Practice questions · Topic 7.4

Unit 7 · Practice for the Topic 7.4 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: Population genetics, summed up

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The gene pool as every copy of a gene; an allele frequency as a share of the copies; evolution measured as a change in allele frequencies; mutation, genetic drift, gene flow and natural selection, each with its tell-tale; the null hypothesis that says no effect.

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 investigation one part at a time, and you can open a hint for each part; the second is a question at the test’s level. Then open the scoring guide and mark your own work against it.

Question 1

A population’s gene pool for a fin-shape gene holds 260 copies of the gene.

How many individuals are in the population?

A. 65 B. 130 C. 260 D. 520

Question 2

Genotypes of the 75 damselflies at the wing-spot gene.

Biologists record the genotype of 75 damselflies at a wing-spot gene with two alleles, W and w. The table gives the counts.

Which of the following is the frequency of allele w?

A. 0.16 B. 0.36 C. 0.56 D. 0.72

Question 3

The shell-color alleles and the number of beetles over 30 generations.

Biologists track a shell-color gene with two alleles, B and b, in an isolated population of ground beetles for 30 generations. The table gives their counts.

Which of the following conclusions do the data support?

A. The population did not evolve at this gene, because its allele frequencies stayed about the same B. The population evolved at this gene, because the number of beetles changed from generation to generation C. The population evolved at this gene, because both alleles were present in every generation D. Whether the population evolved cannot be judged, because the number of beetles kept changing

Question 4

The dandelions in a garden vary in leaf shape. A gardener pulls up every dandelion in the front flower bed and leaves the back bed untouched.

Is the loss of dandelions from the garden random or selective, and why?

A. Selective, because the gardener chose which dandelions to pull B. Selective, because only the dandelions in the front bed were pulled C. Random, because which dandelion was pulled did not depend on its phenotype D. Random, because nothing caused the dandelions to be pulled

Question 5

The frequency of allele R in six enclosures of 20 snails.

Researchers set up six enclosures with 20 snails each, all drawn from one population where allele R is at 0.50, and keep the enclosures apart under the same conditions. The table gives the frequency of R in each enclosure.

Which of the following explains why the six frequencies differ after six generations?

A. Snails carrying R survived better in some enclosures than in others B. The six enclosures started at different frequencies of R C. New mutations changed R into another allele in some enclosures D. Each enclosure’s few breeders were a chance sample of its gene pool

Question 6

How the 24 ponds were stocked.

A researcher stocks 24 ponds with water fleas from one source, as the table shows, and no water flea moves between ponds. She records the frequency of allele M in every pond for 15 generations.

Which of the following best predicts the frequencies of M after 15 generations?

A. The 12 large ponds show a wider range of frequencies than the 12 small ponds B. The 12 small ponds all keep the same frequency, because they all started at 0.50 C. The 12 small ponds show a wider range of frequencies than the 12 large ponds D. Every pond’s frequency of M rises, because M was drawn from the source most often

Question 7

The island’s ground beetles and the frequency of allele A before the storm, among the survivors and after.

A storm floods an island and drowns most of its 480 ground beetles, leaving 45. Which beetles drowned did not depend on their color. The table gives the number of beetles and the frequency of allele A, for a dark shell, before the storm, among the survivors, and in the next two generations.

Which of the following explains the change in the frequency of A between the beetles before the storm and the 45 survivors?

A. The 45 survivors carried a chance sample of the population’s alleles B. Dark-shelled beetles bred more than light-shelled beetles after the storm C. New mutations made extra copies of A during the storm D. The 45 survivors were the only beetles that carried A before the storm

Question 8

The frequency of allele M in the stock and in the four colonies.

Researchers start four colonies of flour beetles by drawing 25 beetles blind for each colony from one large stock where allele M is at 0.50. They keep the colonies apart under the same conditions. The table gives the frequency of M just after setup and after eight generations, when each colony held 40 beetles.

Which of the following explains why the colonies’ frequencies of M differed just after setup?

