Biology Chapter 2: Biological Classification

Biological Classification Overview

• Classification of living organisms began instinctively, evolving to meet human needs for food, shelter, and clothing.
• Aristotle pioneered a more scientific approach using morphology to categorize plants and animals, distinguishing groups based on blood types.
• The Two Kingdom system established by Linnaeus included Plantae and Animalia but was inadequate for classifying all organisms, particularly prokaryotes and eukaryotes.

Development of Classification Systems

• Organisms were traditionally grouped as plants or animals, but many did not fit into these categories due to diverse characteristics.
• Essential criteria for classification expanded to include cell structure, mode of nutrition, reproductive methods, and evolutionary relationships.
• The understanding of plant and animal kingdoms has evolved, with definitions of included groups changing over time among scientists.

Characteristics of the Five Kingdoms

• R.H. Whittaker proposed a Five Kingdom Classification in 1969 consisting of Monera, Protista, Fungi, Plantae, and Animalia.
• Key criteria defining these kingdoms include:
  • Cell Type: Prokaryotic (Monera) vs. Eukaryotic (others).
  • Cell Wall Composition:
    • Monera: Noncellulosic (polysaccharide + amino acid).
    • Fungi: Chitin.
    • Plantae: Cellulose.
    • Animalia: Absent.
  • Nuclear Membrane: Absent in Monera; present in others.
  • Body Organization: Monera and Protista are cellular; Fungi are multicellular/loose tissue; Plantae and Animalia are tissue/organ systems.
  • Mode of Nutrition:
    • Monera: Autotrophic (chemosynthetic/photosynthetic) and heterotrophic (saprophytic/parasitic).
    • Protista: Autotrophic (photosynthetic) and heterotrophic.
    • Fungi: Heterotrophic (saprophytic/parasitic).
    • Plantae: Autotrophic (photosynthetic).
    • Animalia: Heterotrophic (various modes).

Three-Domain System

• A six kingdom classification system divides Kingdom Monera into two domains, with the remaining eukaryotic kingdoms grouped into a third domain.
• This system highlights the significant differences between prokaryotic and eukaryotic organisms, allowing for a more nuanced classification.

Historic Misclassifications

• Earlier systems grouped diverse organisms such as bacteria, algae, fungi, and plants based solely on the presence of cell walls, leading to misleading classifications.
• Distinctions between autotrophic plants and heterotrophic fungi were not recognized until fungi were eventually classified into a separate kingdom due to their distinct characteristics, including wall composition (chitin vs. cellulose).