Classification and Taxonomy

Classification and Taxonomy of Living Organisms

Domains and Kingdoms

  • Classification is essential for organizing and understanding the diversity of life.

  • Organisms are classified into domains and kingdoms based on their characteristics and evolutionary relationships.

Learning Objectives

  • Classify organisms based on their distinguishing characteristics.

  • Discuss hierarchical classification of organisms based on evolutionary relationships.

  • Understand the benchmarks SC.912.L.15.6 and SC.15.4 related to classification.

Classification Importance

  • Biologists classify organisms to deal with approximately 2 million known species and estimate 8 million undiscovered species.

  • Taxonomy is the science of classifying organisms and giving them names.

Historical Changes in Classification

  • 1700s: Linnaeus introduced the two-kingdom system: Plantae and Animalia.

  • 1938: Copeland proposed a kingdom for prokaryotes called Monera.

  • 1977: Woese split Monera into Eubacteria and Archaebacteria.

  • 1959: Haeckel proposed a kingdom for unicellular organisms: Protista.

  • 1969: Whittaker added fungi as a separate kingdom based on their feeding methods.

Criteria for Classification

  1. Presence of nucleus (eukaryotic vs. prokaryotic).

  2. Number of cells (unicellular vs. multicellular).

  3. Energy acquisition (autotrophic vs. heterotrophic).

  4. Special characteristics distinct from other organisms.

Prokaryotes vs. Eukaryotes

  • Prokaryotes: Single-celled, lack a nucleus and membrane-bound organelles, evolved 3.5 billion years ago; includes Eubacteria and Archaebacteria.

  • Eukaryotes: Can be unicellular or multicellular, possess a nucleus and membrane-bound organelles, evolved 1.5 billion years ago; includes all other kingdoms - Animalia, Plantae, Fungi, Protista.

  • Commonalities: Both have cell membranes, genetic material (DNA), ribosomes, and cytoplasm.

Three Domain System

  • Largest classification category includes:

    • Domain Bacteria: Prokaryotic, diverse roles in ecosystems.

    • Domain Archaea: Prokaryotic, found in extreme environments, autotrophic.

    • Domain Eukarya: Includes kingdoms Protista, Fungi, Plantae, and Animalia; all eukaryotic organisms.

Kingdom Characteristics

Kingdom Animalia

  • Multicellular, eukaryotic, heterotrophic; no cell walls or chloroplasts.

Kingdom Plantae

  • Multicellular, nonmotile, photosynthetic autotrophs; cell walls contain cellulose.

Kingdom Fungi

  • Eukaryotic, heterotrophs, cell walls contain chitin; secrete enzymes for digestion, mainly multicellular.

Kingdom Protista

  • Most diverse; includes unicellular and multicellular organisms that can be animal-like (protozoa) or plant-like (algae).

    • Examples: Amoeba (animal-like), Euglena (plant-like).

Kingdom Eubacteria and Archaebacteria

  • Eubacteria: Unicellular prokaryotes with peptidoglycan cell walls; ecologically diverse.

  • Archaebacteria: Unicellular prokaryotes living in extreme environments; autotrophic.

Cladistics and Evolutionary Classification

  • Evolutionary Classification: Groups species by evolutionary descent instead of only physical similarities.

  • Cladistic Analysis: Uses derived characters to construct cladograms reflecting evolutionary relationships.

Reading Cladograms

  • Branch points (nodes) indicate speciation events; each node shows common ancestry.

  • Closely related lineages share recent common ancestors; diverging patterns indicate ancient splits among lineages.

Molecular Biology in Classification

  • DNA similarities help determine classification and evolutionary relationships; closely related species share more similar DNA.

  • Changed perceptions of relationships among species based on molecular evidence leading to revisions in the Linnaean classification system.

Summary

  • Taxonomy is a dynamic field that continuously adapts with new scientific discoveries about organisms’ evolutionary histories and molecular biology, refining our understanding of life's diversity.