A. Each colony’s own conditions favored allele M to a different degree B. New mutations arose in the beetles as each colony was set up C. Drawing 25 beetles removed alleles from the stock D. Each draw of 25 beetles was a chance sample of the stock’s alleles

Question 9

The frequency of allele G in a valley’s population of ground squirrels rose from 0.32 in one year to 0.46 the next. A student says the rise was gene flow.

Which of the following observations, if true, would support the student’s claim?

A. The valley’s population of ground squirrels is small B. Squirrels carrying G survived the winter more often than squirrels without it C. Squirrels from a neighboring valley, where G is at 0.85, arrived that year and bred D. An allele never seen before appeared in one litter that year

Question 10

A biologist claims that gene flow keeps two populations’ allele frequencies alike. She grows twelve pairs of pots of mustard plants. In six pairs a fine screen keeps bees from flying between the two pots; in the other six pairs bees fly freely between them. After ten generations she compares the two pots’ frequencies of a leaf-shape allele in every pair.

Which of the following results would support the claim?

A. The two pots’ frequencies differ by about the same amount in screened pairs and open pairs B. Every pot’s frequency rises over the ten generations C. The two pots’ frequencies differ more in the open pairs than in the screened pairs D. The two pots’ frequencies differ more in the screened pairs than in the open pairs

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 · Scientific Investigation · 6 points

A meadow holds 250 plants of a wildflower. A flower-color gene has two alleles, B and W. The table gives the genotype counts.

Genotypes of the 250 wildflowers at the flower-color gene.

(a) Calculate the number of copies of the flower-color gene in the meadow population. (1 point)

Hint: How many copies of a gene does one diploid plant carry?

Write down the values in the question:

250 plants

Write down the equation:

copies in the pool = 2 \times number of plants

Substitute the values into the equation:

copies in the pool = 2 \times 250 = 500

A full-credit answer: copies in the pool = 2 × 250 = 500

(b) Calculate the frequency of allele W in the meadow population, to two decimal places. (1 point)

Hint: Count the copies of W from the WW plants and from the BW plants before you divide.

Write down the values in the question:

160 BB plants, 70 BW plants, 20 WW plants

250 plants

Write down the equation:

frequency of W = \frac{copies of W}{copies in the pool}

Substitute the values into the equation:

frequency of W = \frac{20 \times 2 + 70 \times 1}{500} = \frac{110}{500} = 0.22

A full-credit answer: copies of W = 20 × 2 + 70 × 1 = 110
frequency of W = 110 ÷ 500 = 0.22

(c) A student claims that the frequency of W is 0.08 because 20 of the 250 plants are white. Evaluate the student’s claim. (1 point)

Hint: Which plants carry a copy of W without being white?

A full-credit answer: The claim is wrong.
The student divided 20 white plants by 250 plants, which gives the share of plants that are WW, not the share of copies of the gene that are W.
Each BW plant carries one copy of W too.
A frequency is a share of the 500 copies, and 110 of them are W.

Check the box for each point your answer earns

the judgement that the claim is wrong AND its ground — 20 ÷ 250 is the share of plants that are WW (a share of individuals), while a frequency is the share of all copies of the gene, so the BW plants’ copies of W must be counted.

Common slip: Saying the student should have divided 20 by 500.

(d) Fifty plants from a second meadow are planted among the 250 and cross with them. The 50 plants carry 10 copies of W among their 100 copies of the gene. Calculate the frequency of W in the meadow after they join, to two decimal places. (1 point)

Hint: Pool the copies of W and pool all the copies before you divide.

Write down the values in the question:

residents: 110 copies of W of 500 copies

arrivals: 10 copies of W of 100 copies

Write down the equation:

frequency of W = \frac{copies of W in residents + copies of W in arrivals}{copies in residents + copies in arrivals}

Substitute the values into the equation:

frequency of W = \frac{110 + 10}{500 + 100} = \frac{120}{600} = 0.20

A full-credit answer: copies of W after joining = 110 + 10 = 120
copies in the pool after joining = 500 + 100 = 600
frequency of W = 120 ÷ 600 = 0.20

(e) Identify the process by which the planted flowers changed the meadow’s gene pool. (1 point)

Hint: What do we call it when individuals move into a population and breed there?

