Interpret cladograms and phylogenetic trees to determine evolutionary relationships based on shared derived characteristics and molecular evidence (TEKS Biology, Reporting Category 3; patterns; systems and system models).

What this topic is asking

The Biology TEKS ask you to interpret cladograms and phylogenetic trees to work out evolutionary relationships, using shared derived characteristics and molecular evidence. For STAAR Reporting Category 3 you need to read what the parts of a cladogram mean (branches, nodes, the order of branching) and judge which organisms are most closely related. This is a patterns and systems and system models topic, and it is one of the most common diagram-based items on the exam.

What a cladogram shows

Reading a cladogram is about following the branches, not about how close the tips happen to be drawn on the page. Two tips drawn near each other are not necessarily closely related; what matters is where their branches join.

Nodes, branches, and common ancestors

So to judge relatedness, trace each organism's branch back until the two branches meet at a node. The more recent that shared node, the closer the relationship. This is the rule behind almost every cladogram question STAAR asks.

Shared derived characteristics

Cladograms are built using shared derived characteristics: traits that arose in a common ancestor and are shared by its descendants. Each new trait marks a branch point, separating the groups that have the trait from those that do not. For example, a cladogram of animals might mark "backbone" at one node (separating vertebrates) and "four limbs" at a later node (separating tetrapods).

Increasingly, these relationships are confirmed with molecular evidence: the more similar two species' DNA or protein sequences, the more recently they shared a common ancestor, which fits where they sit on the tree. This is why the molecular data from the evidence for evolution and the groupings from taxonomy usually agree with a cladogram.

How to read a cladogram on the exam

A STAAR item often shows a cladogram and asks which organisms are most closely related, which share a particular trait, or what a node represents. The method is the same each time:

1. Identify the tips (the organisms) and the nodes (common ancestors).
2. To compare two organisms, find the node where their branches join.
3. The pair whose branches join at the most recent node (closest to the tips) is the most closely related.
4. Traits marked along the tree are shared derived characteristics; every organism above a trait mark has that trait.

Try this

Q1. State what a node on a cladogram represents. [1]

• Cue. A common ancestor shared by the groups that branch from it.

Q2. On a cladogram, species P and Q branch from a recent node, while species R branches near the base. Which two are more closely related, and why? [2]

• Cue. P and Q, because they share a more recent common ancestor (they branch from a node closer to the tips).