SCIENCE · INVESTIGATE AND EXPLAIN

Read common ancestry without using tip order as a ladder

A rooted tree supplies a direction from an ancestral lineage toward descendant lineages. An unrooted tree represents relationships without specifying that direction. An appropriate outgroup and other evidence can help infer a root, but an outgroup is not automatically the ancestor of the ingroup.

Learning outcome: Use all four rotation settings. Draw brackets around cladesAB andABC each time, then trace the A–C path. Identify a non-clade group and explain why its most recent common ancestor also includes an omitted tip.

Starting recap

Reconstruct character-based relationships, preserve topological and sampling uncertainty, and distinguish lineage, species and detection measures of diversity.

All lessons are open directly. Your saved experiments remain when you leave and return.

Try

Rotating A/B and then rotating AB/C can produce the written order((C,(B,A)),D). The AB clade and ABC clade are unchanged. A–B still has two stipulated edges, even though the visual left-to-right sequence changed.

Open the experiment

Predict

If C is drawn next to B after rotation, does that make C and B sister taxa?

Your prediction and reason

Experiment

Use all four rotation settings. Draw brackets around cladesAB andABC each time, then trace the A–C path. Identify a non-clade group and explain why its most recent common ancestor also includes an omitted tip.

Read a clade despite a changed drawing order

Worked teaching model · synthetic values · Fixed rooted topology(((A,B),C),D). Rotating descendants around a node changes presentation only. Unit edge counts are stipulated, not time or molecular distance.

Rotate A/B sister tips: 0 off/on Rotate AB versus C at their node: 0 off/on

Reset keeps your prediction, captures and practice history.

Closeness is defined by common ancestry, not the left-to-right order of tip labels.

All extant tips have histories back to the root; D is not an ancestor of the other living taxa. Unit edge counts are only an illustration and must not be read as dates or a ladder of progress.

Fixed topology, rotated display: (((A,B),C),D). A and B remain sister tips; AB and C share a more recent ancestor than either does with D.

Node rotations preserve descendant sets. Drawing lengths are schematic; the ledger counts stipulated unit edges, not SVG distances or time.

Calculated quantities under stated assumptions
Quantity	Value	Unit
A–B path length	2	stipulated edges
A–C path length	3	stipulated edges
A–D path length	4	stipulated edges
Members of cladeAB	2	tips

Retain prior predictions, settings and quantities in the notebook. These are original supplied/calculated comparisons, not physical specimen observations or laboratory participation.

Closeness is defined by common ancestry, not the left-to-right order of tip labels.

All extant tips have histories back to the root; D is not an ancestor of the other living taxa. Unit edge counts are only an illustration and must not be read as dates or a ladder of progress.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

1. Orient: Read common ancestry without using tip order as a ladder

2. Explain the evidence and inference model

The original fixed topology is(((A,B),C),D). A and B are sister tips; their common ancestor joins C before joining D. Rotating descendants around either internal node changes the drawing but preserves those relationships. Two adjacent labels need not be sisters, and a long terminal branch does not make a living organism more advanced.

A clade includes an ancestor and all its descendants. In this tree A,B form a clade, as do A,B,C. A group containing A and C but excluding B omits a descendant of their most recent common ancestor and is not a clade. Monophyletic groupings differ from paraphyletic or polyphyletic selections.

The ledger uses one unit per edge solely to make paths countable. A–B has two edges, A–C three and A–D four. Unless a tree explicitly encodes time or evolutionary amount, branch length and edge count are not dates. All living tips have evolutionary histories; D is not a primitive living stand-in for the ancestor.

3. Reconstruct a worked example

4. Predict and investigate

Predict first: If C is drawn next to B after rotation, does that make C and B sister taxa?

Retain the complete matrix, topology, branch lengths or detection assumptions with the first prediction and all conflicting results. These original hypothetical data are not actual specimen observations or measured evolutionary histories.

5. Transfer and defend a changed case

Apply the mechanism to a changed case

For the same topology, is the setB,C a clade when A is excluded? Explain without relying on label order.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

No. The most recent common ancestor of B and C also has A as a descendant. A rotation cannot remove A from that ancestry relation.

6. Reflect and preserve uncertainty

Explain which conclusion is conditional on the model, identify an alternative and propose independent discriminating evidence. Use the General Biology II workspace to preserve a first argument, human critique and revision. Public practice and submitted prose do not establish independent phylogenetic, sampling or practical competence.

Supporting scientific reference, checked 30 October 2026. Original prose, numerical packets and diagrams; no source artwork or exercise is reproduced and no organism handling is requested.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

For the same topology, is the setB,C a clade when A is excluded? Explain without relying on label order.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Keep an unknown character state distinct from observed absence Next: Separate locus histories from a single species-tree claim

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