Hint: Calculating the total number of organisms on this planet is a difficult task but scientists have estimated that there are about 8.7 million species on Earth. Grouping all the living organisms into convenient categories is known as classification.
A variety of life forms exist in nature differing in morphology, size, shape, anatomy, habit and habitats. A proper system of classification is must because:
- It is not possible to study every organism out of this rich biodiversity known to science. Thus, a study of one or two organisms of a group gives sufficient information about the essential characteristics of the group.
- Identification is easier once we classify organisms.
- The proper system of classification will help us to study the organisms of the past.
- Relationships among different groups of an organism can be established.
- On the basis of this relationship, one can unmask the puzzle of evolution.
Three main types of classification are:
Artificial classification: Easy observable morphological characters such as habit, habitat, colour, occurrence or absence of any part of an organism’s body etc. E.g. Aristotle categorised animals on the basis of their habitat into aquatic, terrestrial and aerial.
Natural system of classification: Properties such as morphology, cytology(cell structure), phytochemistry(chemicals found in plants), embryology and anatomy are applied to find similarities and affinities among organisms.E.g. Bentham and hooker divided seed plants into three classes namely Dicotyledonae, Gymnospermae and Monocotyledonae.
Phylogenetic classification: It uses phylogenetic trees to classify organisms based on their evolutionary descent and relationship .E.g. Engler and Prantl assorted flowering plants according to the increasing complexity of floral morphology. They considered unisexual flowers with one whorl of perianth and pollinated by wind primitive to bisexual flowers with two whorls of perianth and pollinated by insects.
Note: Phylogeny is an attempt to trace an organism’s evolutionary history and on this trail, one can find out about its evolutionary relationship with another set of animals. This is attempted by creating a phylogenetic tree which may be scaled as in phylogram or unscaled as in cladogram. Both are a diagrammatic representation of an organism’s evolutionary account, but cladograms are usually a predecessor of phylogram. The branches in a cladogram are equal which just inform us that they are related but the extent to which they are related is confirmed by the different lengths of branching used in phylogram.