Systematics, Evolution & Zoogeography |Module 1 Unit 2 | Calicut University Third Semester BSc. Zoology Notes PDF |

 

Module 1: Systematics Unit 2: Classification and Nomenclature

In this study material of systematics, evolution and zoogeography i've covered unit 2 from module 1 which is the major classification systems used to categorise living organisms through out history. 

Summary of what you will learn in Module 1 Unit 2

🌿 Introduction to Classification

Classification is the systematic arrangement of organisms into hierarchical groups based on similarities and differences. Its helps in understanding structural organisation, mode of nutrition, level of complexity, and evolutionary relation.

🏛 Two Kingdom Classification

The earliest scientific system of classification was proposed by Carolus Linnaeus in 1758 in his work Systema Naturae. He divided living organisms into two kingdoms: Plantae and Animalia.

This classification was primarily based on morphological characteristics such as locomotion, mode of nutrition. Plants were considered to be non-motile and autotrophic, while animals were motile and heterotrophic. This two system classification was artificial and lacked evolutionary understanding.

Merits of Two Kingdom Classification

This system was easy to understand and represented the first systematic attempt to organise life scientifically. It clearly distinguished between plants and animals, which are two major groups of macroscopic life.

Demerits of Two Kingdom Classification

Organisms such as Euglena showed both plant-like and animal-like features. Fungi, which lack chlorophyll and possess chitinous cell walls, were incorrectly placed under Plantae. Bacteria, which are prokaryotic, were grouped along with eukaryotic plants. Lichens did not fit neatly into either kingdom.

🌳 Five Kingdom Classification

To overcome the limitations caused by the previous systems of classification, Robert H. Whittaker proposed the five kingdom classification in 1969. This system was more scientific and was based on multiple criteria such as cell type, level of organisation, mode of nutrition, and phylogenetic relationship.

The proposed five kingdoms were: Monera, Protista, Fungi, Plantae, Animalia.

Merits of Five Kingdom Classification

This system clearly separated prokaryotes from eukaryotes. It placed fungi in an independent kingdom and created Protista to accommodate unicellular eukaryotes. For the first time, evolutionary relationships were partially considered in classification.

Demerits of Five Kingdom Classification

Protista remained a heterogeneous group containing algae, protozoans, and slime moulds. archaebacteria and eubacteria were grouped together under Monera despite significant molecular differences. Additionally, viruses, viroids, and prions were not included in any kingdom.

🧬 Cavalier-Smith’s Eight Kingdom Classification

To refine classification based on molecular biology, Thomas Cavalier-Smith proposed the Eight Kingdom Classification. 

The proposed eight kingdoms are: Eubacteria, Archaebacteria, Archezoa, Protozoa, Chromista, Fungi, Plantae, Animalia

🦠 Status of Viruses, Viroids, and Prions

Viruses, viroids, and prions are sub-cellular infectious agents and do not possess complete cellular organisation. They lack independent metabolism and can replicate only inside host cells. Because they are neither prokaryotic nor eukaryotic and do not exhibit true cellular structure, they are not placed in any biological kingdom.

The most accepted theory suggests that viruses originated from “escaped nucleic acids” of cellular organisms. Their exact systematic position remains unsettled, making them unique entities in biology.

📝 Important Exam Questions

• Merits and demerits of two kingdom classification
• Five kingdom classification
• Why are viruses not placed in any kingdom?
• Eight kingdom classification
• Status of viruses in classification
• Comparison between two and five kingdom systems
  
  

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