How do phylogenetic trees represent evolutionary relationships?
Topic 7.9 Phylogeny: interpret and construct phylogenetic trees and cladograms from shared characters and molecular data.
A focused answer to AP Biology Topic 7.9, covering phylogenetic trees and cladograms, shared derived characters, nodes and common ancestors, out-groups, and reading relatedness from a tree, with a worked tree interpretation.
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What this topic is asking
The College Board (Topic 7.9) wants you to interpret and construct phylogenetic trees and cladograms from shared derived characters and molecular data, and to read evolutionary relationships and common ancestors from a tree.
Reading a tree
Building a tree from shared characters
Cladograms versus phylogenetic trees
Trees are hypotheses that can be tested and revised as new data arrive. Modern trees rely heavily on molecular data because DNA and protein sequences provide many independent characters and can compare even very different organisms. When molecular and anatomical evidence agree, the tree is well supported; when they conflict, it signals that a trait may be analogous (from convergent evolution) rather than inherited from a common ancestor.
Try this
Q1. State what a node on a phylogenetic tree represents. [1 point]
- Cue. A common ancestor of the lineages that branch from it.
Q2. Explain how shared derived characters are used to build a cladogram. [2 points]
- Cue. A character present in a group and its common ancestor (but not earlier ancestors) groups those species on the same branch, so species sharing a derived character are placed together.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt, diagram). A cladogram shows four species (W, X, Y, Z) and the shared characters that define each branch. (a) Identify which two species are most closely related and justify your answer using the tree. (b) Explain what a node on the tree represents and how shared derived characters are used to build the tree.Show worked answer →
A 4-point interpret-and-explain FRQ on phylogeny.
(a) Identify and justify (2 points): (1 point) the two species that share the most recent common ancestor (the node closest to their branch tips) are most closely related; (1 point) because fewer branch points separate them, meaning they diverged most recently.
(b) Explain (2 points): (1 point) a node represents a common ancestor from which the branches above it descend; (1 point) shared derived characters (features present in a group and its common ancestor but not in earlier ancestors) group species together, so species sharing a derived character are placed on the same branch.
Markers reward using the most recent common ancestor to judge relatedness and explaining nodes and shared derived characters.
AP 2017 (style)1 marksSection I (multiple choice). On a phylogenetic tree, a node (branch point) represents: (A) a living species today. (B) a most recent common ancestor of the lineages that branch from it. (C) an extinction event. (D) a mutation rate.Show worked answer →
The correct answer is (B).
A node represents the most recent common ancestor of all the lineages that branch off above it. The tips (not nodes) represent species; nodes are ancestral populations, not extinctions (C) or rates (D).
Related dot points
- Topic 7.6 Evidence of Evolution: describe the lines of evidence (fossil, anatomical, molecular, biogeographical) that support evolution.
A focused answer to AP Biology Topic 7.6, covering fossil, anatomical (homologous and vestigial structures), embryological, molecular and biogeographical evidence for evolution, with a worked interpretation of molecular data.
- Topic 7.7 Common Ancestry: describe the structural and molecular features shared by all organisms that indicate common ancestry.
A focused answer to AP Biology Topic 7.7, covering the shared features of all life (DNA, the genetic code, ribosomes, core metabolism, membranes) that indicate common ancestry, and how conserved features reveal deep relationships, with a worked example.
- Topic 7.10 Speciation: explain how reproductive isolation leads to speciation, including allopatric and sympatric speciation.
A focused answer to AP Biology Topic 7.10, covering the biological species concept, reproductive isolation (prezygotic and postzygotic barriers), allopatric and sympatric speciation, and rates of speciation, with a worked example.
- Topic 7.13 Origin of Life on Earth: describe the scientific models for the origin of life, including the RNA world and the evidence supporting them.
A focused answer to AP Biology Topic 7.13, covering models for the origin of life, the formation of organic monomers, the RNA world hypothesis, protocells, the geological timeline, and the evidence behind these models, with a worked example.
- Topic 7.8 Continuing Evolution: explain how ongoing examples such as antibiotic resistance and pesticide resistance show that evolution continues.
A focused answer to AP Biology Topic 7.8, covering observable, ongoing evolution including antibiotic and pesticide resistance, emerging diseases, and how these illustrate natural selection in real time, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)