chapter 3 | Biodiversity part 2




“A species is a group of organisms which can interbreed freely among them and produce fertile offspring, but are reproductively isolated from all other such
groups in nature.” 

In the definition of species we must emphasize “in nature” because two organisms related to two different but closely related species can cross-breed under artificial conditions.

Carolus Linnaeus divided nature into three kingdoms: mineral, vegetable and animal.
 Linnaeus used five ranks in classification: class, order, genus, species, and variety. 
Linnaeus is best known for his introduction of the method still used to formulate the scientific name of every species i.e. binominal nomenclature.


Oldest system  classifies all organisms into two kingdoms i.e. Plantae (organisms that can prepare food from simple inorganic materials and thus can store energy, are autotroph) and Animalia(organisms that cannot synthesize their food and depend on autotrophs or others are heterotrophs ) and is known as two kingdom classification system.
 
According to this system, bacteria, fungi and algae were included in kingdom plantae. Taxonomists found this system unworkable because; many unicellular organisms like Euglena have both plant-like (presence of chlorophyll) and animal-like (heterotrophic mode of nutrition in darkness and lack of cell wall) characters. So there should be a separate kingdom for such organisms. This system also ignores the difference between organisms having prokaryotic and those having eukaryotic cells.

In 1866, Ernst Hackel solved the first objection and proposed a third kingdom i.e. protista to
accommodate euglena-like organisms. He also included bacteria in kingdom protista. In this system,
fungi were still included in the kingdom plantae. This system did not clear the difference between prokaryotes and eukaryotes.

In 1967, Robert Whittaker introduced the five-kingdom classification system. This system is based on;
  • The levels of cellular organization i.e. prokaryotic, unicellular eukaryotic and multicellular eukaryotic
  • The principal modes of nutrition i.e. photosynthesis, absorption, and ingestion.
On this basis, organisms are classified into five Kingdoms: monera, protista, fungi, plantae and
animalia.


In 1988, Margulis and Schwartz modified the five-kingdom classification of Whittaker. They
considered genetics along with cellular organization and mode of nutrition in classification. They
classified the organisms into the same five kingdoms as proposed by Whittaker.








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