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Key concept
The binomial system gives every species a two-part Latin name: the first word is the genus (capitalised) and the second is the species (lower case), both written in italics or underlined.
With millions of species on Earth, common names vary between languages and regions. The binomial system, developed by Carl Linnaeus, provides a universal naming system so scientists worldwide refer to the same organism unambiguously. Cambridge examiners expect you to use the system correctly and explain its advantages.
Rules of Binomial Nomenclature
Every species receives a unique two-part name:
Genus name: always starts with a capital letter. It groups closely related species together.
Species name: always lower case. It identifies the particular species within the genus.
Both parts must be written in italics when typed or underlined when handwritten. For example, humans are Homo sapiens and the domestic dog is Canis familiaris.
After the first mention, the genus can be abbreviated: H. Sapiens, C. Familiaris.
Why Use a Universal System?
Common names cause confusion:
A "robin" in the UK (Erithacus rubecula) is a completely different species from an American "robin" (Turdus migratorius).
Some organisms have dozens of local names: the dandelion has over 30 common names in English alone.
The binomial system solves these problems because it is:
Universal: used by scientists in every country regardless of language.
Unique: each species has one accepted name, preventing ambiguity.
Informative: the genus name immediately tells you which group of organisms the species belongs to.
Linking Binomial Names to Classification
The binomial name is the bottom two levels of the taxonomic hierarchy:
Kingdom → Phylum → Class → Order → Family → Genus → Species
Organisms in the same genus share a recent common ancestor and many structural features. For example, Panthera leo (lion) and Panthera tigris (tiger) both belong to the genus Panthera because they share features like retractable claws and a similar skull structure.
As you move up the hierarchy from species to kingdom, organisms share fewer features and their common ancestor lived further back in evolutionary time.
Try a focused question (2 marks)
A student writes the name of a species as Panthera Leo. State two errors the student has made. (2 marks)
Show the answer and marking guidance
The species name "Leo" should not have a capital letter: it should be "leo".
The name should be written in italics (if typed) or underlined (if handwritten).
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Capitalising the species name: only the genus starts with a capital letter.
Forgetting to italicise or underline the name in written answers.
Writing just the species name without the genus: "sapiens" alone is meaningless.