Read branching relationships rather than tip order

Mechanism before a label

A rooted phylogenetic tree represents hypotheses about common ancestry. A node marks a shared ancestor; sister groups share an immediate common ancestor. Rotating descendants around a node changes the drawing but not the relationships.

For the topology ((A,B),C), A and B share a more recent common ancestor with each other than either does with C. The left-to-right order of tips is irrelevant. Branch lengths carry time or change only when the diagram explicitly defines their scale.

Shared derived characters support clades, but reversals and convergence can mislead a single-character analysis. An outgroup helps polarize changes; it is not an unchanged ancestor or an inferior organism. Compare alternative trees against multiple independent characters.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Hypothesized character changes	Value (changes)
Tree ((A,B),C) required changes	4
Tree ((A,C),B) required changes	6

The initial numerical packet supplies stipulated change totals rather than a character matrix.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Read branching relationships rather than tip order

Supplied synthetic evidence: Original parsimony comparison with stipulated change totals; no character matrix is provided.

Hypothesized character changes: reference condition

Quantity	Value	Unit
Tree ((A,B),C) required changes	4	changes
Tree ((A,C),B) required changes	6	changes

Task: Draw ((A,B),C) twice with different tip orders; identify the same sister pair and explain why C is not an ancestor of A.

No matrix is supplied here, so this packet practices interpretation rather than reconstructing counts.

Compare with the conditional interpretation

The first candidate is more parsimonious for the stipulated counts.

The initial advantage is6−4=2 fewer changes. With the expanded evidence the ordering reverses to8−7=1 fewer for the other tree. Reevaluate a hypothesis when evidence changes.

Case index for this investigation

0. Connect heritable variation to relative reproductive success
1. Distinguish selection by people from inheritance and trade-offs
2. Separate drift, gene flow and mutation from selection
3. Calculate allele frequencies and test equilibrium claims
4. Combine fossils, homology and molecular evidence
5. Read a longitudinal evolution study without inventing observations
6. Read branching relationships rather than tip order
7. Explain reproductive barriers and divergence
8. Relate variation, changing environments and extinction
9. Separate origin-of-life hypotheses from ancestry evidence

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Test your model prediction

Scratch inputs are not saved or graded. Retain your prediction and calculations in the lesson notebook. No simulation output is a measured organism or population.

Rotate A/B around their shared node

Text equivalent: ((A,B),C). A and B share a more recent common ancestor; C is a tip, not the ancestor. Rotation changes drawing order, not ancestry. Branch lengths here are arbitrary, not time or molecular-change scales.

Reset practice

Worked reasoning

The initial advantage is6−4=2 fewer changes. With the expanded evidence the ordering reverses to8−7=1 fewer for the other tree. Reevaluate a hypothesis when evidence changes.

The first candidate is more parsimonious for the stipulated counts.

Apply to a new case

Draw ((A,B),C) twice with different tip orders; identify the same sister pair and explain why C is not an ancestor of A.

Compare your reasoning with the worked response

A and B remain sisters in both drawings. C is a sampled terminal lineage sharing an older ancestor, not automatically the ancestor itself. Rotate branches around nodes, not by reassigning connections.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

The initial numerical packet supplies no matrix, so that packet practices interpretation rather than reconstructing counts. The separate character packet below adds reconstruction. Parsimony is a model criterion, not proof that evolution minimizes changes.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 7.9. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.

Construct a hypothesis from supplied homologous characters

The following original matrix supplies three homologous binary characters, not measured organisms. O is the stipulated outgroup; its state 0 supplies the ancestral-state comparison. Treat the characters as independent, equally weighted and unordered, with no missing observations. Construct a rooted relationship among A, B and C before viewing the explanation.

Taxon	Character 1	Character 2	Character 3
O	0	0	0
A	1	1	0
B	1	1	1
C	0	0	0

Which pair shares derived states rather than merely retaining ancestral zeros? Compare an A–B sister hypothesis with a B–C sister hypothesis. Count the minimum changes required for each character, allowing repeated gains or reversals when needed; do not count a drawn branch rotation as a character change.

Check the initial construction

Characters 1 and 2 each support A–B: one change on their shared ancestral branch explains each. Character 3 needs one change on B's branch. The rooted arrangement O outside (C outside (A,B)) therefore needs three changes. A B–C sister arrangement needs two changes for each of characters 1 and 2 plus one for character 3: five. Shared zeros in O and C do not supply equivalent evidence of a uniquely derived O–C group.

Changed evidence: Replace character 2 by O=0, A=0, B=1, C=1; keep the other columns unchanged. Rebuild the comparison rather than retaining the first answer automatically.

Check conflict rather than force a unique tree

Character 1 favors A–B, but character 2 now favors B–C. The A–B hypothesis needs 1+2+1=4 changes; B–C needs 2+1+1=4. The A–C sister alternative needs 2+2+1=5. Under this small equal-weight analysis, two hypotheses tie. Repeated evolution or reversal can explain a conflicting character, but this matrix alone does not identify which history occurred. More independent characters, justified homology and suitable models could change the inference; a single character or gene tree is not automatically the species history.

Building and interpreting phylogenetic hypotheses, checked 31 October 2026. The character packet is supplied evidence for constructing a hypothesis, not an actual specimen analysis or independently assessed learner result.