Classifying Organisms

Taxonomy

• Taxonomy: science of describing, naming, and classifying living and extinct organisms and viruses
• Systematics: study of biological diversity and the evolutionary relationships among organisms, both extinct and modern
  • Systematists use fossil, molecular, and genetic data to infer evolutionary relationships
• Hierarchical system involving successive levels
• Taxon: each group at any level
• Domain: the highest level
  • All of life belongs to one of 3 domains: bacteria, archaea, and eukarya
• Phylogenetics: reconstructing the evolutionary relationships among organisms

Cladistic Approach

• Cladistic approach: compares homologous traits, also called characters, which may exist in two or more character states
• Shared primitive character or symplesio-morphy: shared by two or more different taxa and inherited from ancestors older than their last common ancestor
• Shared derived character or synapo-morpy: shared by two or more species or taxa and has originated in their most recent common ancestor
  • Basis of the cladistic approach is to analyze many shared derived characters to deduce the pathway that gave rise to those species

Constructing a Cladogram

1. Choose species
2. Choose characters
3. Determine polarity of character states
   • Primitive or derived?
4. Analyze cladogram
   • All species (or higher taxa) are placed on tips in the phylogenetic tree, not at branch points
   • Each cladogram branch point should have a list of one or more shared derived characters that are common to all species above the branch point unless the character is later modified
   • All shared derived characters appear together only once in a cladogram unless they arose independently during evolution more than once

Principle of Parsimony

• Preferred hypothesis is the one that is the simplest for all the characters and their states
• Challenge in a cladistic approach is to determine the correct polarity of events
  • It may not always be obvious which traits are primitive (came earlier) and which are derived (came later in evolution)
  • Fossils may be analyzed to help resolve

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