Taxonomy & Nomenclature — Quick Reference

Nomenclature, Taxonomy, and Classification

  • Taxonomy: science of classifying living beings; origin with Carl von Linnaeus (Linnaeus) (1707–1778).
  • Purpose: provide each organism with a unique name and a defined place to catalog it; aids future identification across biology.
  • Nomenclature: assignment of scientific names to taxa and organisms.
  • Classification: orderly arrangement of organisms into a hierarchy.
  • Identification: discovering and recording traits to recognize/name/classify.
  • Common names exist but are less precise; binomial nomenclature avoids ambiguity.

Binomial Nomenclature

  • The two-name system: genus name followed by species name; genus is capitalized; species is lowercase; both italicized.
  • Abbreviation: after the genus has been stated, the genus name may be abbreviated (e.g., E. coli).
  • Pronunciation guidance is useful for recall; even experts stumble at first.
  • Examples and origins:
    • Staphylococcus aureus: "staphylococcus" = grape-like cluster, "aureus" = golden; a common human pathogen.
    • Lactobacillus san francisco: used as an example of a species name referencing a location.

Taxonomic Hierarchy

  • Ranks from most general to most specific: Domain, Kingdom, Phylum (Division in plants), Class, Order, Family, Genus, Species.
  • Some levels can be added above or below (e.g., superphylum, subclass).
  • Example: Humans vs Protozoa (eukaryotes):
    • Domain: Eukarya
    • Kingdoms: Animalia vs Protista
    • Phylum: Chordata
    • Class: Mammalia
    • Order: Primates
    • Family: Hominidae
    • Genus: Homo
    • Species: sapiens

Evolution, Phylogeny, and the Tree of Life

  • Taxonomy reflects relatedness; phylogeny represents evolutionary relationships.
  • Evolution: hereditary changes over time; natural selection favors beneficial traits.
  • Example: viral evolution during pandemics can enable new host range.
  • Reductive evolution: sometimes evolution proceeds toward lesser complexity.
  • Tree of life visualization: trunk = ancestry; branches = diversification; proximity to root indicates lesser divergence.

From Five Kingdoms to Three Domains

  • Five-kingdom system (Whittaker, 1969): Monera, Protista, Plantae, Fungi, Animalia; based on morphology and nutrition; two cell types (prokaryotic vs eukaryotic).
  • With molecular data, three-domain system proposed by Woese and colleagues: Bacteria, Archaea, Eukarya; highest level is the domain.
  • Archaea resemble bacteria in structure but are more closely related to eukaryotes on a molecular level.

Horizontal Gene Transfer and Modern Views

  • Genes can move horizontally between species, complicating simple, strictly branching trees.
  • Many scientists now use the three-domain framework but acknowledge horizontal exchanges; microbes are often discussed at genus/species level.
  • Note: interpretations of life’s history continue to evolve with new data.

Viruses, Prions, and Taxonomic Limits

  • Viruses and prions are not included in cellular taxonomy or standard evolutionary schemes; they have separate taxonomy discussed elsewhere.

Learning Outcomes (End of Chapter)

  • Differentiate among nomenclature, taxonomy, and classification.
  • Create mnemonic devices for taxonomic categories.
  • Correctly write binomial names (genus capitalized, species lowercase, both italicized).
  • Draw a diagram of the three domains.
  • Explain the difference between traditional (m morphology-based) and molecular (genetic-based) taxonomy.