Practice questions · Topic 7.9

Unit 7 · Practice for the Topic 7.9 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Phylogeny, summed up

Video coming soon

Tips, branches and nodes; who is closest to whom; each node a split; a cladogram or a phylogenetic tree; derived characters and the outgroup; building a tree from a table and from sequences; the molecular clock; when the trees disagree; a tree is a hypothesis.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one case one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Every count of DNA differences in these questions is imagined for the question; the trees show real relationships.

Question 1

A cladogram of four plants; the letter K marks one line.

The cladogram shows four plants. The letter K marks one line of the drawing.

What is the marked line?

A. A tip B. A node C. A branch D. The root

Question 2

A cladogram of four mammals.

The cladogram shows four mammals.

Which mammal is the most closely related to the mongoose?

A. The fossa B. The civet C. The jackal D. The civet and the fossa equally

Question 3

A cladogram of four plants.

The cladogram shows four plants.

Which split happened first?

A. The nutmeg’s lineage splitting from the cinnamon–sassafras lineage B. The cinnamon’s lineage splitting from the sassafras’s C. A cladogram cannot show which of these splits came first D. The black pepper’s lineage splitting from the lineage of the other three

Question 4

Four plants drawn on a tree with a scale in millions of years ago along the bottom; the dates are imagined.

The drawing shows four plants; the dates on its scale are imagined.

Which feature of the drawing makes it a phylogenetic tree rather than a cladogram?

A. The tips lined up at the right B. The scale in millions of years along the bottom C. The filled dots at the branch points D. The thyme printed first, at the top

Question 5

Four characters of four mammals; the pangolin is the outgroup.

The table gives four characters of four mammals. The pangolin is the outgroup.

Which character is a shared derived character of the kudu and the eland alone?

A. Horns with a bony core B. Hooves C. Stripes on the legs D. Gives milk

Question 6

A cladogram of four plants.

The cladogram shows four plants.

Which plant is the outgroup?

A. The durian B. The okra C. The cacao D. The baobab

Question 7

Four characters of four plants; the hornwort is the outgroup.

The table gives four characters of four plants. The hornwort is the outgroup. Biologists build a cladogram from the table.

Which plant do they add to the cladogram last, nearest the tips?

A. The ginkgo B. The watermelon C. The quillwort D. The hornwort

Question 8

The number of DNA positions at which each pair of four plants differs; the counts are imagined.

Biologists compare one stretch of DNA in four plants and count the positions at which each pair differs, as the table shows. The counts are imagined.

Which two plants join at the node nearest the tips?

A. The dill and the coriander B. The fennel and the coriander C. The coriander and the zinnia D. The dill and the fennel

Question 9

Biologists compare one stretch of DNA in two kinds of tinamou, a ground bird of South America, and find 27 differences. From other tinamou lineages with fossil-dated splits they have set the rate at 3 differences per million years. A student says: “The two lineages split exactly 9,000,000 years ago, to the year.” The count and the rate are imagined.

Is the student correct?

A. No: the clock gives an estimate, about 9 million years, because the rate is only roughly steady B. Yes: 27 differences ÷ 3 differences per million years = 9 million years, so the date is exact C. Yes: a rate set from fossil-dated splits makes every date it gives exact D. No: the count should be multiplied by the rate, so the split was 81 million years ago

Question 10

Biologists build two cladograms of five families of fish that live in fast streams. One is built from body shape: four of the families have flattened bodies and sucker-like mouths that grip rocks. The other is built from DNA sequences of six genes. The two cladograms group the families differently.

Which cladogram should biologists accept?

A. The body-shape cladogram B. Neither cladogram C. Both cladograms D. The DNA cladogram

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Analyze Model or Visual Representation · 5 points

Biologists compare one stretch of DNA in four mammals: a beluga, a narwhal, a porpoise and a peccary. For each pair they calculate the percent of positions that differ, the percent divergence, as the table shows; every pair’s value is written twice. Fossils date the split between the beluga’s lineage and the narwhal’s at 3 million years ago. The percentages and the date are imagined for this question.

The percent divergence of one stretch of DNA between each pair of four mammals, every value written twice; the percentages are imagined.

(a) Identify the two mammals that join at the node nearest the tips. (1 point)

Hint: Find the smallest percentage in the table; the two names on its row and column are the pair.

A full-credit answer: The beluga and the narwhal.
Their divergence, 4.2%, is the smallest in the table, so they share the most recent common ancestor.

Check the box for each point your answer earns

the beluga and the narwhal (4.2%, the smallest value in the table).

(b) Calculate the rate of percent divergence per million years between the beluga and the narwhal. Give your answer to two decimal places; the same value to one decimal place is also accepted. (1 point)

Hint: The rate is the percent divergence of the fossil-dated pair divided by the time since their split.

