Practice questions · Topic 7.1

Unit 7 · Practice for the Topic 7.1 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: Introduction to natural selection, summed up

Video coming soon

A population and a species; heritable or acquired; more born than survive; the four steps and the name; fitness counted in offspring; populations evolve, individuals do not; the direction flips when the environment changes.

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 set of data 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.

Question 1

A field guide describes “the moose of North America”. A biologist wants to study one population of moose.

Which of the following groups is one population?

A. All the moose living in one valley this year B. All the moose living in North America this year C. All the moose that have ever lived in one valley D. All the large animals living in one valley this year

Question 2

Two groups of crickets in one meadow look identical. A student says: “They look the same, so they must be one species.”

Which of the following observations would show that the student is wrong?

A. Crickets of one group chirp faster than crickets of the other group do B. The two groups live ten meters apart in the meadow, on patches of grass of different kinds C. The young of each group grow up into adults that look just like their parents did D. Even caged together, a male of either group courts only females of his own group, and no mixed pair ever mates

Question 3

A breeder’s dog has a tightly curled tail; the dog’s littermates have straight tails.

Which observation would show that the curled tail is a heritable difference?

A. The tail curls more tightly when the dog is excited B. Puppies of this dog, raised by another family, grow curled tails C. The tail straightened for a month after an injury and then curled again D. The curl grew tighter as the dog grew from a puppy into an adult

Question 4

Four students explain why the deer in a wood with wolves are faster today than deer were a century ago.

Which of the following explanations is NOT consistent with natural selection?

A. The deer pushed themselves to go faster and faster to escape the wolves, and their fawns were born with the speed they had built up B. Deer that were already faster escaped the wolves more often and left more fawns, which inherited fast legs C. Slower deer were caught by the wolves more often, so a larger share of each generation came from fast parents D. Fawns inherited the alleles of the fast deer that survived the wolves, so fast deer became more common

Question 5

Which of the following is an abiotic factor for the population named?

A. The wolves that hunt a herd of deer B. The caterpillars that eat an oak wood’s leaves C. The depth of the winter snow in a valley where hares live D. The flies that a pond’s frogs eat

Question 6

A hole-nesting bird in one wood over four years: nest holes available, breeding pairs, eggs laid, and young surviving to adulthood.

A kind of bird nests only in tree holes. Researchers counted, in one wood over four years, the nest holes available, the breeding pairs, the eggs laid and the young that survived to adulthood. The table shows the counts.

Which of the following conclusions do the counts best support?

A. Each pair lays only as many eggs as the wood can raise to adulthood, so nearly every egg becomes an adult B. The number of breeding pairs rises and falls by chance, with no limit set by the wood C. Most of the eggs laid survive to adulthood because the wood has a hole for every pair D. More eggs are laid each year than young survive to adulthood, and the nest holes limit the breeding pairs

Question 7

Two leaf-wax types of an annual grass grown in plots beside the sea: plants at the start, plants that set seed, and mean seeds per plant that set seed.

An annual grass of one kind comes in two leaf types, low-wax and high-wax, set by alleles. Researchers grew 80 plants of each type in plots beside the sea, where salt spray reaches the leaves, and counted the plants that set seed and the seeds each produced. The table shows the results.

Which comparison most directly shows that the high-wax type will make up a larger share of the next generation?

A. 60 against 40 plants that set seed B. 840 against 480 total seeds C. 14 against 12 seeds per plant that set seed D. 80 against 80 plants at the start

Question 8

Unbanded and banded land snails released in equal numbers into a dense woodland and into open grassland: the share of each kind surviving one month.

Land snails of one kind have either unbanded shells or shells with dark bands, set by alleles. Researchers released equal numbers of each kind into a dense woodland and into open grassland. In both places, birds hunt the snails by sight. After a month they counted the survivors. The table shows the share of each kind that survived.

Which of the following conclusions fits the results?

A. Banded shells give a better chance of survival wherever a snail lives B. Birds cause snails to grow the pattern that hides them where they live C. Which shell pattern survives better depends on where the snails live D. Unbanded snails survive better in grassland because they change their pattern to match it

Question 9

A valley’s ground squirrels of one kind: the share of early, mid and late wakers in four survey years.

A valley’s ground squirrels of one kind wake from their winter sleep early, mid or late in spring, and the timing is set by alleles. Until 2010 springs were mild. From 2010 a hard frost late each spring killed most of the young born to the earliest wakers. The table shows the share of each waking time over 15 years.

Which of the following best explains the change in the population?

A. Each squirrel put off its own waking, year by year, to keep its young clear of the frost B. The frosts made new alleles for late waking arise in the squirrels that needed them C. Mid and late wakers kept their young through the frosts, so they left more offspring, which inherited their waking time D. The population shrank sharply in the first frosts, and the early-waking share fell by chance alone

Question 10

Slim and deep-bodied fish of one kind in a lake: the share of each type surviving a year with pike absent and with pike present.

Suppose a lake’s fish of one kind are slim or deep-bodied, set by alleles. Every decade or so pike, fish that swallow slim fish whole but cannot swallow deep-bodied ones, arrive in the lake, thrive for several years, then die out from a virus that infects only pike. The table shows the share of each fish type that survives a year when pike are present and when pike are absent.

Which of the following best predicts the share of deep-bodied fish over several pike cycles?

A. It rises until every fish is deep-bodied B. It stays the same across a whole pike cycle C. It falls until every fish is slim D. It rises while pike are present and falls while they are absent

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 · 5 points

A population of lizards on a rocky island has two kinds of male, set by alleles. Large males are strong and win fights over territory; small males avoid fights and hide among the rocks. Researchers followed many males of each kind for their whole lives and recorded how long each lived and how many of his offspring per year survived to breed. The table shows the averages. Evolutionary fitness is counted in the offspring an individual leaves that survive to breed.

