Diversity in Living Organisms

• The system of sorting living organisms into various groups based on their characteristic similarities and differences is called classification.
• To make relevant groups to study various forms, it is important to decide which characteristics decide more fundamental differences. This creates main broad groups of organisms. 
• Within these groups, smaller sub-groups are decided by less important characteristics. This is called hierarchy of characteristics.
• The similarities, differences and the variety among living organisms can be used as criteria for their classification.
  
  

Importance of Classification 

• Classification allows us to understand diversity better.
• It helps in the identification of living organisms as well as in understanding the diversity of living organisms.
• Classification helps us to learn about different kinds of plants and animals, their features, similarities and differences.
• It enables us to understand how complex organisms evolve from simpler organisms.
• To understand and study the features, similarities and differences between different living organisms, they are grouped under different categories.
• Classification is a tool which helps us to deal with a great diversity of living forms. 
• It is essential to understand the inter-relationships among the different groups of organisms.
• Classification forms a base for the development of other biological sciences.

• The principles of classification help us to trace the evolutionary relationships of the species around us. 
• The similarities among the organisms allow us to group them into broad groups. The grouping can be done based on a certain set of characteristics.
  
  
• Characteristics are details of the appearance or behaviour, a particular form or a function of an organism. 
• As time passed, certain characteristics began to influence the body design of organisms. When a new body design came into existence, changes began to occur in it which in turn brought about new changes. 
• The characteristics which came into existence earlier are likely to be more primitive than those which arose later. 
• Most of the organisms have arisen by the accumulation of changes in their body design. The organisms which possess these favourable changes survive better in the long run. 
• In 1859, Charles Darwin first described this concept of evolution in his book The Origin of Species.
• Certain groups of organisms have ancient body designs and are referred to as primitive or lower organisms.  Some organisms have acquired their body designs relatively recently and are called advanced or higher organisms. 
• The terms primitive and advanced do not clearly reflect the evolution of organisms, and hence, we often refer to them as older and younger organisms, respectively. 
• There is a strong possibility that complexity within the organisms increases with an increase in the evolutionary time. Hence, we can say that older organisms are relatively simpler, while younger organisms are more complex.
• We can build up small groups of species with recent common ancestors, then super groups of these groups with more distant common ancestors and so on. At some point in tracing back the history of the Earth, we find that non-living materials must have given rise to life on the Earth.

Systems of Classification

 Two-Kingdom Classification

• The two-kingdom classification was proposed by Carolus Linnaeus. 
• He categorised and classified the living organisms on the basis of nutrition and mobility. The living organisms were classified into Kingdom Plantae and Kingdom Animalia.
  
   

• All fungi were non-photosynthetic and enjoyed very distinct mode of nutrition (absorptive; osmotrophic).
• Some of the algae have lost their photosynthetic ability (e.g., some euglenophycophytes), lacked cell wall, moved freely and thus were considered closely related with protozoa.
• Other algal forms exhibiting more extensive organizational development and totally photosynthetic nature were considered more closely related with plants. 

• The five-kingdom classification was proposed by R.H. Whittaker in 1969. Whittaker classified these five kingdom basis on their cell structure, mode of nutrition, mode of reproduction.
  
   

Six-Kingdom Classification

• Carl Woese proposed the six- kingdom classification. Woese found that the six kingdoms naturally cluster into three main categories, based on the sequence of 16s ribosomal RNA genes. He called these categories as domains of life. 
• These domains are Bacteria, Archaea, and Eukarya.
  
   

Five-Kingdom System of Classification 

• The Five-kingdom Classification proposed by Whittaker is the most widely used classification system. According to this system, based on their cell structure, mode of nutrition and body organisation, organisms are divided into five kingdoms: 

• Kingdom Monera
• Kingdom Protista
• Kingdom Fungi
• Kingdom Plantae
• Kingdom Animalia

• Later, modifications suggested by Woese divided Kingdom Monera into Archaebacteria or Archae and Eubacteria or Bacteria.
• In 1892, Margulis and Schwartz revised the Five-kingdom Classification. It includes one prokaryotic kingdom—Prokaryotae—and four eukaryotic kingdoms—Protista, Fungi, Plantae and Animalia.
  
  
  
  
  
  
  
  
  
  
  
  

Drawbacks of Five-Kingdom Classification