Common Ancestry, Evidence & Branching Trees — Practice · Grade 7 Science

Chapter: Evolution, Evidence & Life History

What this practice covers

By the end of Common Ancestry, Evidence & Branching Trees you should be able to:

8 questions, written answer and multiple choice, each with a worked explanation.

Vocabulary used: common ancestor, homologous structure, vestigial structure, phylogenetic tree, node.

Problem 1

Does the same function alone prove close ancestry?
Answer and explanation

No. Similar functions can arise through different histories; structural and other evidence must be compared.

Problem 2

Why is “vestigial means no function whatsoever” too strong?
Answer and explanation

Reduced ancestral roles can coexist with remaining or changed functions.

Problem 3

Why is one short sequence an incomplete relationship test?
Answer and explanation

It may not represent the whole history and can be affected by sampling and different rates; additional regions and other evidence help.

Problem 4

Does rotating B and C around their node change their relationship?
Answer and explanation

No. The shared node and connections remain the same; visual left-right order is not the ancestry criterion.

Problem 5

Illustrative tree: A splits from a lineage before B and C split from each other. No times, branch-length scale, or actual species identities are supplied. Identify the closest pair in the model, explain why C need not descend from living B, and state what timing cannot be inferred.
Answer and explanation

B and C share the more recent common ancestor in the stated topology. C is not represented as descending from living B; both connect through an ancestral lineage. Exact dates or elapsed times cannot be inferred because no time scale is supplied. The diagram is a hypothesis whose supporting evidence would need examination.

Find the shared B–C node.

Treat living B and C as tips, not parent and child.

Respect the missing time scale.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

Two living tips are drawn next to each other but share a deeper ancestor than another pair. Which determines relatedness in the model?
Answer and explanation

Shared branching nodes determine the modeled relationship, not distance between printed tip labels.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

Which determines the represented relationship?
  1. Shared branching nodes
  2. Alphabetical tip order
  3. The prettiest color
  4. Every tip’s horizontal spacing
Answer and explanation

Shared branching nodes

Topology and shared nodes represent relationships; display choices need a stated meaning.

Problem 8

Why combine fossils, anatomy, and genetic evidence?
Answer and explanation

Different observations can test related aspects of a relationship hypothesis. Agreement strengthens it while limitations or contradictions guide further investigation.

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