Two kinds of male lizard in one population: average years lived and average offspring per year that survive to breed.

(a) Describe the difference between the two kinds of male in how long they live and in the offspring they leave each year. (1 point)

Hint: Read the two rows of the table.

A full-credit answer: Large males live longer, 4.0 years against 3.0, but leave fewer offspring that survive to breed each year, 2.0 against 5.0 for small males.

Check the box for each point your answer earns

large males live longer but leave fewer surviving offspring per year.

Accept with or without the values (4.0 against 3.0 years; 2.0 against 5.0 offspring per year).

Common slip: Writing only that large males win the fights. The table has no column for fights; describe the two columns it has.

(b) Calculate the average number of offspring a large male leaves in his lifetime that survive to breed. (1 point)

Hint: The per-year count is for one year, and a large male lives 4.0 years.

Write down the values in the question:

years lived = 4.0

surviving offspring per year = 2.0

Write down the equation:

lifetime offspring = years lived \times surviving offspring per year

Substitute the values into the equation:

lifetime offspring = 4.0 \times 2.0 = 8

A full-credit answer: A large male leaves 8 offspring in his lifetime that survive to breed.

(c) Calculate the average number of offspring a small male leaves in his lifetime that survive to breed. (1 point)

Hint: Use the small male’s years lived and his surviving offspring per year.

Write down the values in the question:

years lived = 3.0

surviving offspring per year = 5.0

Write down the equation:

lifetime offspring = years lived \times surviving offspring per year

Substitute the values into the equation:

lifetime offspring = 3.0 \times 5.0 = 15

A full-credit answer: A small male leaves 15 offspring in his lifetime that survive to breed.

(d) Determine which kind of male has the higher evolutionary fitness. (1 point)

Hint: Fitness is counted in one kind of thing. Which of your two lifetime totals counts it?

A full-credit answer: The small males have the higher evolutionary fitness, because a small male leaves about 15 offspring that survive to breed and a large male about 8.

Check the box for each point your answer earns

small males, on the ground that 15 lifetime surviving offspring is more than 8.

Common slip: Choosing the large males because they live longer and win fights. Fitness is counted in surviving offspring, and the large males leave fewer.

(e) Predict how the share of large males in the population changes over many generations, if conditions stay as they are. (1 point)

Hint: Which kind of male leaves more offspring, and what do those offspring inherit?

A full-credit answer: Over many generations the share of large males falls, because small males leave more offspring that survive to breed, and those offspring inherit the alleles for small size.

Check the box for each point your answer earns

the share of large males falls (small males become more common), because small males leave more surviving offspring and the kind of male is inherited.

Common slip: Predicting that large males become more common because they win the fights. Winning fights counts only if it leads to more surviving offspring, and here it does not.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

A lake holds a population of perch. Adult body length is a heritable trait. Thirty years ago a fishing club began netting the lake every summer with nets that catch only fish longer than 25 cm, and it kept every fish it caught. The graph shows the population’s average adult length, year by year, since the fishing began.

Average adult length of the lake’s perch, in centimeters, over the thirty years since netting began.

(a) Describe the change in the perch population shown by the graph. (1 point)

A full-credit answer: The population’s average adult length fell from about 30 cm to about 22 cm over the thirty years.

Check the box for each point your answer earns

the average adult length fell (with or without the values, about 30 cm to about 22 cm).

Common slip: Writing “the perch got smaller”. Say what was measured: the population’s average adult length, and the direction of the change.

(b) Explain how the netting produced this change by natural selection. (1 point)

A full-credit answer: Perch already varied in adult length, and the length was set by alleles.
The nets caught and removed fish longer than 25 cm, so shorter adults escaped and bred more often than long ones.
Their offspring inherited the alleles for shorter length.
So short adults made up a larger share of each new generation, and the average fell.

Check the box for each point your answer earns

variation in length already present and heritable; the nets removed long fish, so short fish survived and bred more; their offspring inherited short length, so the average fell.

Common slip: Writing that the perch “adapted” to the nets. Name the steps: existing variation, the nets removing long fish, short fish breeding more, offspring inheriting.

(c) A student writes: “The perch grew less because they needed to stay small enough to slip through the nets.” Evaluate the student’s explanation. (1 point)

A full-credit answer: The student’s explanation is not supported: it is the Lamarckian form.
No perch chose or changed its own length; a fish’s adult length was set by its alleles.
The nets removed the long fish, and the short fish that were already there bred more.
The population changed, not any one fish.

Check the box for each point your answer earns

the judgement (not supported) AND the ground (no fish changed itself by need; short fish already existed and the nets sorted them; the population changed, not the individual).

Accept with or without naming the Lamarckian form.

Common slip: Saying the student is right because the fish are smaller now. The outcome is right; the cause the student gives (need) is not how the change happened.

(d) Predict how the perch population would change over the next thirty years if all netting stopped. (1 point)

A full-credit answer: The average adult length stops falling.
The nets were the reason short fish left more offspring than long fish.
With the nets gone, long fish are no longer removed, so short fish no longer leave more offspring than long ones.
If long fish now leave more offspring, the average rises again.

Check the box for each point your answer earns

the average length stops falling, because the nets no longer remove long fish (the reason short fish left more offspring is gone).

Accept either prediction with its reason: the average stays near 22 cm, because nothing now favors short or long adults; OR the average rises back toward 30 cm, because long fish now survive and can leave more offspring.

Common slip: Predicting that the average keeps falling. Selection favors whatever does best in the current conditions, and without the nets short length no longer does best.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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