Lecture Notes: Taxonomy and Classification of Organisms

Introduction to Taxonomy

• Scientists classify organisms based on relatedness.
• Classification helps in identifying and naming organisms logically.
• Taxonomy aims to create a phylogenetic system.

Traditional Classification

• Previously, organisms were classified into five kingdoms:
  • Plants
  • Fungi
  • Animals
  • Protists
  • Prokaryotes
• This classification was based on phenotypic characteristics.

Carl Woese's Three Domain System

• In 1990, Carl Woese proposed a new classification system using molecular clocks.
• He suggested using genetic information (instead of phenotypic traits) for classification.
• Ribosomal RNA (rRNA) was key:
  • rRNA evolves slowly, acting as a molecular clock.
  • Similar rRNA sequences indicate evolutionary relatedness.

The Three Domains

• Woese's classification resulted in three major domains:
  • Eukaryotes: Includes protists, animals, plants, fungi.
  • Bacteria: Traditional bacteria group.
  • Archaea: Previously grouped with prokaryotes but genetically closer to eukaryotes.

Significance of Woese’s Discovery

• Compared to Einstein's impact on physics.
• Allowed measurement of evolutionary history using genetic analysis.
• Eukaryotes are less genetically diverse compared to bacteria.

Archaea

• Known as extremophiles; survive in extreme environments.
• Found in human guts as methanogens.
• Minimal relevance to the class, but important to recognize as a third domain.

Modern Bacterial Classification

• Historically based on phenotype (shape, staining, growth conditions).
• Phenotypic classification is inaccurate for bacteria due to simplicity.
• Modern classification relies on genetic analysis and DNA sequencing.
• Ribosomal RNA analysis enhances classification accuracy.

Conclusion

• Current classification of life uses the three-domain system.
• Genetic tools provide a more accurate understanding of evolutionary relationships.