The big idea: The traditional system places every species in a fixed ladder of ranks, each rank inside the one above.
The points to remember
- Above the kingdoms sits the domain.
- Then, from widest to narrowest: kingdom, phylum, class, order, family, genus, species.
- Each group at one rank is a taxon (plural taxa), such as the family Canidae.
- Every taxon holds one or more taxa of the rank below.
- The genus and species together make the binomial name: Canis lupus.
Remember it as: King Philip Came Over For Good Soup: kingdom to species.
Example: the wolf and the dog: The domestic dog and the grey wolf share every rank down to the genus Canis: so close that the dog is often treated as a subspecies of the wolf.
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The big idea: Evolution produces a branching tree with endless gradations. A fixed ladder of ranks cannot always follow it.
The points to remember
- Ranks can cut a clade in two: class Aves (birds) is set apart from class Reptilia, yet birds evolved from reptiles.
- Ranks are arbitrary: where an order ends and a family begins is a judgement, not a fact.
- Taxa of one rank are not equal: a family of beetles may be far older than a family of mammals.
- Some organisms fit no rank neatly: Euglena has chloroplasts like a plant but swims and eats like an animal.
- Cladistics groups organisms into unranked clades instead: a change of approach that is a paradigm shift.
Real example: crocodiles and birds: Crocodiles share a more recent common ancestor with birds than with lizards, yet the traditional ranks put crocodiles and lizards in one class and birds in another.
In an answer:
Name the mismatch: Do not just say the hierarchy is 'old'. Say what it gets wrong: its ranks do not always match the pattern of divergence in evolution.
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How it is tested: Paper 2 shows an organism's classification as a table with gaps: name the domain and the missing rank. Learn the ranks in order.
IB-style questionIdentify[2 marks]
The garden snail is classified as: Domain ......; Kingdom Animalia; Phylum Mollusca; Class Gastropoda; ...... Stylommatophora; Family Helicidae; Genus Cornu; Species aspersum. Identify the domain and the missing taxon.
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Ranks, not names: 'Missing taxon' asks for the rank (order, family), not the group's own name.