TL;DR:
A cladogram is a branching diagram. Each branch point stands for a common ancestor, and relatedness is read from how recently two groups share one.
Groups are defined by shared derived traits, new features that appeared in a common ancestor and were inherited by its descendants.
Do not read a cladogram left to right as a ladder of progress, and do not judge relatedness by which tips sit next to each other.
A cladogram looks like a tree turned on its side, with animal names at the tips. Test questions ask things like: is a frog more closely related to a lizard or a shark? The answer comes from the branching, not from how the animals look. Once you know how to read the branch points, these questions become routine.
The idea in plain words
A cladogram is a diagram of evolutionary relationships. The tips are the groups you are comparing. The branch points, called nodes, each stand for a common ancestor from which the branches beyond it descended. The line from the root to the tips is time, in a loose sense, but branch length usually means nothing.
A clade is a branch and everything on it: an ancestor and all of its descendants. To find a clade, you cut one branch and take what falls off.
Why traits build the tree
The branching is built from shared derived traits. A derived trait is a new feature that evolved in an ancestor and was passed to its descendants. All of the descendants have it, and animals outside the clade do not. A feature that appears in every group in the diagram, such as a backbone in an all-vertebrate tree, does not help sort anything.
Worked example: a cladogram of five animals
We compare a lamprey, a shark, a frog, a lizard and a mouse. The table lists which of four traits each animal has.
Animal | Jaws | Four limbs | Amnion | Hair |
|---|---|---|---|---|
Lamprey | no | no | no | no |
Shark | yes | no | no | no |
Frog | yes | yes | no | no |
Lizard | yes | yes | yes | no |
Mouse | yes | yes | yes | yes |
Build the tree by adding groups from the broadest trait to the narrowest.
Jaws are shared by shark, frog, lizard and mouse, but not lamprey. So the lamprey branches off first, and the other four form a clade.
Four limbs are shared by frog, lizard and mouse, but not shark. So the shark splits off next.
Amnion is shared by lizard and mouse, but not frog. So the frog splits off next.
Hair appears only in the mouse, so the mouse is on its own branch from the node it shares with the lizard.
Written in brackets, the tree is (lamprey, (shark, (frog, (lizard, mouse)))). Each pair of brackets is a node, and each node is a common ancestor.
Answering questions from it
Is the frog closer to the lizard or the shark? The frog and lizard share an ancestor at the four-limb node. The frog and shark only share the older jaw node. So the frog is more closely related to the lizard.
Which two animals are most closely related here? Lizard and mouse, which branch from the most recent node.
Which trait defines the clade of lizard and mouse? The amnion.
Where did hair evolve? On the branch leading to the mouse, after the split from the lizard.
Why the lamprey is included
The lamprey is the outgroup, a group that branched off before all the others. It lacks the new traits, so it shows you which traits are derived. Without an outgroup you cannot tell whether jaws are a new feature or an old one shared by everything. When you build a tree, pick the animal that lacks the most traits and treat it as the outgroup.
One more habit helps. Redraw the tree with the node at the four-limb step flipped so the mouse sits next to the shark. Nothing about the relationships has changed, but it now looks different. If your reading changes when you flip a node, you were reading by neighbours, not by branches.
How to check your answer
For relatedness, trace both groups back until their lines meet. The more recent that meeting point, the closer the relationship.
Every trait should be placed on one branch and be present in all descendants on it.
Count nodes: a tree with 5 tips and no branches of three or more has
5 − 1 = 4nodes. Ours has four: jaws, limbs, egg and the final split.Redraw the tree with branches flipped. If the relationships are unchanged, you read it correctly.
Common mistakes
Reading by neighbours. Tips can be spun around any node. Two tips sitting next to each other are not necessarily closest relatives. Follow the branches.
Reading it as a ladder. The mouse is not 'more evolved' than the lamprey. Both have had the same time to evolve since their common ancestor.
Thinking one living group came from another. The frog did not evolve from the shark. They share an ancestor.
Using a trait that every animal has. It cannot split the group.
Where you will use it next
Real cladograms use many traits and, in modern biology, DNA sequences. The reading skill is the same. For the process that produces new traits in the first place, see how natural selection works.
Frequently asked questions
Does the length of a branch matter?
In most school cladograms it does not. Only the order of the branching counts, unless the diagram says its branch lengths show time or amount of change.
Is a cladogram the same as an evolutionary tree?
Close. A cladogram shows branching order. Other trees may also show time or amount of change.
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