Taxonomy and Phylogeny Overview

Taxonomy and Classification of Organisms

  • Definition of Taxonomy
    Taxonomy is the science of classifying organisms based on shared characteristics and organizing them in hierarchical categories.

  • Historical Method of Classification

    • Initially based on morphological similarity (physical forms and structures).
    • Example: Carolus Linnaeus developed binomial nomenclature (genus and species names).

  • Example of Classification

    • Scaly foot organism question: Is it a snake, lizard, or worm?
    • Determines classification based on shared characteristics:
      • Legless lizards vs. snakes:**
      • Characteristics like fused eyelids and a highly mobile jaw indicate it's a lizard.

Levels of Taxonomic Classification

  • Hierarchical Structure
    • Domain
    • Most general classification; e.g., all members of the domain Eukarya have eukaryotic cells.
    • Kingdom
    • E.g., Kingdom Animalia, characteristics include heterotrophic metabolism and lack of cell walls.
    • Phylum
    • E.g., Phylum Chordata includes animals with spinal structures.
    • Class
    • E.g., Class Mammalia, characterized by hair and mammary glands.
    • Order
    • E.g., Order Carnivora includes carnivorous mammals, such as felids (cats).
    • Family
    • E.g., Family Felidae includes all cat species.
    • Genus and Species
    • Smallest grouping, e.g., Panthera pardus for leopards.

Modern Taxonomy Principles

  • Cladistics

    • Uses shared derived characters to separate organisms into groups based on evolutionary significance.
    • Example distinctions:
    • Mammals based on hair and mammary glands, vs. birds which do not share these traits.

  • Phylogenetic Trees and Cladograms

    • Visual representation of evolutionary relationships
    • Show branching of species and shared characteristics:
    • Nodes represent shared derived characteristics.
    • Sister taxa have a common ancestor, but one does not evolve from the other.

Key Concepts in Phylogenetic Analysis

  • Out Group

    • Group outside of the study group that helps to root the tree.

  • Reading Trees

    • Each branch reflects evolutionary relationships among organisms; older nodes are closer to the root.
    • Polytomy indicates uncertainty in relationships when multiple branches come off a node.

  • Molecular Phylogenetics

    • Analyzes genetic data to classify organisms.
    • Looks at genetic similarities and differences to establish evolutionary timelines,
    • Molecular Clock:
    • Predicts divergence times based on genetic changes between species.
    • Example: If two bird species differ at multiple gene loci, this might suggest a divergence 5 million years ago.

Homology vs. Analogy

  • Understanding evolutionary relationships requires distinguishing between:
    • Homology: Similar traits due to shared ancestry.
    • Analogy: Similar traits due to convergent evolution (similar adaptations to environmental pressures without common ancestry).