A phylogenetic tree is a branching diagram that shows hypothesized evolutionary relationships among lineages and uses fossils or a molecular clock to show the amount of change over time, with nodes marking the most recent common ancestor of any two groups.
A phylogenetic tree is a branching diagram that maps out how organisms are related through shared ancestry. Each branch is a lineage, and each node where branches split represents the most recent common ancestor of the groups above it (EK 7.9.B.1). The big thing that sets a phylogenetic tree apart from a plain cladogram: it shows time and amount of change. Branch lengths are calibrated by fossils or a molecular clock, so a longer branch means more evolutionary change has piled up (EK 7.9.A.2).
Here's the part to internalize: a tree is a hypothesis, not a fact carved in stone. You build it from evidence (morphology of living or fossil species, plus DNA and protein sequence similarities), and when new evidence shows up, the tree gets revised (EK 7.9.B.2, EK 7.9.B.3). The out-group is the lineage least closely related to everything else on the tree, and it acts as your reference point for rooting the diagram (EK 7.9.A.3).
This term lives in Topic 7.9 Phylogeny inside Unit 7: Natural Selection, and it carries two learning objectives. AP Bio 7.9.A asks you to describe the evidence used to infer evolutionary relationships, and AP Bio 7.9.B asks you to use trees and cladograms to infer relatedness. Phylogenetic trees are where the whole unit's story about descent with modification gets drawn out visually. If natural selection and speciation explain how lineages change, the tree is the picture of what those changes produced. It ties straight into the Evolution big idea: common ancestry is the thread connecting all life, and the tree is how biologists represent it.
Keep studying AP Biology Unit 7
Cladogram (Unit 7)
A cladogram is the same branching idea stripped of time. It shows the order of splits but not how much change happened or when, so the branch lengths mean nothing. A phylogenetic tree adds the calibration that makes branch length meaningful.
Molecular Clock (Unit 7)
The molecular clock is the ruler that gives a phylogenetic tree its time scale. By assuming mutations accumulate at a roughly steady rate, you can estimate how long ago two lineages split, which is exactly what a tree's branch lengths display.
Out-group (Unit 7)
The out-group is the lineage least related to the rest of the tree, and it acts as your anchor. Comparing the in-group to the out-group tells you which traits are ancestral and which are newly evolved, which is how you decide where branches go.
Convergent Evolution (Unit 7)
Convergent evolution is the trap that messes up trees built only on appearances. When unrelated lineages independently evolve a similar trait, it can look like shared ancestry, so DNA and protein data are used to avoid being fooled by surface resemblance.
On the multiple-choice section, expect stems that hand you a tree and ask you to read it. A shorter distance between humans and chimps than between humans and lemurs signals a more recent common ancestor, often supported by greater DNA or protein sequence similarity. You'll also get the classic comparison question: a phylogenetic tree beats a cladogram when the researcher needs when divergence happened or how much change occurred, because only the tree shows a time scale. A trait that appears in several lineages but is absent in their most recent common ancestor points to convergent evolution, not shared inheritance. On the free-response side, the 2018 Long FRQ Q1 used a phylogenetic tree of bear populations built from mitochondrial DNA, asking you to interpret relatedness and reason about adaptation, and the 2024 SRFRQ Q5 framed speciation mechanisms that trees illustrate. The skill to practice: read nodes as common ancestors, treat the diagram as a testable hypothesis, and justify relatedness with sequence or morphological evidence.
Both are branching diagrams of evolutionary relationships, and that's why they get mixed up. The difference is the axis: a phylogenetic tree shows the amount of change over time, calibrated by fossils or a molecular clock, so branch lengths are meaningful. A cladogram shows only the branching order with no time scale and no measure of how different the groups are. If branch length matters in the question, you're dealing with a tree, not a cladogram.
A phylogenetic tree is a branching diagram of hypothesized evolutionary relationships, and the nodes mark the most recent common ancestor of the groups they connect.
The defining feature versus a cladogram is time: trees show the amount of change over time calibrated by fossils or a molecular clock, while cladograms do not.
Trees are built from morphological similarities of living or fossil species and from DNA and protein sequence data, with molecular data often the most reliable.
The out-group is the lineage least closely related to everything else and serves as the reference point for the rest of the tree.
A phylogenetic tree is a hypothesis that gets revised as new evidence comes in, not a permanent fact.
It's a branching diagram showing hypothesized evolutionary relationships among lineages, where nodes are common ancestors and branch lengths show the amount of change over time, calibrated by fossils or a molecular clock. It maps to Topic 7.9 in Unit 7.
No. Both show branching relationships, but only a phylogenetic tree shows a time scale and the amount of evolutionary change through its branch lengths. A cladogram shows the order of splits with no time or difference between groups.
Trace back to the shared node. Two species sharing a more recent common ancestor (a node closer to the tips) are more closely related, which is why humans and chimps sit closer together than humans and lemurs.
Because convergent evolution produces similar traits in unrelated lineages, a trait shared across distant branches but absent in their common ancestor can falsely suggest close relatedness. DNA and protein data help avoid being fooled by surface similarity.
A hypothesis. Trees represent testable ideas about relatedness and get revised whenever new morphological or molecular evidence appears, which is exactly the framing the CED expects in EK 7.9.B.3.