The binomial system — Genus species
Every organism has one internationally agreed two-part Latin name. The genus comes first with a capital letter; the species epithet comes second in lower case; both are italicised, or underlined in handwriting — Homo sapiens, Panthera leo, Aquila chrysaetos. Latin is used because it is neutral, stable and understood worldwide, so where common names differ between languages the binomial still names exactly one species.
Dichotomous keys split in two, every step
A dichotomous key identifies an unknown organism through paired contrasting statements — "dichotomous" means divided into two, so each step offers exactly two choices. To use one: start at step 1, pick the statement that fits, follow the instruction, repeat until a name appears. To build one: split on the most obvious observable feature (wings / no wings), then subdivide until every organism ends alone on its own path.
Species = interbreed and fertile offspring
Two elements, both required for the marks: organisms that can interbreed, and that produce fertile offspring — offspring able to reproduce in turn. A horse (Equus caballus) and a donkey (Equus asinus) do interbreed, but the mule they produce is sterile, so they are different species sharing one genus. "Look alike", "can reproduce" and "same genus" all score zero.
Drawn from real examiner reports.
"Organisms that look the same" is not a species
Appearance is not the criterion. A Chihuahua and a Great Dane look nothing alike yet are one species; two beetles can look identical and still be two. "Organisms that can reproduce" is too vague — nearly everything reproduces — and "organisms in the same genus" is wrong because one genus holds many species. All three versions score zero.
Flagged s23 P12 Q2 · w22 P13 Q2 · s22 P13 Q2
"Can interbreed" alone is half the marks
Stopping at "organisms that can interbreed" earns 1 of the 2 marks and is the commonest loss on this definition. It is the fertility of the offspring that draws the species line: horse and donkey interbreed, but the mule cannot breed, so the parents are different species. Say both halves every time — interbreed and fertile offspring.
Flagged s23 P12 Q2 · w22 P13 Q2 · s22 P13 Q2
Genus asked — don't write the binomial
If the question says "state the genus of Aquila chrysaetos", the answer is Aquila, one word. Handing back the whole binomial is marked wrong, not treated as generous. The reverse also loses marks: asked for the binomial, both words are needed. Find the level word in the question and underline it before you write.
Flagged s23 P11 Q2 · w22 P31 Q1b · s22 P31 Q2
The second word is never the genus
Asked for the genus of Homo sapiens, candidates write sapiens. The genus is Homo — it comes first and carries the capital letter. Memory aid: Genus gets the Great capital and goes first, species stays small and second. The genus is also the part close relatives share — Panthera leo, Panthera tigris and Panthera pardus are all Panthera.
Flagged s23 P11 Q2 · w22 P31 Q1b · s22 P31 Q2
Answering at the wrong level of the hierarchy
Asked for a kingdom, candidates give Homo (a genus) or "mammal" (a class); asked for the phylum of an insect they write "Insecta" (a class) rather than Arthropoda. "Vertebrates" is not a kingdom at all. Learn the ladder — Kingdom → Phylum → Class → Order → Family → Genus → Species — and the five IGCSE kingdoms: Animalia, Plantae, Fungi, Protoctista, Prokaryotae.
Flagged s22 P41 Q1a · w22 P31 Q1
A key with three options is not dichotomous
Two faults cost marks when constructing keys. Offering three or four options at a step breaks the definition — every step must be a clean split into two. And subjective features ("large", "dark") are not credited, because a reader cannot apply them consistently; use countable or present/absent features such as number of legs, wings, scales or a shell.
(Extended) Similar means closer, not distant
The rule runs one way only: the more similar two species' DNA base sequences or amino-acid sequences, the more closely related they are, and the more recently they shared a common ancestor. Reversing it loses every mark. Molecular data is also preferred to appearance because convergent evolution can make unrelated species look alike.
(Extended) Direction first, then the ancestor
Two sentences earn the marks: "the greater the similarity in DNA or amino-acid sequence, the more closely related the species", then "so they diverged from a common ancestor more recently". Compare the same protein across species — cytochrome c or haemoglobin.
Spot the level before you write
Genus only → first word, capital (Panthera). Species epithet only → second word, lower case (leo). Full binomial → both words, italicised or underlined (Panthera leo). Decide which of the three is being asked for before your pen moves.
Name the tool exactly: "dichotomous key"
Examiners flagged answers of "genus", "binomial key", "branching diagram" and "identification chart". Only dichotomous key earns the mark, so learn it as a fixed phrase rather than trying to describe the thing.
Walk the key, don't jump to the answer
Read both statements at a step before choosing, jot down which branch you took, and only then move on — most errors come from matching a feature you expected rather than the one described. At the end, check the named organism back against every feature you were given.
Species: a group of organisms that can interbreed with each other to produce fertile offspring. Both elements are essential for full marks — interbreeding alone (without the word "fertile") scores only partial credit.
Classic example: a horse and a donkey can mate but their offspring (the mule) is sterile. Therefore horse and donkey are different species despite their similar body plan.
Binomial system: the internationally agreed method of naming organisms using a two-part Latin name (Genus species).
| Part | Rule | Example |
|---|---|---|
| Genus | First word; capital first letter; italicised or underlined | Homo |
| species | Second word; lower case; italicised or underlined | sapiens |
Latin is used because it is neutral and internationally recognised, so scientists worldwide use identical names for the same organism.
Dichotomous key: an identification tool built from paired, contrasting statements that divide organisms into two groups at each step. The word "dichotomous" means "divided into two." Always name it as a dichotomous key in exam answers.
Classification hierarchy (broadest → most specific):
Memory phrase: King Paul Came Over For Good Soup.
The five kingdoms recognised at IGCSE: Animalia, Plantae, Fungi, Protoctista, Prokaryotae (bacteria).
Define the term species.
1. State the definition of a species. [2]
