There are millions of species and no ecologist can remember them all. Classification is how biologists file them into groups, and a dichotomous key is the tool you use in the field to work out what you are actually holding. Both turn up in Paper 1, and the key questions are free marks if you follow the numbers properly.
📚 What you need to know
Classification means sorting species into groups based on shared features.
Each species gets a binomial name: genus first with a capital letter, then species with a small letter, both in italics.
Taxonomists identify organisms using reference collections, DNA surveys and dichotomous keys.
A dichotomous key is a series of paired statements, each with two choices.
Keys are quick but limited: they only cover the species they were built for, and they rely on visible features.
Why bother classifying?
Grouping species together does two useful jobs. First, it lets scientists anywhere in the world talk about the same organism without arguing over local names. A “buzzard” means different birds in different countries; Buteo buteo means one bird everywhere.
Second, it lets you predict. If you find an unknown animal and can place it in a group, you already know a lot about it — how it breathes, how it reproduces, roughly what it eats — before you have studied it at all.
The hierarchy of groups
The groups fit inside each other like boxes inside boxes. A big group near the top holds a huge number of organisms that only share a few basic features. A small group near the bottom holds few organisms, but they are very alike.
Only the last two levels, genus and species, are used in an organism’s scientific name.
Writing a binomial name
Every species has a two-part name made from its genus and its species. There are three rules and examiners do check them:
🧩 How to write a scientific name
Genus first, with a capital letter. For the grey wolf that is Canis.
Species second, all lower case. That gives Canis lupus.
Italics, or underline if you are handwriting. In an exam you cannot italicise, so underline both words.
A genus usually holds several closely related species. Canis also contains the coyote and the golden jackal, which is why they look and behave so much alike. Our own genus, Homo, once held several species but now holds only Homo sapiens.
A quick memory hook: the genus is the surname and the species is the first name — except biologists write the surname first, like a school register.
Three ways taxonomists identify an organism
Method
How it works
Strength and weakness
Reference collections
The unknown specimen is compared with named, preserved specimens in a museum or herbarium, or with photographs in an app.
Fast and cheap, but only as good as the collection and the expert using it.
DNA surveys
The DNA sequence of the specimen is compared with sequences already stored in large databases.
Very precise, and it separates species that look identical — but it needs a lab and money.
Dichotomous keys
The user answers a series of paired questions about visible features until a name is reached.
Usable in the field with no equipment, but limited to the species it was written for.
How a dichotomous key works
“Dichotomous” just means splitting into two. At each numbered step you are given two statements and only one of them can be true for your organism. That choice either sends you to another number or gives you a name.
Written keys use numbers instead of branches, but the shape of the thinking is exactly the same.
Worked examples
WORKED EXAMPLE
Using a written key
Use the key below to identify an animal found in a stream that has a soft body, no shell, and eight legs.
Step
Statement
Go to
1a
Body covered by a hard shell
freshwater mussel
1b
Body not covered by a hard shell
go to 2
2a
Six legs
go to 3
2b
More than six legs
water mite
3a
Wings folded over the back
water boatman
3b
No wings
mayfly nymph
Step 1: start at 1 and check the shellno shell, so 1b is true1b → go to 2Step 2: count the legseight legs is more than six, so 2b is true2b → name givenwater mitenever skip a step — always start at 1 and follow the route
WORKED EXAMPLE
Spotting an error in a name
A student writes the scientific name of the domestic cat as “felis Catus”. Identify two mistakes and write the name correctly. [3]
Mistake 1: the genusthe genus must start with a capital letter, so “felis” is wrongMistake 2: the speciesthe species must be lower case, so “Catus” is wrongCorrect versionFelis catus (italicised or underlined)the capital letters are the marks — a third mark often comes from the italics
Where keys let you down
Keys are useful, but you must be able to criticise them:
Limited scope. A pond key will not identify a beetle from a hedge. If your organism is not in the key, the key will still give you an answer — a wrong one.
Variation within a species. Young, damaged or unusually coloured individuals may not match the statements.
Only visible features. Behaviour, habitat and DNA are ignored, yet these often separate very similar species.
Slow and fiddly. A beginner may take a long time and still need an expert to check the answer.
Link this to fieldwork. If a question asks how you would improve the reliability of a species survey, “check identifications against a reference collection or a second observer” is a strong, specific answer.
💡 Exam tip
Learn the eight levels in order. A mnemonic is faster than trying to reason it out under time pressure.
In key questions, write down the route you took (1b, 2b) — method marks are often available even if the final name is wrong.
Read both statements at a step before choosing. They are usually opposites, so one must be false.
When asked for a limitation of keys, give a specific one, not just “it can be wrong”.
DNA surveys are the answer whenever a question mentions species that look alike.
Underline both parts of a binomial name in handwritten answers.
⚠ Common mix-up
Capitalising the species name. It is always lower case, even when it comes from a person’s or place’s name.
Starting a key in the middle. Always begin at step 1, however obvious the answer looks.
Thinking a genus holds only one species. Most genera hold several closely related species.
Confusing classification with identification. Classifying is filing species into groups; identifying is working out which species you have.
Saying keys are unreliable because they are old. The real limitations are scope, variation and reliance on visible features.
Writing the whole name in capitals. That breaks the naming rules and can cost the mark.
Up next: What Controls Population Size — now that you can name the species, the next question is what decides how many of them there are.
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