A full-credit answer: The process is gene flow.

Check the box for each point your answer earns

gene flow. Accept ‘migration followed by breeding’.

Common slip: Naming migration alone, without the breeding that moves the alleles.

(f) Explain why the frequency of W in the meadow changed when the planted flowers crossed with the residents. (1 point)

Hint: Compare the arrivals’ share of W with the meadow’s share before they joined.

A full-credit answer: The planted flowers carried W at 10 of 100 copies, a share of 0.10, which is lower than the meadow’s 0.22.
Their copies joined the meadow’s pool, so the pool now holds more copies with a lower share of W.
So the frequency of W fell, toward 0.10.

Check the box for each point your answer earns

the arrivals’ copies carried W at a lower share (0.10) than the meadow’s, so pooling them lowered the frequency of W (toward 0.10).

Common slip: Saying the frequency fell because the meadow now has more plants.

Free-response score: 0 of 6

Free response 2 · Conceptual Analysis · 4 points

A heatwave dries most of a lake and kills all but 30 of its 4,000 water snails. Which snail survived did not depend on its alleles. The lake refills, and over 15 years the population grows back to its old size. A second lake nearby holds many more of the same snails, and no snail moves between the lakes. The table gives the frequency of allele K in each population.

The frequency of allele K in the first lake before the heatwave, among the 30 survivors, and in the second lake of about 6,000 snails.

(a) Describe how the 30 survivors’ frequency of allele K came to differ from the first lake’s before the heatwave. (1 point)

A full-credit answer: The heatwave killed whichever snails it reached, and which snail survived did not depend on its alleles.
So the 30 survivors carried a chance sample of the lake’s copies of the gene, and by chance 36 of their 60 copies were K, 0.60, where the lake’s frequency was 0.40.

Check the box for each point your answer earns

the 30 survivors were a chance (random) sample of the lake population’s alleles, so their frequency of K differed by chance. Accept ‘the bottleneck effect’ named together with the chance sample.

Common slip: Saying snails carrying K survived the heat better.

(b) Explain why the population that grew back from the 30 survivors is likely to hold fewer alleles at many of its genes than the lake population did before the heatwave. (1 point)

A full-credit answer: Thirty snails carry 60 copies of each gene.
An allele carried by few snails before the heatwave was likely missing from those 60 copies.
The population grew back from those copies alone, and a missing allele arises again only by mutation, so the recovered population holds fewer alleles.

Check the box for each point your answer earns

the 30 survivors carried only 60 copies of each gene, so rare alleles were likely absent from them, and the recovered population descends from those copies only. Accept with or without ‘a lost allele returns only by mutation or by arrivals’.

Common slip: Saying the population has fewer alleles because it is still small.

(c) Predict whether the frequency of allele K is more likely to change by chance over the next five years in the first lake, as it grows back from the 30 survivors, or in the second lake. (1 point)

A full-credit answer: The first lake’s frequency of K is more likely to change by chance.

Check the box for each point your answer earns

the first lake (the population growing back from the 30 survivors).

Common slip: Predicting the second lake because 0.35 is further from 0.60.

(d) Justify your prediction. (1 point)

A full-credit answer: The first lake’s population is small for years after the heatwave, so the snails that breed each year are a chance sample of few copies.
A small sample often misses the lake’s frequency, so K swings by chance.
The second lake’s thousands of breeders are close to a perfect sample, so its frequency barely moves.

Check the box for each point your answer earns

the recovering population is small, so its breeders are a chance sample that swings the frequency (genetic drift is strongest in small populations), while the large lake’s breeders are close to a perfect sample. Accept ‘drift is stronger in the smaller population’ with the size named.

Common slip: Justifying with the second lake’s lower frequency rather than its size.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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