The big idea: Living things are sorted into a few big groups, and each group has a short list of features that gives it away.
If you can spot those tell-tale features, you can place an organism in its group — even one you have never seen before.
You need to recognise the animal phyla, the vertebrate classes and the main plant groups from their features.
- Classification
- Sorting living things into groups based on shared features.
- Phylum
- A major group within a kingdom — for animals, e.g. chordates, molluscs, arthropods.
- Class
- A group inside a phylum — for vertebrates, e.g. mammals, birds, fish, amphibians, reptiles.
- Vertebrate
- An animal with a backbone (a column of bones along its back).
- Invertebrate
- An animal without a backbone.
| Big group | An example | One give-away feature |
|---|---|---|
| Mammals | dog | fur/hair + feeds young on milk |
| Birds | robin | feathers + a beak |
| Flowering plants | rose | true roots, stems and flowers |
| Mosses (bryophytes) | moss | no true roots, no flowers |
| Molluscs | snail | soft body, often a shell |
The major groups you must recognise — vertebrate classes, key animal phyla and the main plant groups — each paired with its one tell-tale feature.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
To recognise a group, look for its identifying features — the small set of characteristics that members of that group share and other groups do not.
Start with the vertebrate classes (animals with a backbone).
| Vertebrate class | Identifying features |
|---|---|
| Mammals | fur or hair; feed young on milk (mammary glands); usually give birth to live young |
| Birds | feathers; a beak (no teeth); lay hard-shelled eggs; front limbs are wings |
| Fish | scales; gills; fins; live in water |
| Amphibians | moist smooth skin; live partly in water and partly on land; lay jelly-covered eggs in water |
| Reptiles | dry scaly skin; lay leathery-shelled eggs on land |
Next, the main plant groups. The key contrast is between flowering plants (angiosperms) and mosses (bryophytes).
Flowering plants (angiosperms)
- Have true roots, stems and leaves
- Have vascular tissue to carry water
- Reproduce using flowers and seeds
- Can grow tall (e.g. a tree)
Mosses (bryophytes)
- No true roots (only thread-like rhizoids)
- No vascular tissue — stay small and low
- No flowers or seeds — reproduce by spores
- Need a damp place to live
Finally, some common animal phyla that often appear. You assign an animal to a phylum from features such as its body symmetry, its gut and any hard parts.
- Cnidarians
- Soft jelly-like body with stinging cells and a single body opening (one gut hole) — e.g. jellyfish, sea anemones.
- Molluscs
- Soft body, often protected by a shell, usually with a muscular foot — e.g. snails, octopuses.
- Annelids
- Long bodies built from many similar ring-like segments — e.g. earthworms.
- Arthropods
- A hard external skeleton (exoskeleton) and jointed legs — e.g. insects, spiders, crabs.
- Chordates
- Have a nerve cord along the back; the vertebrates (animals with a backbone) belong here.
Groups share an ancestor — and how we read that: Organisms are not just sorted by looks — they are grouped because they share an ancestor.
When two organisms are placed in the same taxonomic rank (the same genus, or the same family or order), it means they share characteristics inherited from a common ancestor.
So if two mosses are put in the same order, they must have inherited features in common — that shared order is telling you they came from a common ancestor.
- Common ancestor
- An organism from the past that two or more present-day groups both descended from.
- Taxonomic rank
- A level in the classification system. From broad to narrow: kingdom → phylum → class → order → family → genus → species.
- Genus
- A narrow rank just above species. Organisms in the same genus are closely related and share many inherited features from a recent common ancestor.
- Cladogram
- A branching tree diagram that shows how groups are related through common ancestors.
- Node (branch point)
- A point on a cladogram where a branch splits in two. Each node represents a common ancestor of everything beyond it.
A cladogram is the branching tree we use to show these relationships. Reading one is a common exam task, so learn the two rules below.
How to read a cladogram
- Each branch point (node) is a common ancestor — the point where one line splits into two.
- Groups that meet at a more recent (closer) node are more closely related.
- To compare two groups, trace back to the node where their lines join — the nearer that shared node, the closer the relationship.
Cladograms are built from molecular evidence: Modern classifications and cladograms are not drawn from looks alone — they are built from molecular evidence.
Scientists compare a molecule that all the organisms share and count the differences:
- the amino-acid sequence of a shared protein (for example haemoglobin or myoglobin), or
- the base sequence of DNA.
The fewer the differences in the sequence, the more recently the two groups shared a common ancestor — so the more closely related they are.
IB-style question — reading a cladogram
A cladogram shows four reptile-related groups. Crocodiles and birds branch from one node. Turtles join the tree at an earlier (deeper) node, and squamates (lizards and snakes) branch off earliest of all. Which group are crocodiles most closely related to, and how does the cladogram show this? [2]
Model answer
- Remember the rule: groups that meet at a more recent (closer) node are more closely related.
- Find the node nearest to crocodiles. Crocodiles and birds share the most recent branch point (node), so that node is their common ancestor.
- Answer: crocodiles are most closely related to birds, because they share the most recent common ancestor (they branch from the closest/most recent node on the cladogram).
Final answer
Birds — crocodiles and birds branch from the most recent common node, so they share the most recent common ancestor and are the most closely related of the groups shown.
Crocodiles and birds meet at the most recent node (highlighted), so they are the closest relatives; squamates branch off earliest.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Read it like a checklist: Don't try to memorise everything about each group. Learn two or three give-away features per group and match the organism against them.
Stinging cells + one gut opening → cnidarian. Shell + soft body → mollusc. Feathers + beak → bird. Fur + milk → mammal.
Watch the wording: An identifying (or recognition) feature must be one that tells the group apart from others.
'Has eyes' is true of many animals, so it is not an identifying feature of birds. 'Has feathers' is — only birds have feathers.
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
How this is tested: Paper 1A shows you an organism (or a short feature list) and asks you to assign it to a phylum or class — a single best-answer choice.
Paper 2 asks you to outline or state identifying features of a named group (often two features for two marks), or to give a feature one plant group has that another lacks.
Mark schemes want distinctive features — name ones that actually separate the group from others.
IB-style question — identifying a mammal
A student observes an unfamiliar animal in a zoo. It is covered in fur and the keeper says the mother feeds her young on milk. Outline two features that identify this animal as a mammal. [2]
Model answer
- Pick features that are identifying for mammals — ones other classes do not share. Two clear ones here are fur/hair and feeding young on milk.
- Feature 1 — it is covered in fur (hair). Hair is a feature of mammals and is not found in birds, fish, reptiles or amphibians.
- Feature 2 — the mother feeds her young on milk (produced by mammary glands). Producing milk for the young is unique to mammals.
- Answer the command term (outline two features): this animal is a mammal because it (1) is covered in fur/hair and (2) feeds its young on milk from mammary glands.
Final answer
It is a mammal because (1) it is covered in fur/hair, and (2) the mother feeds her young on milk from mammary glands — both are identifying features of mammals.
If it were a 'draw' question: This skill is not examined as a drawing task — it asks you to name or assign groups, not to draw an organism. So the answer is always a precise feature list, like the two features above, rather than a labelled sketch.