Hint: A variety of life forms exist in nature differing in morphology, size, shape, anatomy, habit, and habitats. Grouping all the living organisms into convenient categories is known as classification. Artificial, natural and phylogenetic are the three principal systems of classification.
The natural system of classification is a type of classification in which properties such as morphology, cytology(cell structure), phytochemistry(chemicals found in plants), embryology, and anatomy are applied to find similarities and affinities among organisms. These characteristics help in bringing out the maximum number of similarities as well as dissimilarities. Homology is also considered among diverse organisms. It is the relationship of comparable structures having been derived from a common form. For e.g. the forearm of different land vertebrates has the same pentadactyl (five fingers or toes)constitution but different externally. It has evolved to meet an organism’s requirement like grasping in human beings, running in horses, swimming in whales and flight in birds or bats.
In the case of biochemicals, molecular homology helps to establish a relationship between organisms by studying the similarities and dissimilarities in biomolecules such as DNA, RNA, and certain proteins.
E.g.Bentham and Hooker published their classification of seed plants in the three-volume publication ‘Genera Plantarum’.
Note: The other two systems of classification are:
Artificial classification: Easy observable morphological characters such as habit, habitat, color, occurrence, or absence of any part of an organism’s body etc. E.g. Aristotle categorized animals on the basis of their habitat into aquatic, terrestrial and aerial.
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.