Levels of Biological Classification

Two Kingdom Classification System

Proposed by Carolus Linnaeus in $1758$, this system established the foundation for modern taxonomy by categorizing all living things into two kingdoms using anatomical characteristics and motility as primary criteria.

Kingdoms include:

• Plantae (Regnum Vegetabile):

  • Autotrophic organisms (green plants, mosses, ferns, multicellular seaweeds).

  • Diverse microbes including bacteria, fungi, and lichens.

  • Possess a cell wall, use starch for reserve food, and are fixed in soil.

• Animalia (Regnum Animale):

  • Heterotrophic organisms (multicellular animals, unicellular protozoans).

  • Lack a cell wall and chlorophyll, show proportional growth, respond to stimuli, and can move.

  • Store reserve food as glycogen.

Merits: Initiated systematic classification methods. Demerits: Failed to distinguish between eukaryotes and prokaryotes. Did not accommodate dual characteristic organisms like Euglena or lichens.

Three Kingdom Classification System

Introduced by Ernst Haeckel in $1866$, this system added a third kingdom:

• Kingdom Protista: For single-celled organisms intermediate between plants and animals, focusing on those lacking tissue differentiation.

Carl Woese’s Three-Domain System

Introduced in $1990$, based on molecular evidence.

• Domains:

  1. Archaea: Prokaryotes with unique biochemistry.

  2. Bacteria: Prokaryotes (eubacteria) with peptidoglycan cell walls.

  3. Eukarya: Organisms with membrane-bound nuclei, includes kingdoms Protista, Fungi, Plantae, and Animalia.

Five Kingdom Classification System

Established by R.H. Whittaker in $1969$ based on various criteria:

1. Kingdom Monera: Unicellular prokaryotes.

2. Kingdom Protista: Unicellular eukaryotes, mainly aquatic.

3. Kingdom Fungi: Multicellular eukaryotes, chitin cell walls.

4. Kingdom Plantae: Multicellular eukaryotic autotrophs.

5. Kingdom Animalia: Multicellular eukaryotes without cell walls.

System Evaluation:

• Merits: More homogeneous groups and reflects phylogenetic relationships better than previous systems.

• Demerits: Heterogeneous groups present in Monera and Protista; viruses are excluded.