Early Systems
- Instinctive Classification:
- Since ancient times, humans classified living organisms based on their utility for food, shelter, and clothing, not on scientific criteria.
- Aristotle's Classification:
- Aristotle was the first to attempt a more scientific basis for classification. He categorized plants into trees, shrubs, and herbs based on their morphological characteristics and divided animals into those with red blood and those without.
Two Kingdom System by Linnaeus
- Plantae and Animalia:
- During Linnaeus' time, a two-kingdom system was used, classifying all living organisms into either Plantae or Animalia.
- This system did not account for differences between eukaryotes and prokaryotes, unicellular and multicellular organisms, or photosynthetic (green algae) and non-photosynthetic (fungi) organisms.
Limitations of the Two Kingdom System
- Inadequacy:
- Many organisms did not fit neatly into the Plantae or Animalia categories.
- The two-kingdom system lacked differentiation based on cell structure, nature of cell walls, modes of nutrition, habitats, reproduction methods, and evolutionary relationships.
Advances in Classification
- Need for a New System:
- The two-kingdom system's inadequacies led to the development of more comprehensive classification systems that considered a broader range of characteristics
Whittaker's Five Kingdom Classification (1969)
R.H. Whittaker proposed a five kingdom classification:
- Monera: Prokaryotic organisms
- Protista: Unicellular eukaryotic organisms
- Fungi: Organisms with chitin in their cell walls
- Plantae: Multicellular, photosynthetic organisms with cellulosic cell walls
- Animalia: Multicellular organisms without cell walls
Criteria for Classification
- Cell structure
- Body organization
- Mode of nutrition
- Reproduction
- Phylogenetic relationships
Evolution of Classification Systems
Earlier systems placed bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under 'Plants' due to the presence of cell walls. However, this classification was problematic as it grouped prokaryotic bacteria with eukaryotic organisms. The fungi were later separated due to their chitinous cell walls, differing from the cellulosic walls of green plants.
Whittaker’s Classification: Addressing Earlier Misclassifications The five-kingdom system addressed these issues by:
- Placing fungi in a separate kingdom due to their unique cell wall composition and heterotrophic mode of nutrition.
- Grouping all prokaryotic organisms under Kingdom Monera.
- Placing unicellular eukaryotic organisms in Kingdom Protista.
Modern Classification: Three-Domain System
The three-domain system further refines classification by dividing Kingdom Monera into two domains, resulting in a six-kingdom classification:
- Bacteria
- Archaea
- Eukarya (includes Protista, Fungi, Plantae, Animalia)
Importance of Phylogenetic Classification
Modern classification aims to reflect not only morphological, physiological, and reproductive similarities but also evolutionary relationships.
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