Practice Phylogenetic Trees 1 Answer Key

Practice Phylogenetic Trees – Answer Key

Phylogenetic trees are visual representations of evolutionary relationships among organisms. Below is a comprehensive answer key for a typical “Practice Phylogenetic Trees 1” worksheet. In many biology courses, instructors provide practice trees for students to draw or interpret. They help students grasp concepts such as common ancestry, divergence, and homology. The key includes explanations for each question, the correct tree layout, and the reasoning behind each branch arrangement. This resource can be used by teachers to grade student submissions or by students to self‑check their work Most people skip this — try not to..

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1. Identify the Outgroup

Question:
Which organism should be chosen as the outgroup for a tree that includes Homo sapiens, Pan troglodytes, Macaca mulatta, Gorilla gorilla, and Pongo abelii?

Answer:
Outgroup: Pongo abelii (Sumatran orang‑utan)

Reasoning:
An outgroup is a lineage that diverged earlier than the ingroup members. In this set, Pongo abelii is the most distantly related species; it diverged before the common ancestor of the great apes (Hominidae). Using Pongo as the outgroup provides a clear root for the tree and helps students see the direction of evolution.

2. Construct the Tree (Step‑by‑Step)

Question:
Draw a phylogenetic tree for the same five species, labeling all branches with the correct order of divergence.

Answer Key – Tree Diagram (ASCII Representation)

          ┌───── Homo sapiens
          │
      ┌───┤───── Pan troglodytes
      │   │
      │   └───── Gorilla gorilla
  ┌───┤
  │   │
  │   └───── Macaca mulatta
┌───┤
│   │
│   └───── Pongo abelii

Explanation of Branching Order

1. Root at the base of Pongo abelii (outgroup).
2. First divergence: Pongo separates from the lineage leading to all other species.
3. Second divergence: Macaca mulatta (rhesus macaque) branches off next, as it is a prosimian relative of the hominoids.
4. Third divergence: Gorilla gorilla splits off before the split between Homo and Pan.
5. Final split: Homo sapiens and Pan troglodytes share the most recent common ancestor, reflecting their close genetic relationship.

3. Determine the Most Recent Common Ancestor (MRCA)

Question:
For the pair Homo sapiens and Pan troglodytes, what is the MRCA, and what taxonomic level does it represent?

Answer:
MRCA: The common ancestor of Homo and Pan, which belongs to the family Hominidae (great apes).
Taxonomic Level: Family (Hominidae) That's the part that actually makes a difference..

Why this matters:
Identifying the MRCA helps students understand how taxonomic ranks reflect evolutionary history. The MRCA of Homo and Pan predates the divergence of Gorilla and Pongo, so the common ancestor is placed at the family level rather than at the genus or species level That's the part that actually makes a difference..

4. Calculate Genetic Distance (Illustrative Example)

Question:
Assume the following genetic distances (in substitutions per site) between the species pairs:

Which pair is most closely related?

Answer:
Pair: Homo sapiens – Pan troglodytes (distance 0.015) Surprisingly effective..

Interpretation:
The smallest genetic distance indicates the closest relationship, confirming the tree’s branching order where H. sapiens and P. troglodytes share a recent common ancestor It's one of those things that adds up..

5. Identify Synapomorphies

Question:
Which morphological trait is a synapomorphy for the clade containing Homo sapiens, Pan troglodytes, and Gorilla gorilla?

Answer:
Trait: The presence of a fully developed sagittal crest in adult males (though reduced in humans) Took long enough..

Explanation:
A synapomorphy is a shared derived characteristic that evolved in the last common ancestor of a group. The sagittal crest is absent in Macaca and Pongo, making it a defining feature for the clade that includes Homo, Pan, and Gorilla.

6. Interpret Branch Lengths

Question:
In a branch‑length tree, the branch leading to Macaca mulatta is twice as long as the branch leading to Pan troglodytes. What does this imply?

Answer:
Implication:
A longer branch indicates a greater amount of evolutionary change (more substitutions) or a longer time since divergence. Because of this, Macaca mulatta has undergone more genetic change or diverged earlier than Pan troglodytes relative to their common ancestor It's one of those things that adds up..

7. Use Molecular Clock Hypothesis

Question:
If the molecular clock rate is estimated at 1.5 × 10⁻⁸ substitutions per site per year, estimate the divergence time between Homo sapiens and Pan troglodytes using the genetic distance 0.015 Surprisingly effective..

Answer:

1. Total substitutions per site = 0.015.
2. Rate per year = 1.5 × 10⁻⁸.
3. Divergence time = 0.015 / (1.5 × 10⁻⁸) ≈ 1,000,000 years (1 Ma).

Conclusion:
The split between humans and chimpanzees likely occurred about one million years ago, aligning with fossil and genetic evidence Less friction, more output..

8. Evaluate Tree Accuracy

Question:
A student’s tree places Macaca mulatta as sister to Homo sapiens. Why is this incorrect?

Answer:
Reason:
Macaca belongs to the family Cercopithecidae (Old World monkeys), which diverged earlier than the great apes. Placing Macaca next to Homo ignores the well‑established phylogenetic hierarchy and misrepresents the evolutionary history. The correct sister group for Homo sapiens is Pan troglodytes.

9. Discuss Bootstrap Support

Question:
What does a bootstrap value of 95% indicate for a particular branch in the tree?

Answer:
A bootstrap value of 95% means that, in 95% of the resampled datasets, that branch appeared in the same configuration. This high value indicates strong statistical support for the relationship depicted by that branch.

10. Summarize Key Takeaways

Answer Key Summary

• Outgroup selection clarifies the root and direction of evolution.
• Branching order reflects genetic distances and morphological synapomorphies.
• MRCA identification links taxonomic ranks to evolutionary history.
• Genetic distances provide quantitative measures of relatedness.
• Branch lengths inform on the amount of change or time elapsed.
• Molecular clock calculations estimate divergence times.
• Bootstrap values assess the reliability of inferred relationships.
• Common misconceptions (e.g., misplacing Macaca) highlight the importance of understanding evolutionary frameworks.

Final Notes for Instructors and Students

• Double‑check the placement of the outgroup; a misplaced outgroup can invert the entire tree.
• Cross‑validate genetic data with morphological traits to reinforce learning.
• Use visual aids (color‑coded branches, labeled nodes) to help students see patterns.
• Encourage critical thinking: ask why a certain branch arrangement makes sense or not, prompting deeper engagement with evolutionary concepts.

By mastering these elements, students will not only answer practice questions correctly but also develop a solid understanding of how phylogenetic trees are constructed and interpreted in modern biology And it works..