Compare tree hypotheses with character evidence

A tree is a relationship hypothesis

The four labels A, B, C and D in the lab represent fictional taxa. AB | CD groups A and B on one side of an internal branch and C and D on the other. Rotating a drawing does not change that split. This tree is unrooted: it supplies neither the direction of time nor the location of an ancestor.

For each binary character, the tool tries both possible states at each of the two internal nodes. Four assignments are possible. It counts a change on an edge whenever its endpoints differ, keeps the smallest count and adds those minima across characters.

Explain one character by hand

For A=0, B=0, C=1 and D=1, AB | CD needs one change between internal nodes. AC | BD requires at least two. The concordant dataset has two characters with the first grouping and one different only in A. The conflicting dataset favors AB for one character and AC for another, leaving a tie. The uninformative dataset favors no split.

Parsimony seeks fewer changes, but convergence, reversals and character dependence can complicate inference. The synthetic score is conditional evidence, not proof of history or a clock. Do not count two correlated measurements of one feature automatically as independent confirmations. Background: OpenStax, evolutionary relationships.

Classify by ancestry

A clade includes an ancestor and all its descendants. On a rooted tree, find the most recent common ancestor of selected tips, then check for omitted descendants. This test depends on the tree hypothesis and its sampling.

In the separate exercise below, Cedar and Dune share Ancestor 2, which also has Elm as a descendant. Adding Elm repairs the group under this hypothesis. Brook and Elm alone omit Cedar and Dune. Follow branches, not the ordering of labels. Background: OpenStax, cladistics.

Test a group on a rooted tree

This is a fixed teaching hypothesis for five fictional taxa. Predict whether Cedar and Dune form a clade before reading the result. Select taxa to include in your group; then trace their shared ancestor and every descendant shown.

Taxa in your proposed group

AsterBrookCedarDuneElm

Reverse drawing orderClear selectionReset tree exercise

Not a clade in the displayed tree. Most recent common ancestor: Ancestor 2. All sampled descendants: Cedar, Dune, Elm.

Omitted descendants: Elm.

Try Cedar, Dune and Elm together, then Brook and Elm alone. Reverse the drawing order: the relationships and answer should stay the same. This tests membership under the given hypothesis; it does not infer a tree from observations.

Selections are temporary. Record your proposed group, omitted descendants and reasoning in the saved lesson reflection. This exercise adds no separate completion requirement.

Names organize observations

Taxonomy supplies names and classifications; systematics investigates relationships and diversity. Commonly taught ranks run from domain through kingdom, phylum, class, order, family and genus to species. A two-part species name combines a capitalized genus with a lowercase specific epithet, conventionally italicized: Homo sapiens. The epithet alone is not the full species name.

Bacteria, Archaea and Eukarya are domain names in the familiar three-domain classification. A rank is not a fixed amount of genetic difference or elapsed time. Classifications can change as evidence changes. Naming a group does not prove a tree. Background: OpenStax, organizing life.

Apply: a fictional field guide puts Aster and Elm in “floating organisms” because both were observed at a pond surface. Is that an observation category, a species name, or a demonstrated clade?

Compare your classification reasoning

It is a useful observation category, but not a two-part species name or evidence of a clade. In this tree Aster and Elm share the root; Brook, Cedar and Dune are also descendants. The behavior category and ancestry category answer different questions.

Compare evidence with different sources of error

Fossils, geographic distributions, anatomy and molecular sequences provide different observations relevant to evolutionary explanations. Homology is similarity inherited from shared ancestry; convergence can generate similarities independently. Specify the character: bird and bat forelimb bones share an inherited structural pattern, while powered-flight adaptations evolved separately. Similar function alone cannot establish a particular relationship. Background: OpenStax, evidence for evolution.

This dossier is invented for reasoning practice, not measured data from named species. Compare H1, “Cedar and Dune are closest relatives,” with H2, “Dune and Elm are closest relatives.” The rooted exercise supplies H2; the dossier asks how observations might test it.

These observations should not be added into a universal certainty score. Independence is a question about how evidence arises, not how many publications contain it. A gene history can differ from a species history. A fossil can constrain a timing claim conditional on its assignment without choosing among every living-tip relationship. Background: OpenStax, molecular comparisons and tree inference.

Write an evidence brief: choose the better-supported hypothesis in the dossier, explain the paddle alternative, identify duplicated evidence, and propose an observation that could change your conclusion.

Compare an evidence-based response

H2 currently has support from joint anatomy and the sequence comparison, subject to the stated checks. The paddles admit a convergence explanation. Repeating the same file checks reproducibility; it is not another biological sample. Another well-chosen genomic region and independently examined specimens could test whether the pattern persists. Conflicting results should trigger examination of methods and evolutionary explanations. If its age and assignment hold, the fossil challenges the 8-million-year origin claim without proving direct ancestry.

Transfer to an unfamiliar case

A new fictional taxon, Fern, is sampled. Two hypotheses place it either inside the Dune–Elm group or on a separate branch outside Ancestor 2. Explain why the five-tip exercise cannot decide between them. Specify observations to collect, checks on sample identity, and how the claimed clade would change under each placement.

Record your response and selected groups in the saved lesson reflection. Distinguish testing membership on a supplied tree from inferring a tree from observations. The dossier supplies guided practice; a study of actual organisms requires traceable specimens or datasets and review of methods.