2. Systematics of Living Organisms part 03 - Units of Classification

 

2. Systematics of Living Organisms part 03 - Units of Classification


Units of Classification :
Species :
  • Species is the principal natural taxonomic unit, ranking below a genus and denoted by latin binomial (considered as the basic) unit of classification. 
  • It is a group of organisms that can interbreed under natural condition to produce fertile offspring. 
  • It was thought to be an indivisible, stable and static unit. 
  • However in the modern taxonomy, subdivision of species such as sub-species, varities and populations are seen and given more importance.

Genus : 
  • Genus is a taxonomic rank or category larger than species used in the biological classification of living and fossil organisms. 
  • Genus is a group of species bearing close resemblance to one another in their morphological characters but they do not interbreed. 
  • e.g. Tiger, Leopard, Lion all three belong to same genus  Panthera. 
  • They have common characters yet are different from each other because their genus is same but species is different. 
  • Another example is genus Solanum.
  • Brinjal and potato both belong to this genus.

Family : 
  • It is one of the major hierarchial taxonomic rank. 
  • A family represents a group of closely related genera. 
  • e.g. genera like Hibiscus, Gossypium, Sida, Bombax are included in same family Malvaceae. 
  • Cat also belongs to family of leopards, tigers and lions, family Felidae but dog belongs to different family Canidae.


Cohort / Order : 
  • It is taxonomic rank used in the classification of organisms and recognised by nomenclature codes. 
  • An order is a group of closely related families showing definite affinities. 
  • Order thus is a step above family in taxonomic hierarchy. 
  • Members belonging to same order but different families may show very few dis similarities. 
  • e.g. family - Papavaraceae, Brassicaceae, Capparidaceae, etc with parietal plancentation are grouped in order Parietales. 
  • Families of dogs and cats though are different, they belong to same order Carnivora.

Class : 
  • The class is the distinct taxonomic rank of biological classification having its own distinctive name. 
  • A group of higher taxonomic rank than order. 
  • Class is the assemblage of closely allied orders. 
  • Orders Carnivora and order Primates belong to class Mammalia. 
  • Thus monkeys, gorillas, gibbons (Primates) and dogs, cats, tigers (Carnivora) belong to same class.

Division / Phylom : 
  • The division is a category composed of related classes 
  • e.g. division. Angiospermae includes two classes - Dicotyledonae and Monocotyledonae.
  • In animal classification division is a sub-unit of Category / Phylum.

Sub-kingdom : 

  • Different divisions having some similarities form sub-kingdom.
  • e.g. The divisions Angiospermae and Gymnospermae will tIt is the highest taxonomic category composed of different subkingdoms.
  • e.g. sub-kingdom Phanerogams and Cryptogams form the Plant kingdom or Plantae which includes all the plants while all animals are included in kindom Animalia.
  • The taxonomic categories we have considered so far are broad categories.
  • Scientists have added sub-categories to these in order to place organisms in more scientific manner. 
  • As we go higher in taxonomical ladder, number of common characters go on decreasing.
  • If we are comparing two organisms that are related to each other only at division or phylum level, their classification may become difficult.

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