Practice questions · Topic 7.6

Unit 7 · Practice for the Topic 7.6 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: Evidence of evolution, summed up

Video coming soon

Five kinds of evidence; fossils, extant and extinct; the deeper layer is older; decay as a clock and an age from half-lives; which clock for which age; where species live; transitional fossils in order; homologous, analogous and vestigial structures; counting sequence differences and ranking by them; independent lines of evidence agreeing.

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 sequence table and every count in these questions is imagined for the question.

Question 1

Four studies of pines, each labeled by a student with the kind of evidence it gives.

A student sorts four studies of pines by the kind of evidence each gives, as the table shows.

Which study has the student labeled wrongly?

A. Counting an allele’s copies in a pine population every year for 30 years B. Reading the order of bases in one gene from a pine and a fir C. Mapping which mountain range each pine species grows in D. Finding pine cones preserved in a rock layer

Question 2

Five undisturbed rock layers with three fossils marked; layer 1 is the bottom layer.

In the drawing, layer 1 is the bottom layer, and the layers are undisturbed. Three fossils are marked. No date in years has been measured on any layer.

Which statement do the layers support?

A. The tooth is the oldest of the three fossils, because it lies in the top layer B. The leaf is about 20 million years old C. The three fossils are all about the same age D. The shell is older than the leaf, and the leaf is older than the tooth

Question 3

The percentage of an isotope still left, against the number of half-lives passed.

The graph shows the percentage of an isotope still left in a sample against the number of half-lives that have passed.

After how many half-lives is 25% of the isotope still left?

A. 1 B. 2 C. 3 D. 4

Question 4

A seed from a buried storage pit held 1,600 carbon-14 atoms when its plant died and holds 200 now. Carbon-14’s half-life is 5,730 years.

How old is the seed?

A. 5,730 years B. 11,460 years C. 17,190 years D. 22,920 years

Question 5

Carbon-14’s half-life is 5,730 years, and a lab can measure only what is left of it.

Which is the oldest age carbon-14 can put on a bone?

A. About 5,000 years B. About 50,000 years C. About 5 million years D. About 50 million years

Question 6

Biologists claim that the finches of an island descend from colonists that came from the nearest mainland.

Which observation is geographical evidence for the claim?

A. The island’s finches most resemble the finch species of the nearest mainland B. The island’s finches hold the same wing bones as every other bird C. The island’s rock is about 2 million years old D. The island finch’s protein differs from the mainland finch’s at 2 positions

Question 7

A line of descent is known from two fossils: a 30-million-year-old form with 5 toes on each foot, and a 10-million-year-old form with 1 toe on each foot. Biologists then find a transitional fossil from this line, 20 million years old.

Which toe count would biologists predict for the transitional fossil?

A. 1 toe on each foot B. 3 toes on each foot C. 5 toes on each foot D. 7 toes on each foot

Question 8

A bat and a bird both fly, and both have a streamlined body. Underneath, each wing holds one upper bone, two lower bones, wrist bones and finger bones.

Which likeness is evidence that bats and birds descend from a common ancestor?

A. Both wings push against air to fly B. Both animals have a streamlined body C. Both animals are most active in the same hours of the day D. The same bones in the same order in both wings

Question 9

A vine that feeds on other plants has leaves that are tiny scales. A biologist wants to decide whether the scales are a vestigial structure.

Which comparison decides it?

A. Whether the vine’s relatives have full-sized leaves that catch light B. Whether the scales are of no use at all to the vine today C. Whether the scales are built on a different plan from a full-sized leaf D. Whether the vine grows faster than its relatives

Question 10

Three lizards compared with the ridge lizard: the percentage of a 900-base stretch of DNA and of one protein that match; the percentages are imagined for this question.

A lab compares three lizard species with the ridge lizard, using a stretch of DNA and one protein. The table gives the percentage of each molecule that matches the ridge lizard’s.

Which species shares the most recent common ancestor with the ridge lizard?

A. The rock lizard B. The shore lizard C. The dune lizard D. The rock lizard and the shore lizard equally

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 Data · 6 points

A fossil bone lies in a layer of sandstone between two layers of volcanic rock, as the drawing shows. Layer 1 is the bottom layer, and the layers are undisturbed. Each volcanic layer holds a slow isotope with a half-life of 100 million years; the table gives the share of the isotope still left in each volcanic layer.

A fossil bone in a sandstone layer between two volcanic layers; layer 1 is the bottom layer.

(a) Calculate the number of half-lives that have passed since layer 3 formed. (1 point)

Hint: Start at 100% and halve until you reach the share the table gives for layer 3; count the halvings.

The share of the slow isotope still left in each volcanic layer; the isotope’s half-life is 100 million years.

half-lives

Write down the values in the question:

isotope left in layer 3 = 50% of the amount when it formed

Count the halvings:

100% → 50%: one halving, so 1 half-life

A full-credit answer: Layer 3 holds 50% of its isotope.
100% → 50% is one halving.
So 1 half-life has passed.

