Phylogenetic Trees Practice Worksheet

Let’s go through each question one by one, step by step. We’ll use the phylogenetic trees shown to figure out relationships between organisms based on shared nodes (branching points). Remember: the more recent the common node two organisms share, the more closely related they are.

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Question 1:
*In the diagram to the right, which node represents the most recent common ancestor for organism B and C?*

Look at the tree with Organisms A, B, C, D.
- Node 1 is the oldest — it’s where everything splits from.
- Node 2 is where B and C split from each other — so that’s their *most recent* common ancestor.
- Node 3 is where C and D split — not relevant for B and C.

✔ So, Node 2 is correct.

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Question 2:
*Which node represents the most recent common ancestor for A and C?*

Trace back from A and C:
- A branches off early at Node 1.
- C branches off later at Node 2, but to get to A, you have to go all the way back to Node 1.
- There’s no node between Node 1 and the tips that connects only A and C — so their most recent common ancestor is Node 1.

Wait — the answer written says “A”. That doesn’t make sense. Nodes are labeled 1, 2, 3 — not letters. Probably a typo in the student’s answer.

Actually, looking again: the question asks for the *node*, not an organism. The correct answer should be Node 1, because that’s the last point where A and C’s lineages were together before splitting.

But the student wrote “A” — that’s incorrect. Let’s fix this.

✔ Correct answer: Node 1

*(Note: If the worksheet expects “A” as shorthand for something else, that would be confusing — but based on standard phylogeny reading, it’s Node 1.)*

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Question 3:
*Which organism is B more closely related to, A or C? Explain.*

Compare how recently B shares a node with A vs. C.
- B and C share Node 2 → very recent.
- B and A only share Node 1 → much older.

So B is closer to C.

✔ Answer: C — because B and C share a more recent common ancestor (Node 2) than B and A do (Node 1).

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Question 4:
*Which organism is B more closely related to, C or D? Explain.*

B and C share Node 2.
B and D? To connect B and D, you go back to Node 1 — same as with A.
C and D share Node 3 — which is even more recent than Node 2.

But the question is about B’s relationship to C vs. D.

B and C → Node 2
B and D → Node 1 (since D comes off after Node 3, which is after Node 2)

So B is closer to C than to D.

The student wrote: “c and d because they share node 2 and node 1” — that’s misleading. It implies B is equally close to both, which isn’t true.

Actually, B is closest to C. Then C is closest to D. But B is NOT as close to D as it is to C.

✔ Better explanation: C — because B and C share Node 2, while B and D only share the older Node 1.

*(Student’s answer is partially right but imprecise — we’ll clarify.)*

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Now moving to Questions 5–10 with different trees.

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Question 5:
*Which tree above shows a different evolutionary history from the others? Explain the difference.*

We’re given four small trees labeled 1, 2, 3, 4 — all showing relationships among A, B, C, D.

Tree 1: B and C are sisters; then D; then A.
Tree 2: B is alone; then C and D are sisters; then A.
Tree 3: A and B are sisters; then C; then D.
Tree 4: B and C are sisters; then D; then A — same as Tree 1!

Wait — actually, let’s redraw mentally:

Tree 1: ((B,C),D),A
Tree 2: (B,(C,D)),A
Tree 3: ((A,B),C),D
Tree 4: ((B,C),D),A ← same as Tree 1

So Trees 1 and 4 are identical. Tree 2 has C and D as sisters. Tree 3 has A and B as sisters.

The student said: “1 because c did not come after b or directly evolve from b, c evolved from d”

That’s not quite accurate. In Tree 1, C does NOT evolve from D — they are separate branches. Also, Trees 1 and 4 are the same.

Actually, Tree 3 is the odd one out — because in Trees 1, 2, and 4, A is always the outermost (least related to the rest), but in Tree 3, D is outermost.

Wait — let’s check branching order:

Standard way: look at who shares the most recent node.

In Trees 1, 2, 4: A is sister to the group (B,C,D) — meaning A branched off first.

