AP® Biology · Unit 7 · Topic 7.9
Building a Cladogram from Shared Derived Characters
A cladogram is built by sorting traits into ancestral versus derived using an outgroup, then grouping species by shared derived characters, which mark each branch point on the tree.
Before you can build a cladogram, you need an outgroup: a species that split off from the rest long before any of the species you're actually studying (the ingroup) diverged from each other. The outgroup acts as a reference. Any trait the outgroup shares with some ingroup members is ancestral, meaning it was already present before the group started splitting. Any trait the outgroup lacks but some ingroup members have is derived, a newer trait that evolved after the split. Only derived traits are useful for building the tree, because only they can mark where one lineage broke off from another.
To build the tree from a character table, find traits shared by some but not all ingroup species. Each shared derived character defines a branch point, called a node: every species with that trait sits on one side of the node, descended from a common ancestor that evolved the trait; every species without it sits outside that branch. Work from traits shared by the fewest species (marking recent, narrow splits near the tips) to traits shared by more species (marking older, deeper splits near the root). A trait shared by every single species in the group is useless for branching, because it doesn't separate any subgroup from another; it just confirms the trait is ancestral for the whole group. Once the tree is built, the most recent common ancestor of any two species is the node where their two branches first join going backward.
This connects directly to speciation: each node on a cladogram represents a population that split into two lineages, the same process covered when populations diverge into separate species.
Key terms
- Outgroup
- A species outside the group being studied, used as a reference to tell ancestral traits from derived ones.
- Ingroup
- The set of species whose evolutionary relationships the cladogram is meant to show.
- Ancestral character
- A trait present before the group under study began to diverge, shown by its presence in the outgroup.
- Shared derived character
- A newly evolved trait present in some but not all members of the group, used to mark a branch point.
- Most recent common ancestor
- The node on a cladogram where the lineages leading to two chosen species last shared a single ancestor.
Watch it explained
- How it worksHow does a shared derived character mark a branch point on the tree?
- Worked exampleWalk me through turning a character table of 5 species and 4 traits into a cladogram.
- Worked exampleFind the most recent common ancestor of two species from a completed cladogram.
- Common mistakeWhy doesn't a trait shared by every species in the group help place any branch?
Check yourself
End of topic.