The big idea: Common names are messy — a 'robin' in Britain is a different bird from a 'robin' in North America.
So biologists give every species one agreed scientific name made of two words. This is called binomial nomenclature ('binomial' = two names).
Example: the domestic cat is Felis catus everywhere in the world, in every language.
| Part of the name | What it is | How it is written |
|---|---|---|
| First word | the genus (a group of closely related species) | Capital first letter, italic |
| Second word | the species (the specific kind within that genus) | lower-case, italic |
| Both together | the full binomial name of one species | italic (or underlined if hand-written) |
The two-part name comes from the bottom of the classification ladder: the genus and species rungs together make the binomial (e.g. Panthera leo).
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Read the name like this: In Felis catus, the first word (Felis) is the genus and the second word (catus) is the species.
The genus is the bigger group; the species is the exact kind inside it.
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The system was created by Carl Linnaeus in the 1700s. He grouped organisms that looked alike into the same genus — this is the morphological species concept (judging a species by its physical features / appearance).
The two-part name tells you two things at once: the species itself, and the genus (the group of close relatives) it belongs to.
- Binomial nomenclature
- The system of naming each species with a two-part Latin name: genus + species.
- Genus
- A group of closely related species — it is the FIRST word of the name.
- Species
- One particular kind of organism — named by the SECOND word, which only has meaning alongside the genus.
- Morphological species concept
- Linnaeus's idea of grouping organisms into species by their shared physical features (their form/appearance).
How to WRITE a binomial name
- Genus capitalised: Panthera
- Species lower-case: leo
- Whole name in italics: Panthera leo
- If hand-written, underline each word instead
Common mistakes to avoid
- Do not capitalise the species word (not Panthera Leo)
- Do not leave it un-italicised in print
- Do not use the species word on its own — it needs its genus
- Do not swap the order — genus always comes first
Why the genus matters most: Two species that share the same first word (same genus) are more closely related than two species in different genera.
So Panthera leo (lion) and Panthera tigris (tiger) are close relatives — both Panthera — while Felis catus (cat) sits in a different genus.
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How this is tested: On Paper 1A (multiple choice) a common question gives you a list of binomial names and asks which two species are most closely related — the answer is the pair that share the same genus (same first word).
On Paper 2 you may be asked to state the genus an organism belongs to from its binomial name, or to outline Linnaeus's morphological species concept.
IB-style question — which species are most closely related?
A student records four beetle species: Carabus auratus, Carabus violaceus, Harpalus affinis and Pterostichus niger. State which two species are the most closely related and explain your answer. [2]
How to score both marks
- Read the first word of each name (the genus). The genera here are Carabus, Carabus, Harpalus and Pterostichus.
- Find the pair that shares a genus. Only Carabus auratus and Carabus violaceus have the same first word (Carabus).
- Answer the command term. The most closely related pair is Carabus auratus and Carabus violaceus, because sharing the same genus means they are more closely related than species placed in different genera.
Final answer
Carabus auratus and Carabus violaceus — they share the same genus (Carabus), so they are more closely related than the species in other genera.
The reasoning that scores: Mark 1: identifying the pair that shares a genus (Carabus auratus + Carabus violaceus).
Mark 2: explaining that a shared genus means more closely related than species in different genera. Don't be distracted by similar-looking species words.