Evolution: Phylogenetic Trees and Cladistics Worksheet

Question 1

In a phylogenetic tree, what does a node represent, and what do the branch tips usually represent?

Accepted Answer

A node represents a common ancestor where one lineage split into two or more lineages. The branch tips usually represent living species, extinct species, or larger groups being compared.

Question 2

A tree shows that humans and chimpanzees share a more recent common ancestor with each other than either shares with gorillas. Which two groups are sister taxa?

Accepted Answer

Humans and chimpanzees are sister taxa because they share the most recent common ancestor with each other compared with gorillas.

Question 3

Explain the difference between an ancestral trait and a derived trait in cladistics.

Accepted Answer

An ancestral trait is a feature inherited from a distant common ancestor and found in older lineages. A derived trait is a newer feature that evolved within a specific lineage and can help identify a clade.

Question 4

Use this character information: Species A has a backbone only. Species B has a backbone and four limbs. Species C has a backbone, four limbs, and amniotic eggs. Species D has a backbone, four limbs, amniotic eggs, and hair. Which species is most closely related to Species D?

Accepted Answer

Species C is most closely related to Species D because it shares the greatest number of derived traits with Species D: a backbone, four limbs, and amniotic eggs.

Question 5

A cladogram places a fish, frog, lizard, and mouse in that order from the outgroup toward the most derived branch. The derived traits appear in this order: jaws, lungs, amniotic egg, hair. Which trait separates lizard and mouse from fish and frog?

Accepted Answer

The amniotic egg separates lizard and mouse from fish and frog because it appears after lungs but before the split between lizard and mouse.

Question 6

Why is an outgroup included when building or interpreting a phylogenetic tree?

Accepted Answer

An outgroup is included as a comparison group that is related to the organisms being studied but branched off earlier. It helps identify which traits are ancestral and which traits are derived.

Question 7

Two organisms both have wings: a bird and a butterfly. Explain why wings in this case may not be strong evidence that birds and butterflies are close relatives.

Accepted Answer

Bird wings and butterfly wings may be analogous structures because they have similar functions but different evolutionary origins. Similar traits caused by convergent evolution do not always show close common ancestry.

Question 8

A phylogenetic tree shows Species X branching off first, then Species Y, while Species Z and Species W share the most recent node. Which species is least closely related to Species W?

Accepted Answer

Species X is least closely related to Species W because it branched off earliest and shares the most distant common ancestor with Species W among the species listed.

Question 9

What is a clade?

Accepted Answer

A clade is a group that includes a common ancestor and all of its descendants. Clades are also called monophyletic groups.

Question 10

A group includes reptiles and birds but excludes mammals. If the common ancestor of reptiles, birds, and mammals is included in the group, is this group a clade? Explain.

Accepted Answer

This group is not a clade if it includes the common ancestor of reptiles, birds, and mammals but excludes mammals. A clade must include the ancestor and all of its descendants.

Question 11

Two trees show the same relationships among A, B, C, and D. In one tree, B and C are drawn on opposite sides of a node compared with the other tree. Does rotating branches around a node change the evolutionary relationships? Explain.

Accepted Answer

Rotating branches around a node does not change the evolutionary relationships. The relationships are determined by the pattern of shared nodes, not by the left-to-right order of the branch tips.

Question 12

A character matrix shows that Species 1, 2, and 3 all have a vertebral column. Species 2 and 3 have a hinged jaw. Only Species 3 has a bony skeleton. Which trait is the most derived among these traits?

Accepted Answer

The bony skeleton is the most derived trait in this matrix because it appears only in Species 3 after the other shared traits.

Question 13

Explain how the principle of parsimony is used in cladistics.

Accepted Answer

The principle of parsimony favors the phylogenetic tree that requires the fewest evolutionary changes. It is used because the simplest explanation that fits the evidence is often preferred unless more evidence suggests otherwise.

Question 14

A DNA comparison shows that Species A and Species B differ by 4 nucleotide changes, Species A and Species C differ by 18 changes, and Species B and Species C differ by 20 changes. Which two species are most closely related based on this evidence?

Accepted Answer

Species A and Species B are most closely related based on this evidence because they have the fewest nucleotide differences.

Question 15

A phylogenetic tree has branch lengths proportional to genetic change. Branch A is much longer than Branch B. What does the longer branch length suggest?

Accepted Answer

The longer branch length suggests that more genetic change occurred along Branch A than along Branch B. It does not necessarily mean that the organism on Branch A is more advanced.

Evolution: Phylogenetic Trees and Cladistics

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