the shape and size of the top layer. Similarly, the characteristics at the next level will decide sub-groups based on the fundamental one. In this way, a hierarchy of classification is built.
  • Organisms are classified based on some of the following characteristics:
    • Prokaryotic or Eukaryotic cell.
    • Unicellular or Multicellular.
    • Autotrophs (Photosynthetic) or Heterotrophs (Non-photosynthetic).
    • The level of organization and development of organs.

Body design of a plant and an animal is the fundamental characteristic of classification. The next level of the hierarchy is, whether the plant is a tree or a shrub. More subgroups will be formed based on different characteristics.

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