5.1 Taxonomy and Classification
Key Takeaways
- The Linnaean hierarchy runs from broad to specific in eight ranks: Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species, often remembered as "Did King Phillip Come Over For Good Soup."
- Carl Woese's three-domain system (Bacteria, Archaea, Eukarya), based on 16S/18S rRNA sequence analysis, replaced the older five-kingdom system because prokaryotes split into two fundamentally distinct lineages.
- Binomial nomenclature names every species with an italicized Genus + species pair (e.g., *Homo sapiens*); the genus is capitalized and the specific epithet is lowercase.
- Modern phylogenetics organizes life into monophyletic clades using molecular evidence (DNA, rRNA, protein sequences) rather than purely morphological similarity.
- Archaea are more closely related to Eukarya than to Bacteria, a finding from molecular evidence that overturned the older Monera kingdom.
Why Classification Matters on the Praxis
Classification gives biologists a shared vocabulary for the roughly 1.5 million described species and the millions more yet to be named. On the Praxis Biology test, expect questions that ask you to (1) order the taxonomic ranks, (2) interpret a scientific name, (3) place an organism in the correct domain or kingdom, or (4) explain why molecular data reshaped 20th-century taxonomy.
The Linnaean Hierarchy
Carl Linnaeus (1707-1778) introduced the hierarchical system biologists still use. Each rank is a set nested inside the next-larger set: every species belongs to exactly one genus, every genus to one family, and so on.
| Rank | Example (Human) | Example (Domestic Dog) |
|---|---|---|
| Domain | Eukarya | Eukarya |
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Mammalia |
| Order | Primates | Carnivora |
| Family | Hominidae | Canidae |
| Genus | Homo | Canis |
| Species | sapiens | familiaris |
Mnemonics that survive every biology classroom:
- Did King Phillip Come Over For Good Soup
- Dear King Philip Came Over For Good Spinach
The ranks move from most general (Domain) to most specific (Species). Members of the same genus share more recent ancestry than members of the same family.
Binomial Nomenclature
Linnaeus's two-name system gives every species a universal, language-independent label:
- Format: Genus species (italicized; genus capitalized; specific epithet lowercase)
- Handwritten: underline both parts when you cannot italicize
- Abbreviation: after first use, the genus may be shortened (e.g., H. sapiens)
- Authority: the original describer's surname sometimes follows the name (e.g., Homo sapiens Linnaeus, 1758)
A scientific name is unique worldwide, while common names ("robin" means different birds in Europe and North America) are ambiguous.
The Three-Domain System
In 1977, Carl Woese compared small-subunit ribosomal RNA (16S rRNA in prokaryotes, 18S rRNA in eukaryotes) sequences and showed that the kingdom Monera was not a natural group. He proposed three domains:
| Domain | Cell type | Key features |
|---|---|---|
| Bacteria | Prokaryotic | Peptidoglycan cell walls; ester-linked membrane lipids; circular DNA |
| Archaea | Prokaryotic | Pseudopeptidoglycan or protein walls; ether-linked lipids; extremophiles |
| Eukarya | Eukaryotic | Membrane-bound nucleus and organelles; linear chromosomes; histones |
Key molecular finding: Archaea share more derived features with Eukarya (similar transcription machinery, RNA polymerases with multiple subunits, histone-like proteins) than they do with Bacteria. Bacteria branched off the tree of life first.
From Five Kingdoms to Three Domains
The older five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia) collapsed because:
- Monera was polyphyletic - Bacteria and Archaea are not each other's closest relatives.
- Protista is paraphyletic - it is a grab-bag of unrelated eukaryotic lineages.
- Molecular data trump morphology when ancestry is the question.
Modern textbooks may show six kingdoms (splitting Monera into Bacteria and Archaea while keeping the rest) or shift fully to domain-based phylogeny with multiple eukaryotic supergroups (Excavata, SAR, Archaeplastida, Amoebozoa, Opisthokonta).
Cladistics and Phylogenetic Trees
Modern systematics emphasizes monophyletic clades - a clade is an ancestor plus all its descendants. Three classification styles:
- Monophyletic (clade): includes the common ancestor and every descendant - the only group type cladistics accepts.
- Paraphyletic: includes the ancestor and some descendants (e.g., "reptiles" excluding birds).
- Polyphyletic: groups based on convergent traits, excluding the common ancestor (e.g., "warm-blooded animals" treating mammals + birds as one group).
Shared derived characters (synapomorphies) - like feathers in birds or hair in mammals - define clades. Shared ancestral characters (plesiomorphies) do not.
High-Yield Praxis Pitfalls
- Italicization rules: "Escherichia coli" is correct; "Escherichia Coli" is wrong (lowercase species epithet).
- Order of ranks: confusing Class and Order is a classic distractor; remember the C of Class comes before the O of Order in the mnemonic.
- Viruses are not in any domain. They are acellular and excluded from cellular taxonomy.
- Archaea ≠ Bacteria. Even though both are prokaryotic, they are as distinct from each other as either is from you.
A student is given the scientific name Quercus alba (white oak). Which of the following correctly identifies the genus and species and follows the conventions of binomial nomenclature?
A teacher tells a class that birds are reptiles. Cladistically, which type of group does the traditional (non-cladistic) "reptiles" - which excludes birds - represent?