The big idea: A cladogram is a branching tree-diagram that shows the most probable evolutionary relationships between groups of organisms. Its whole job is to show who is related to whom, and how recently they shared an ancestor.
Read it like a family tree of life
- Each branch is a lineage.
- Each branch point (a node) is a common ancestor that split into two.
- The further left you go (towards the root), the further back in time you are.
A cladogram of four invented reptile/amphibian groups. Each branch point (node) is a common ancestor; the node nearest the tips is the most recent, so the two groups sharing it are the most closely related.
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- Clade
- A group that contains an ancestor and ALL of its descendants — one whole 'twig' of the tree of life.
- Cladogram
- A branching tree-diagram showing the most probable evolutionary relationships among clades, based on the best current evidence.
- Node
- A branch point on a cladogram. It represents a common ancestor that diverged (split) into two separate lineages.
- Divergence
- The splitting of one ancestral lineage into two — this is what each node marks.
- Root
- The deepest (oldest) node, on the far left — the common ancestor of every group on the cladogram.
Node = ancestor, not a living thing: A node is not one of the organisms you are comparing — it is the point in the past where two of them last shared an ancestor.
The living/named groups sit at the tips (the ends of the branches). The ancestors sit at the nodes inside the tree.
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Reading a cladogram comes down to one rule: the more recently two groups share a common ancestor, the more closely related they are.
'How recently' is shown by the position of the node where their branches meet. A node near the tips is recent; a node near the root is ancient.
Step-by-step: find a group's closest relative
- Start at the tip of the group you care about.
- Trace its branch back towards the root until you reach the first node it shares with another branch.
- Whatever joins it at that nearest shared node is its closest relative — they share the most recent common ancestor.
- To compare two groups, find the node where their branches finally meet: a recent meeting = closely related; a deep (near-root) meeting = distantly related.
- The root (far-left, deepest node) is the oldest ancestor — the one shared by everything on the tree.
To find a group's closest relative, trace its branch back to the nearest node it shares with another group. The deepest node on the left is the ROOT — the oldest common ancestor of every group on the tree.
Interactive diagram
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Order of branching = order of splitting: The order in which branches split tells you the order of divergence.
Branches off near the root (early)
- Sits on its own long branch and split away first.
- It is the most distantly related to the rest.
- In the diagram: the river newt branches off earliest, so it is the outsider of the four.
Branches off near the tips (late)
- These groups split most recently.
- They are the most closely related.
- In the diagram: the two skinks branch last, so they are the closest pair.
| More closely related | More distantly related | |
|---|---|---|
| Where their branches meet | At a RECENT node (near the tips) | At a DEEP node (near the root) |
| Most recent common ancestor | Younger / more recent | Older / further back in time |
| Position on the tree | Branches that join up soon | Branches that join only near the root |
| Example (diagram above) | Spotted skink + banded skink | River newt vs the skinks |
A worked reading: Take the cladogram above (spotted skink, banded skink, garden gecko, river newt) and answer three questions.
Three questions, three nodes
Closest relatives?
The two skinks: their branches meet at the most recent node.
Is the gecko closer to the skinks or to the newt?
To the skinks. The gecko joins the skinks at a node that is more recent than the deep node it shares with the newt.
Most distantly related of all?
The river newt: it only meets the others at the root, the oldest node.
Recent node = close relatives; the root = the oldest link.
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How this is tested: A very common HL task gives you a cladogram and asks you to deduce relationships from it.
What you may be asked
- Which two groups are most closely related.
- Which group is the most distantly related.
- Which two share the most recent common ancestor.
- Explain why a cladogram can change: it is a hypothesis based on current evidence.
Justify with the nodes: Always justify your answer by referring to the nodes. Name the node where the branches meet and say whether it is recent (near the tips) or deep (near the root). Stating the answer without the node reasoning usually loses the explanation mark.
Exam-style question — deduce relationships from a cladogram
Using the cladogram, (a) identify the two groups that are most closely related, and (b) deduce which group is the most distantly related to the others. Justify each answer. [4]
How to score all four marks
- (a) Identify the closest pair. The spotted skink and banded skink are most closely related.
- (a) Justify. Their branches meet at the most recent node (nearest the tips), so they share the most recent common ancestor.
- (b) Identify the outsider. The river newt is the most distantly related to the other three.
- (b) Justify. Its branch joins the others only at the root — the deepest (oldest) node — so it shares only a very ancient common ancestor with them and diverged first. (Award 1 mark per correct identification and 1 per valid node-based justification, max 4.)
Final answer
(a) The two skinks are most closely related — their branches meet at the most recent node, so they share the most recent common ancestor. (b) The river newt is most distantly related — it joins the others only at the root (the oldest node), having branched off first.
Why this scores full marks: Every claim is tied to a node: the closest pair share the most recent node, and the outsider only meets the others at the root.
A common way to lose marks: Giving the right groups but no node reasoning loses marks: 'they look closest' is not an explanation. Always say which node their branches meet at and whether it is recent or deep.
A cladogram is a hypothesis: A cladogram is not a fixed fact. It is the best current hypothesis of how the groups are related, drawn from the evidence available.
Why a cladogram can change
- Early trees were built mainly from physical features (morphology).
- Today most are built from DNA and protein sequence data, which is far more reliable.
- If new evidence appears (e.g. better sequence data), a cladogram can be revised: branches may be rearranged.
- Being testable and revisable is exactly what makes it good science.
A cladogram shows four species of cat: the cheetah branches off first, then the puma, and the lion and leopard meet at the most recent node. Identify the two most closely related species, giving a reason.
Model answer plan
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When a new species is added to a cladogram:
A newly sequenced bacterium, X, is placed on a cladogram in the same clade as species P, while species Q is in a different clade. Identify whether X is more closely related to P or to Q, giving a reason.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Parsimony in one line: Asked how two cladograms are chosen between? Parsimony: pick the one that needs the fewest changes.