Biology
Taxonomy
Classifying life — from kingdoms to species
Taxonomy is the science of naming, defining, and classifying organisms. Linnaean system (1735) established hierarchical classification: domain, kingdom, phylum, class, order, family, genus, species. Modern: includes phylogenetic relationships (cladistics, molecular phylogeny). Three domains (Woese 1990): Bacteria, Archaea, Eukarya. Six kingdoms in school: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, Animalia. Modern alternative: cladistic groups based on evolutionary relationships. Binomial nomenclature: Genus species (Italics; e.g., Homo sapiens). ~1.5-2 million species described; estimated 8.7 million on Earth.
- Founded byCarolus Linnaeus, 1735
- HierarchyDomain, Kingdom, Phylum, Class, Order, Family, Genus, Species
- DomainsBacteria, Archaea, Eukarya (Woese 1990)
- Binomial nomenclatureGenus + species (e.g., Homo sapiens)
- Species described~1.5-2 million
- Estimated total~8.7 million on Earth
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Why taxonomy matters
- Communication. Universal naming.
- Conservation. Protecting endangered species.
- Biodiversity. Cataloging life.
- Evolution. Phylogenetic relationships.
- Drug discovery. Finding related organisms with useful compounds.
- Agriculture. Crop identification, breeding.
- Education. Foundation of biology.
Common misconceptions
- Taxonomy stable. Frequent reclassification.
- Species clearly defined. Multiple definitions.
- All life classified. Most species unknown.
- Taxonomy = phylogeny. Different but related.
- Older Linnaean enough. Cladistics modern approach.
- Common names unique. Many synonyms; binomial avoids.
Frequently asked questions
What's the Linnaean hierarchy?
Hierarchical classification. Domain (Bacteria, Archaea, Eukarya) → Kingdom → Phylum → Class → Order → Family → Genus → Species. Each level: more inclusive higher; more specific lower. Example for humans: Eukarya → Animalia → Chordata → Mammalia → Primates → Hominidae → Homo → sapiens. Mnemonic: "Dear King Philip Came Over For Good Soup."
What's binomial nomenclature?
Two-part scientific name. Genus + species. Italicized in print. Genus capitalized; species lowercase. Examples: Homo sapiens (humans), Canis lupus (gray wolf), Felis catus (domestic cat). Universal across languages — same name in any country. Avoids confusion with common names (which vary). Established by Linnaeus; still used.
How are species defined?
Multiple definitions. (1) Biological species concept (Mayr): can interbreed and produce fertile offspring. Doesn't work for asexual organisms or extinct ones. (2) Morphological: defined by shared physical traits. Old approach. (3) Phylogenetic: defined by shared evolutionary history. (4) Ecological: defined by ecological niche. Different definitions for different purposes.
What are the three domains?
Carl Woese (1990) proposed three-domain system based on rRNA sequences. (1) Bacteria — typical bacteria (E. coli, etc.). (2) Archaea — distinct from bacteria; many in extreme environments (thermophiles, halophiles); also gut microbes. (3) Eukarya — eukaryotes (plants, animals, fungi, protists). Replaced two-domain (prokaryote/eukaryote) view.
What's cladistics?
Phylogenetic classification based on shared derived characteristics (synapomorphies). Groups together organisms by common ancestry. Result: clades — branches of evolutionary tree. Aim: only natural groups (monophyletic — all descendants of common ancestor). Avoids: paraphyletic groups (excluding some descendants; e.g., "reptiles" excluding birds), polyphyletic (multiple unrelated groups). Modern approach.
How are phylogenies constructed?
Multiple methods. (1) Morphological: traditional; structural traits. (2) Molecular: DNA, RNA, protein sequences. Most accurate now. (3) Combining: morphology + molecules. Methods: maximum parsimony (minimize evolutionary changes), maximum likelihood (statistical), Bayesian inference. Software (RAxML, MrBayes) used. Provides: branching structure of evolutionary tree.
How many species are there?
~1.5-2 million described to date. Recent estimate: ~8.7 million on Earth (most undescribed). Most undescribed: insects, microbes, deep-sea organisms. Rate of discovery: ~15,000 new species per year. Major taxonomic groups: arthropods (~1.2 million described; estimated 5+ million); plants (~390,000 species), bacteria (~10,000 described; estimated millions). Vast biodiversity unknown.