PRELIMS

Classification of Organisms

Classification categories millions of organisms into manageable groups. Linnaeus is called the 'Father of Taxonomy' (Author of Systema Nature).

Five Kingdom Classification (Whittaker, 1969)

Based on cell complexity, mode of nutrition, and phylogenetic relationship.

1. Monera: Unicellular prokaryotes (Bacteria, Cyanobacteria, Archaebacteria). Microscopic and filamentous.
2. Protista: Unicellular eukaryotes (usually aquatic). Autotrophic, parasitic, or saprophytic. (e.g., Diatoms, Flagellates, Protozoa, Euglena - placed between plants and animals).
3. Fungi: Non-green, saprophytic plants. Cell walls made of Chitin. (e.g., Mushroom, Mucor).
4. Plantae: All plants (except some Algae, Diatoms, Fungi).
5. Animalia: All multicellular animals (Metazoans) except Protozoans.

Two Kingdom vs Five Kingdom vs Three Domain

• Linnaeus (Two Kingdom, 1758): Divided all organisms into Plantae and Animalia. Failed to distinguish prokaryotes from eukaryotes or autotrophs from heterotrophs among microbes.
• Whittaker (Five Kingdom, 1969): Monera, Protista, Fungi, Plantae, Animalia (see above) — based on cell structure, thallus organisation, mode/source of nutrition, and phylogenetic relationships.
• Carl Woese (Three Domain System, 1977/1990): Based on differences in ribosomal RNA (rRNA) sequences, divides life into Archaea, Bacteria, and Eukarya. Splits Monera into Archaea (extremophiles/ancient) and Bacteria (true bacteria/Eubacteria).

Binomial Nomenclature

Every organism is given two international names:

• Genus: Always starts with a Capital letter (e.g., Homo).
• Species: Starts with a small letter (e.g., sapiens).

Scientific Names of Common Organisms

• Man: Scientific Name: Homo sapiens. Organism: Wheat. Scientific Name: Triticum aestivum
• Frog: Scientific Name: Rana tigrina. Organism: Pea. Scientific Name: Pisum sativum
• Cat: Scientific Name: Felis domestica. Organism: Gram. Scientific Name: Cicer arietinum
• Dog: Scientific Name: Canis familiaris. Organism: Mustard. Scientific Name: Brassica campestris
• Cow: Scientific Name: Bos indicus. Organism: Mango. Scientific Name: Mangifera indica
• Housefly: Scientific Name: Musca domestica. Organism: Rice. Scientific Name: Oryza sativa

UPSC Relevance

The classification of life forms is a mandatory pillar for GS-III Biology.

• Taxonomy: The shift from Linnaeus's 2-kingdom system to Whittaker's 5-kingdom system is a classic conceptual theme.
• Monera vs Protista: Distinguishing between prokaryotic and eukaryotic unicellular life is fundamental to microbiology.
• Scientific Names: While less common in Main exam, Prep for Prelims involves knowing names of common crops (Wheat, Rice) or species of high conservation interest.