How to build a cladogram from a trait table
- Find the outgroup: the organism with none of the derived traits. It branches off first.
- Count how many organisms have each trait. The trait most of them share goes nearest the base.
- Work outward. Each trait marks the branch leading to exactly the organisms that have it.
- Two organisms that share a trait no one else has are each other's closest relatives. A trait only one organism has goes on its own branch.
Each table fits exactly one cladogram. Turning branches around a node does not change it, and the checker accepts any rotation.
Example: five invented animals
| Trait | Yarrel | Jorvel | Pellux | Glarrow | Ulmark |
|---|---|---|---|---|---|
| Scaly legs | lacks it | lacks it | lacks it | lacks it | has it |
| Mane | has it | lacks it | has it | has it | has it |
| Webbed toes | has it | lacks it | has it | lacks it | has it |
| Ear tufts | lacks it | lacks it | has it | lacks it | has it |
Jorvel has no derived traits, so it is the outgroup. Each trait then marks one group, from the base outward:
- Mane: all except Jorvel
- Webbed toes: Yarrel, Pellux and Ulmark
- Ear tufts: Pellux and Ulmark
- Scaly legs: Ulmark
Open this example in the practice tool
Reading a cladogram and a table of differences
Two organisms are more closely related when their lines meet at a more recent node. How near they sit on the page means nothing. The difference tables follow a steady molecular clock, so pairing the fewest differences always gives the key; real data are messier.
Middle school sets match NGSS MS-LS4-2. AP Biology sets follow Topic 7.9 of the AP Biology Course and Exam Description (fall 2025): traits gained or lost, the outgroup, nodes as common ancestors, and trees from DNA and protein data.
Questions
How do you make a cladogram from a table of traits?
Start with the outgroup, which has none of the traits. Sort the traits by how many organisms have them. The most widely shared one goes on the first branch after the outgroup, the next inside it, until the last pair splits.
What is the outgroup in a cladogram?
The organism least closely related to all the others. It lacks every derived trait in the table, so it branches off at the base.
How do you tell which organisms are most closely related?
Follow their lines back toward the base. The pair whose lines meet first, at the most recent node, shares the most recent common ancestor. Do not go by who is drawn next to whom: branches can turn around a node without changing anything.
Is a cladogram the same as a phylogenetic tree?
Both show how groups are related. The AP Biology course description says a phylogenetic tree can show the amount of change over time, from fossils or a molecular clock, while a cladogram does not show time or how much the groups differ.
How do you build a tree from amino acid or DNA differences?
Join the two species with the smallest number. Treat them as one group, using the average of their differences to each other species, and repeat. The species with the most differences to all others is the outgroup.
Can a trait be lost?
Yes. The AP course description says traits gained or lost can both be used. In AP sets a loss like “Lost eyespots” is a derived trait: the outgroup and the common ancestor still had eyespots.
Sources, checked October 24, 2026: College Board, AP Biology Course and Exam Description, effective fall 2025 (PDF, revised June 2026), Topic 7.9 Phylogeny, EK 7.9.A.2, 7.9.A.3, 7.9.B.1 and 7.9.B.2; NGSS MS-LS4-2, Biological Evolution: Unity and Diversity.