Phylogeny worksheet - Free Printable

Let’s solve each question based on the phylogenetic tree provided.

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First Phylogenetic Tree (with timeline)

1. Which organism is the ancestor of all of the other organisms on this tree?

→ A

Explanation: In a phylogenetic tree, the root (the bottom-most node) represents the most ancient common ancestor of all organisms shown in the tree. Organism A is at the very base (100 million years ago), and all branches originate from it.

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2. Which organisms (species) are still on Earth presently?

→ J, K, M, O, P

Explanation: The tree has a “Present Time” line at the top. Any branch that extends to the present time line without ending before it represents a species that is still alive today. Looking at the diagram, J, K, M, O, and P reach the present time line. Others (like H, I, L, N, G, F, E, D, C, B) end before present → extinct.

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3. Approximately, when did species A have a speciation event? Which organisms resulted from this event?

→ Approximately 75 million years ago. Organisms B and E resulted from this event.

Explanation: At 75 million years ago, the branch from A splits into two main lineages: one leading to B (and eventually C, D, etc.) and the other to E (leading to F, G, etc.). This split is the first major speciation event after A.

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4. Which organisms went extinct? How do you know?

→ B, C, D, E, F, G, H, I, L, N

Explanation: These organisms’ branches terminate *before* reaching the “Present Time” line. In phylogenetic trees with timelines, if a branch ends before the present, it means that lineage died out (extinct). Only J, K, M, O, P reach the present.

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5. Which organism is the most recent common ancestor of J, K and M?

→ I

Explanation: To find the most recent common ancestor (MRCA) of J, K, and M, trace back their lineages until they converge at the most recent shared node. J and K share a common ancestor at I. M also branches off from the same lineage as I (from node I → M). So I is the MRCA of J, K, and M.

*(Note: If we look carefully, J and K branch from I, and M branches from a node just after I — but since I is the last node shared by all three, I is correct.)*

Actually, let’s double-check:

- J and K come from I.
- M comes from a branch that diverges from the same lineage after I (but not from I directly — wait, looking at the tree again: From I, one branch goes to J/K, another to M? No — actually, M branches from a point *after* I, but I is still the MRCA of all three because I is the last node before the lineages diverge into J, K, and M.

Wait — correction: Actually, F might be the MRCA? Let’s retrace:

- All three (J, K, M) descend from I? Or from earlier?

Looking at the tree:

- A → B → ... → G → I → then splits to J, K, and also to M? Not exactly.

Actually, from the tree:

- I leads to J and K.
- M branches off from a different lineage — from L, which comes from F.

Wait — let’s map it properly:

From A:
- Branch 1: A → B → C, D
- Branch 2: A → E → F → G → I → J, K
- Also from F → L → M
- And F → N → O, P

So J and K come from I (which is from G → F).

M comes from L → which is from F.

Therefore, the most recent common ancestor of J, K, and M is F, because F is the last node where all three lineages (J/K via I, and M via L) converge.

✔ Correct Answer: F

How do we know? By tracing back the lineages of J, K, and M — they all meet at node F. I is only the MRCA of J and K, not M. F is older than I and is the node where the lineage leading to M (via L) and the lineage leading to J/K (via G, I) diverged.

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6. When did the speciation event occur that gave rise to organisms J and K?

→ Approximately 25 million years ago

Explanation: Look at the timeline. The branch splitting into J and K occurs right at the 25 million years ago mark. That’s when the lineage from I split into J and K.

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Second Phylogenetic Tree (without explicit timeline, but labeled)

7. Which species is the common ancestor of all of the other species?

→ K

Explanation: In this tree, K is at the bottom (root), and all other species (A, B, C, D, E, F, G, H, I, J) branch off from it or its descendants. Therefore, K is the common ancestor of all others.

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8. Explain why species B and C are more related than species A and C.

→ Because B and C share a more recent common ancestor (node F) than A and C do. A and C share a more distant common ancestor (node K).

Explanation: In phylogenetics, “more related” means sharing a more recent common ancestor.

- B and C both branch from node F → their MRCA is F.
- A branches from K directly; C branches from F, which comes from K → so A and C’s MRCA is K.
- Since F is more recent than K (closer to the present), B and C are more closely related to each other than either is to A.

This reflects evolutionary closeness: fewer evolutionary changes occurred between B and C since their divergence from F than between A and C since their divergence from K.

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## ✔ Final Answers Summary:

First Tree:
1. A
2. J, K, M, O, P
3. ~75 million years ago; B and E
4. B, C, D, E, F, G, H, I, L, N — because their branches end before present time
5. F *(Most Recent Common Ancestor of J, K, M)*
6. ~25 million years ago

Second Tree:
7. K
8. B and C share a more recent common ancestor (F) than A and C do (whose MRCA is K).

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Let me know if you’d like a visual walkthrough or diagrams!