Biological Classification Systems

• Biological classification involves identifying differences among organisms and placing them into groups, reflecting significant features and relationships.
• Aristotle first proposed biological classification, using morphological characteristics to classify plants into trees, shrubs, and herbs.
• Aristotle classified animals into two groups based on the presence or absence of red blood.
• The purpose of biological classification is to organize known plants into categories for easier study.

Classification Systems

• Carolus Linnaeus proposed a 2-kingdom system, dividing organisms into Plantae and Animalia.
• Ernst Haeckel proposed a 3-kingdom system: Plantae, Protista, and Animalia.
• Copeland introduced a 4-kingdom system: Plantae, Protista, Monera, and Animalia.
• Whittaker proposed a 5-kingdom system: Plantae, Protista, Monera, Fungi, and Animalia.
• Carl Woese proposed a 6-kingdom system: Plantae, Protista, Monera, Fungi, Archaebacteria, and Animalia.

Two Kingdom Classification System

• Linnaeus classified all organisms into Kingdom Plantae and Kingdom Animalia.
• Kingdom Plantae included autotrophic, fixed organisms.
• Kingdom Animalia included motile, heterotrophic organisms.

Limitations of the Two-Kingdom System

• Unicellular and multicellular forms were grouped together despite organizational differences.
• Bacteria and cyanobacteria (prokaryotes) were included under plants.
• Lichens, symbiotic organisms, were included in plants.
• Fungi, non-photosynthetic heterotrophs, were included under Plantae.
• The system failed to distinguish between prokaryotes and eukaryotes.

Five Kingdom Classification System

• R.H. Whittaker proposed the five-kingdom classification in 1969.
• The five kingdoms defined by Whittaker are Monera, Protista, Fungi, Plantae, and Animalia.
• Criteria for classification included cell structure, thallus organization, mode of nutrition, reproduction, and phylogenetic relationships.

Key Points of Five Kingdom Classification

• Earlier systems grouped bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under 'Plants.'
• A unifying characteristic that all organisms contain include cell walls in all cells.
• The system grouped together prokaryotic bacteria and eukaryotic blue-green algae.
• Unicellular and multicellular organisms like Chlamydomonas and Spirogyra were grouped together under algae.
• Fungi have chitin in their walls, while green plants have cellulosic cell walls. Fungi were placed in a separate kingdom (Kingdom Fungi) when wall composition was considered.
• All prokaryotic organisms were grouped under Kingdom Monera.
• Unicellular eukaryotic organisms were placed in Kingdom Protista.
• Kingdom Protista includes Chlamydomonas and Chlorella, which were previously placed in Algae within Plants, and Paramecium and Amoeba which were previously placed in the animal kingdom.

Merits of Five Kingdom Classification

• Exhibits phylogenetic relationships between groups of organisms.
• Prokaryotes are classified in a separate kingdom based on genetic, cellular, reproductive, and physiological differences.
• Fungi were grouped under a separate kingdom, and protozoa were excluded from kingdom Animalia.