% per million years

Write down the values in the question:

percent divergence, beluga–narwhal = 4.2%

time since the split = 3 million years

Write down the equation:

rate = \frac{percent divergence}{time since the split}

Substitute the values into the equation:

rate = \frac{4.2}{3} = 1.40 % per million years

A full-credit answer: The beluga and the narwhal differ at 4.2% of positions, and fossils date their split at 3 million years ago.
4.2 ÷ 3 = 1.40% per million years.

(c) Calculate how long ago the porpoise’s lineage split from the beluga’s, using the rate from part (b). Answers within 0.5 million years are accepted. (1 point)

Hint: Divide the beluga–porpoise divergence by the rate from part (b), not by the fossil date.

million years

Write down the values in the question:

percent divergence, beluga–porpoise = 8.4%

rate = 1.40% per million years

Write down the equation:

time since the split = \frac{percent divergence}{rate}

Substitute the values into the equation:

time since the split = \frac{8.4}{1.40} = 6.0 million years

Write the answer as an estimate:

about 6 million years ago

A full-credit answer: The beluga and the porpoise differ at 8.4% of positions.
8.4 ÷ 1.40 = 6.0 million years.
So the two lineages split about 6 million years ago.

(d) Identify the outgroup, and justify its placement as the first branch off the root. (1 point)

Hint: Which mammal’s row holds the largest values against every other mammal?

A full-credit answer: The peccary.
Its divergence from every other mammal is the largest in the table, 20.1% to 20.4%.
The mammal most different from all the others is the least closely related to them, so its lineage branches first.

Check the box for each point your answer earns

the peccary, because its DNA differs most from every other mammal’s (20.1% to 20.4%), so it is the least closely related and branches first.

(e) Explain why the time in part (c) is an estimate rather than an exact date. (1 point)

Hint: Think about what the clock assumes about the rate, and whether DNA really changes at exactly that rate.

A full-credit answer: The clock treats the rate as steady.
The real rate is only roughly steady: it differs from gene to gene and from lineage to lineage.
So the time the division gives is an estimate.

Check the box for each point your answer earns

the rate at which differences build up is only roughly steady (it varies between genes and between lineages), so the calculated time is approximate.

Free-response score: 0 of 5

Free response 2 · Analyze Model or Visual Representation · 4 points

The phylogenetic tree shows five birds, built from DNA sequences of two genes. Its scale gives the time since each split in millions of years. The dates are imagined for this question.

A phylogenetic tree of five birds drawn on its side, with a scale in millions of years ago along the bottom; the dates are imagined.

(a) Estimate, in millions of years, the age of the most recent common ancestor of the barbet and the honeyguide. Answers within 2 million years are accepted. (1 point)

million years

A full-credit answer: The barbet’s and the honeyguide’s lines join at the node above 20 on the scale.
So their most recent common ancestor lived about 20 million years ago.

(b) Identify the bird most closely related to the puffbird. (1 point)

A full-credit answer: The jacamar.
Its line joins the puffbird’s at the node above 30, higher than any node the puffbird shares with another bird.

Check the box for each point your answer earns

the jacamar.

(c) Identify two birds whose positions on the tree could be switched while every relationship the tree shows stays the same. (1 point)

A full-credit answer: The barbet and the honeyguide, or the jacamar and the puffbird.
Each pair is the two branches at one node, so swapping them changes no relationship.
The two pairs are themselves the two branches at the node above 50, so the whole barbet–honeyguide pair could be switched with the whole jacamar–puffbird pair.

Check the box for each point your answer earns

the barbet and the honeyguide, OR the jacamar and the puffbird, OR the whole barbet–honeyguide pair switched with the whole jacamar–puffbird pair (the two branches at the node above 50).

(d) Suppose biologists then sequence four more genes, and all four place the barbet beside the puffbird. Determine whether biologists should redraw the tree, and justify your answer. (1 point)

A full-credit answer: They should redraw the barbet’s branch beside the puffbird’s.
A tree is a hypothesis about relationships, built from the evidence available.
Four independent genes contradict one placing, the barbet’s, and agree with each other.
So biologists redraw the branch the new evidence contradicts and keep the rest.

Check the box for each point your answer earns

yes, redraw the barbet’s branch (beside the puffbird), because a tree is a hypothesis and four genes that agree with each other and contradict a placing revise it; the branches the new evidence does not contradict stay. Also award the point for: combine all six genes and rebuild the tree from them, with the same reason (the tree is revised to fit the larger body of evidence). ‘No: the two genes already gave a tree’ earns no point.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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