Classification and dichotomous keys
The binomial system, the five kingdoms, and identifying organisms with a key.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.2.1Describe the binomial system of naming species and the classification hierarchy.
- 1.2.2Construct and use dichotomous keys based on observable features.
5 minute read
Classification and dichotomous keys
There are millions of species. Classification is the filing system that stops that being useless.
The binomial system
Every species gets a two-part Latin name: the genus first, with a capital letter, then the species, in lower case. Humans are Homo sapiens. The name is written in italics, or underlined when handwritten. This matters because common names are ambiguous — a "robin" is a different bird in Britain and America — while a binomial name is unique and understood worldwide.
The hierarchy
Organisms are grouped in a nested sequence: kingdom, phylum, class, order, family, genus, species. Each level down is a smaller, more closely related group. Species are grouped together because they share features, and modern classification takes this further: the more similar the DNA base sequences of two species, the more closely related they are, and the more recently they shared a common ancestor.
Dichotomous keys
A dichotomous key identifies an unknown organism through a series of paired, contrasting statements — "di" meaning two. At each step you choose the option matching your specimen, which sends you to another pair or gives you a name. A good key uses only visible, unambiguous features: "has six legs" works; "is large" does not, because large compared with what?
Think of it like this
A dichotomous key is a game of twenty questions where every question has exactly two answers, and each answer halves the number of possibilities still in play.
Worked examples
Method, step by step
Two species share the same genus name. What does this tell you about how closely related they are, compared with two species in different genera?
- 1The genus is the level immediately above species in the hierarchy.
- 2Sharing a genus means they were grouped at a lower, more specific level.
- 3The lower the shared level, the more features and DNA they share.
They are more closely related — they share more features and a more recent common ancestor than two species that only share a higher grouping such as family or order.
Common misconceptions
- Writing the species name with a capital letter — only the genus is capitalised, so it is *Homo sapiens*, never *Homo Sapiens*.
- Thinking classification is based on outward appearance alone. Similar-looking species can be unrelated; DNA sequence comparison is the more reliable modern evidence.
- Using vague features in a key, such as "big" or "pretty" — a key only works if every choice is unambiguous to a stranger.
In the exam
- If asked why the binomial system is used internationally, the mark is for it giving each species one unique name that avoids the confusion of local common names.
- When constructing a key, use paired opposite statements and only features that can actually be seen in the picture or specimen provided.