(b) Calculate the age of layer 3, in million years. (1 point)

Hint: Multiply your number of half-lives from part (a) by the half-life.

million years

Write down the values in the question:

number of half-lives = 1

half-life = 100 million years

Write down the equation:

age = number of half-lives \times half-life

Substitute the values into the equation:

age = 1 \times 100 million years = 100 million years

A full-credit answer: Age = number of half-lives × half-life.
1 × 100 million years = 100 million years.

(c) Calculate the age of layer 1, in million years. (1 point)

Hint: Count the halvings for layer 1 from the table, then multiply by the half-life.

million years

Write down the values in the question:

isotope left in layer 1 = 25%

half-life = 100 million years

Count the halvings:

100% → 50% → 25%: two halvings, so 2 half-lives

Write down the equation:

age = number of half-lives \times half-life

Substitute the values into the equation:

age = 2 \times 100 million years = 200 million years

A full-credit answer: Layer 1 holds 25% of its isotope: 100% → 50% → 25% is two halvings, so 2 half-lives.
2 × 100 million years = 200 million years.

(d) Explain why the fossil must be younger than layer 1 and older than layer 3. (1 point)

Hint: Which layer settled first in an undisturbed cliff: the one beneath or the one above?

A full-credit answer: In undisturbed layers the deeper layer is the older.
The sandstone holding the fossil settled after layer 1, beneath it, and before layer 3, above it.
So the fossil is younger than layer 1 and older than layer 3.

Check the box for each point your answer earns

the deeper layer is the older, so the sandstone settled after layer 1 and before layer 3.

(e) Determine the range within which the fossil’s age lies. (1 point)

Hint: Use your two ages from parts (b) and (c) and where the fossil’s layer sits between them.

A full-credit answer: Layer 1 is 200 million years old and layer 3 is 100 million years old.
The fossil settled between the two.
So its age lies between 100 million and 200 million years.

Check the box for each point your answer earns

between 100 million and 200 million years, resting on the two layers’ ages and the fossil’s position between them.

(f) Explain why the lab reads the fossil’s age from the volcanic layers rather than from carbon-14 in the bone. (1 point)

Hint: Compare carbon-14’s half-life with the fossil’s age. How much carbon-14 could be left?

A full-credit answer: Carbon-14’s half-life is 5,730 years.
After about 50,000 years too little carbon-14 is left to measure.
The bone is over 100 million years old, so it holds no carbon-14 to measure.
The slow isotope in the volcanic rock is still there to measure.

Check the box for each point your answer earns

carbon-14 decays too fast (half-life 5,730 years), so after about 50,000 years too little is left to measure, while the volcanic rock’s slow isotope is still measurable.

Free-response score: 0 of 6

Free response 2 · Conceptual Analysis · 4 points

Suppose biologists study a line of sea-going lizards known from two fossils and one living species, as the table shows. The living species’ protein differs from a land lizard’s at 4 positions and from a sea snake’s at 20; the counts are imagined for this question.

Three forms in one line of sea-going lizards, from the oldest fossil to the living species.

(a) Describe the change in the hind limbs from the oldest form to the living species. (1 point)

A full-credit answer: From the oldest form to the living species, the hind limbs changed from four walking legs, through short legs with paddle-shaped feet, to flippers with the leg bones inside.

Check the box for each point your answer earns

the hind limbs went from walking legs (40 cm) through short paddle-footed legs (15 cm) to flippers, in age order.

(b) Explain how the 15-million-year-old fossil is evidence that one line of descent changed over time. (1 point)

A full-credit answer: The 15-million-year-old fossil’s limbs sit partway between the oldest form’s walking legs and the living species’ flippers.
A fossil whose traits sit partway between an older form and a later form is a transitional fossil.
So the series shows one line of descent changing step by step, from legs to flippers.

Check the box for each point your answer earns

the 15-million-year form is a transitional fossil (its limbs are partway between walking legs and flippers), so the line changed step by step over time.

(c) Determine which living animal, the land lizard or the sea snake, shares the more recent common ancestor with the sea-going lizards. (1 point)

A full-credit answer: Fewer differences mean a more recent common ancestor.
The living sea-going lizard differs from the land lizard at 4 positions and from the sea snake at 20.
So the land lizard shares the more recent common ancestor with the sea-going lizards.

Check the box for each point your answer earns

the land lizard, because 4 differences is fewer than 20, so less time has passed since the two lines split.

(d) Explain why the fossils and the sequence counts agreeing make the case for a land-living ancestor stronger than either line alone. (1 point)

A full-credit answer: The fossils and the sequences were measured in different ways.
So a mistake in one line does not carry into the other.
The two lines agree: the fossils show legs becoming flippers, and the sequences place the land lizard as the closest living relative.
For both to be wrong, each would have to be wrong for its own reason, so their agreement makes the case stronger.

Check the box for each point your answer earns

the two lines are independent (measured in different ways), so both being wrong is far less likely than one being wrong; their agreement strengthens the case.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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