Phylogenetic trees and cladograms — reading and building them

Phylogenetic trees and cladograms appear on the AP Biology exam every year, both as multiple-choice stimuli and in free-response questions that ask you to read a tree, build one from data, or explain what a node means. The skill looks simple until a drawing rotates two branches or a table of traits has to be turned into a branching order. This material trains both directions: reading a tree correctly and building one from evidence.

Every tree is also described in words — which lineage branches off first, which one next, which two split last — and every data set is written into the question; where the exam would print a tree, a drawing is given too, as on the exam. You learn to read relationships from nodes rather than from how close two names sit on the page: which pair shares the most recent common ancestor, why one lineage is equally related to two sister lineages however the drawing is arranged, and why a tip on a tree is never the ancestor of the others. You identify the outgroup as the lineage that branches off first and is least closely related to the rest.

The building questions start from evidence. A character table for four vertebrates, with a vertebral column, jaws, four limbs and an amniotic egg, is turned into a nested branching order, and you explain why a trait that every member shares cannot separate them. A table of amino acid differences in one protein is turned into a cladogram by grouping the most similar pair first. You compare two candidate trees and choose the one that explains the data with the fewest evolutionary changes, and you see why wings in bats and birds are analogous traits produced by convergent evolution rather than evidence of close relationship.

Two questions address what trees are: a phylogenetic tree calibrated with fossils or a molecular clock places branch points in time, while a cladogram shows only the branching pattern; and every tree is a hypothesis that is revised when strong new evidence, such as DNA sequence data, conflicts with it.

The material offers a quiz and a flashcard deck. The quiz explains each answer in terms of nodes and shared derived characters. The flashcards cover the vocabulary the exam uses: node, most recent common ancestor, clade, outgroup, sister groups, shared derived and shared ancestral characters, molecular clock, the cladogram versus the phylogenetic tree, convergent evolution, analogous traits and the principle of choosing the tree with the fewest changes.

• Identify the most recent common ancestor of two lineages from the nodes of a tree
• Recognize that rotating branches does not change relationships
• Identify the outgroup, shared derived characters and shared ancestral characters
• Build a cladogram from a character table or from sequence differences
• Distinguish phylogenetic trees from cladograms and treat trees as revisable hypotheses

Practice material written by Zestly, based on the College Board AP Biology course framework (topic 7.9 Phylogeny, with links to 7.6 evidence of evolution and 7.10 convergent evolution).