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What is a cladogram?

A cladogram is a branching diagram that shows how a group of organisms are related by common ancestry. It answers one question: in what order did these lineages split? Here is a plain definition, the parts of a cladogram, and how to read one.

A plain definition

A cladogram is a diagram that shows how a set of organisms are related by descent from shared ancestors. It is drawn as a series of lines that split at points called nodes. Each split says that the two groups beyond it share an ancestor which nothing else on the diagram shares, and the whole picture is a hypothesis about the order in which the lineages divided.

The name comes from clade, meaning a branch of the tree of life. The cladogram is the drawing; the relationships it proposes are what biologists actually care about.

One thing a cladogram leaves out on purpose is any sense of scale. It shows branching order and nothing more. It does not show how long ago a split happened, and it does not show how much a lineage has changed since. That is the single most important idea to keep in mind while reading the rest of this page.

The parts of a cladogram

Five words cover almost everything you will meet:

Tips (also called leaves or terminals) are the organisms being compared, at the ends of the branches.
Nodes are the branch points. Each one stands for the most recent common ancestor of everything beyond it.
Branches are the lines that join ancestors to their descendants.
Root is the base of the whole diagram, the common ancestor of every organism shown.
Clade is a node plus every descendant of that node. Snip a branch off cleanly and what falls away is a clade.

The parts of a cladogram. The outgroup, in blue, is a taxon known to branch off first; it roots the tree and gives it direction. A clade is any node together with everything descended from it.

One more term is worth knowing straight away: the outgroup. It is a taxon you already know branched off before all the others, added on purpose so the diagram has a clear starting point. Without an outgroup you can still see which organisms cluster together, but you cannot tell which end of the diagram is the beginning.

How to read one

Two organisms are more closely related when they share a more recent common ancestor. To compare any two tips, trace back along the branches from each until the two paths meet. The pair whose paths join at the nearer node are the closer relatives. It is a mechanical procedure, and it always works.

The mistake nearly everyone makes is reading the tip order across the page as though it showed relatedness. It does not. Every node is a free hinge, so the branches can be rotated without changing a single relationship.

Both pictures state exactly the same relationships: A and B are each other's closest relatives, and C sits outside that pair. Rotating the branches at a node changes how the tree looks, not what it means.

Because of that hinge, one cladogram can be drawn in many different-looking ways. If you can turn one drawing into another purely by rotating branches at their nodes, they are the same tree. The longer guide on how to read a cladogram works through this in more detail.

What the branches do and do not tell you

In a plain cladogram, branch length carries no meaning. The lines are stretched or squeezed only so the diagram fits and stays legible, and the tips are usually lined up in a neat column. A long branch is not more time and not more change.

Diagrams that do use branch length have other names. A phylogram scales branches to the amount of evolutionary change, and a chronogram scales them to time. If a tree has a scale bar or a time axis, the lengths mean something; if the tips are flush and there is no scale bar, they do not. That is the heart of the cladogram versus phylogenetic tree distinction.

How a cladogram is built

Cladograms are built from shared derived characters: features that appeared in an ancestor and were passed down to its descendants. You score each taxon for a set of characters, present or absent, group taxa by the characters they share, and let the nested groups become nested clades. The character shared by the most taxa marks the earliest split, and the one shared by the fewest marks the most recent.

The full walkthrough, including how to choose an outgroup and how to settle conflicts with parsimony, is in how to make a cladogram. You can also build one here in the browser by typing a character table and letting the tool draw the tree for you.

Build yours in the browser, no signup, no install.

Open the cladogram maker

Frequently asked questions

What is a cladogram in simple terms?

It is a branching diagram that shows how organisms are related by descent. Lines split at nodes, each node is a shared ancestor, and the pattern of splits shows the order in which the groups diverged. It shows branching order only, not time and not amount of change.

What is the difference between a cladogram and a phylogenetic tree?

A cladogram shows branching order only, with branch lengths that mean nothing. A phylogenetic tree is the general term and can also use branch lengths to show evolutionary change or time. Every cladogram is a phylogenetic tree, but not every tree is a cladogram.

What are the main parts of a cladogram?

Tips (the organisms at the branch ends), nodes (the branch points, each a common ancestor), branches (lines of descent), the root (the base, the ancestor of everything shown), and clades (a node plus all of its descendants). An outgroup is often added to root the tree.

What does a node on a cladogram represent?

A node is the most recent common ancestor of everything that branches off beyond it. The characters marked at a node are the traits inferred to have been present in that ancestor.

Why does a cladogram need an outgroup?

The outgroup is a taxon known to have branched off before the rest. It roots the diagram and shows which character states are ancestral and which are derived. Without one you can see which taxa cluster together but not which group split off first.