In Tree 3: D is sister to (A,B,C) — so D branched off first.

Therefore, Tree 3 shows a different history — because in the others, A is the earliest branch; in Tree 3, D is.

Student’s answer focused on Tree 1 being different — but actually Tree 3 is the outlier.

✔ Correct answer: Tree 3 — because in Trees 1, 2, and 4, organism A is the first to branch off, while in Tree 3, organism D is the first to branch off. This changes the evolutionary relationships.

*(Student’s reasoning was confused — we correct it.)*

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Question 6:
*What characteristic do all of the organisms in the tree to the right have in common?*

Looking at the vertebrate tree: Lampreys, Sharks, Ray-finned fish, Lungfish, Amphibians, Land vertebrates.

At the base, there’s a trait marked: “Vertebrae” — and every organism is above that line.

So yes — all have vertebrae.

✔ Answer: vertebrae

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Question 7:
*What characteristic is common to only amphibians and land vertebrates?*

Look at the traits listed along the branches.

After lungfish, the next trait is “Choanae” — then “Bony ribs and shoulder girdle” — then “Fingers and toes”.

Amphibians and land vertebrates both have “Fingers and toes” — and nothing below them has that.

Lungfish and below don’t have fingers/toes.

✔ Answer: fingers and toes

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Question 8:
*What characteristic(s) do sharks and lungfish have in common?*

Sharks: have jaws and vertebrae.
Lungfish: also have jaws and vertebrae — plus more (like bony ribs, etc.)

Common traits: both are above “Jaws” and “Vertebrae” lines.

Ray-finned fish also have those — but the question is just sharks and lungfish.

✔ Answer: jaws and vertebrae

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Question 9:
*Who is the ray-finned fish more closely related to – sharks or lungfish? Explain.*

Ray-finned fish and lungfish share a more recent common ancestor than ray-finned fish and sharks.

Why? Because ray-finned fish and lungfish both come after the “Jaws” node, and they share additional nodes like “Bony ribs...” — whereas sharks branched off earlier, right after “Jaws”.

So ray-finned fish and lungfish are closer.

✔ Answer: lungfish — because they share more recent nodes and derived traits (like bony structures) that sharks don’t have.

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Question 10:
*Are lungfish more closely related to amphibians or land vertebrates? Explain.*

Look at the tree:

Lungfish → then Amphibians → then Land Vertebrates.

Lungfish shares a node with Amphibians (the one leading to “Choanae” and beyond).
Land vertebrates are a subgroup within the lineage that includes amphibians — so lungfish is equally distant to both? Wait.

Actually, in cladistics, if Amphibians and Land Vertebrates form a clade (which they do — Tetrapods), then lungfish is the sister group to that entire clade.

So lungfish is equally related to amphibians and land vertebrates — because both amphibians and land vertebrates descend from the same ancestor that lungfish split from.

The student wrote: “Both because they share the same nodes and the same amount of traits” — that’s essentially correct.

✔ Answer: Both — because lungfish is the sister group to the clade containing both amphibians and land vertebrates. They share the same most recent common ancestor with lungfish.

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## Final Answers (Corrected & Clarified):

1. Node 2
2. Node 1 *(not "A")*
3. C — because B and C share a more recent common ancestor (Node 2) than B and A do (Node 1).
4. C — because B and C share Node 2, while B and D only share the older Node 1.
5. Tree 3 — because in Trees 1, 2, and 4, organism A is the first to branch off, while in Tree 3, organism D is the first to branch off.
6. vertebrae
7. fingers and toes
8. jaws and vertebrae
9. lungfish — because they share more recent nodes and derived traits (like bony structures) that sharks don’t have.
10. Both — because lungfish is the sister group to the clade containing both amphibians and land vertebrates. They share the same most recent common ancestor with lungfish.

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Final Answer:
1. Node 2
2. Node 1
3. C
4. C
5. Tree 3
6. vertebrae
7. fingers and toes
8. jaws and vertebrae
9. lungfish
10. Both