Practice questions · Topic 7.11

Unit 7 · Practice for the Topic 7.11 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: Variations in populations, summed up

Video coming soon

How much variation a population holds; why a mixed population survives a bad year; one clone, one blight; useless now, needed later; justifying the claim; genetic diversity is not species diversity.

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 case that opens with ‘Suppose’ is imagined for the question.

Question 1

Two marjoram populations: the number of alleles each carries at three genes.

Suppose marjoram, a small herb, grows on two old field terraces. The table below gives each population’s number of alleles at three genes.

Which population is the more genetically diverse?

A. The lower-terrace population B. The upper-terrace population C. The two populations are equally diverse D. The two cannot be compared until the plants on each terrace are counted

Question 2

Suppose a new fungus reaches a population of sea spurge, a beach plant, that carries eleven alleles at a typical gene. Three plants in a hundred survive the fungus.

Which of the following do the surviving plants have in common?

A. A change the fungus made in them as it spread B. A place at the edge of the beach, where the fungus arrived last C. A larger size than the plants that died D. An allele that let them resist the fungus, carried before the fungus came

Question 3

Suppose a disease kills 96% of a population of brittlestars, sea animals related to starfish. The survivors breed, and within ten years the population is as large as it was. Then a second, different disease reaches the population.

Which of the following describes the rebuilt population when the second disease arrives?

A. The rebuilt population is less likely than the original population to contain survivors of the second disease B. The rebuilt population is more likely than the original population to contain survivors of the second disease C. The rebuilt population carries as many different alleles as the original population carried D. Each brittlestar in the rebuilt population is immune to the second disease

Question 4

Suppose water soldier, a floating plant, carries an allele for larger leaves. In the sunny pond, plants with the allele set more seed than the other plants. In the shaded pond, they set less seed. In the weedy pond, they set the same amount of seed as the other plants. In the windy pond, they set more seed.

In which pond is the large-leaf allele a deleterious allele?

A. The sunny pond B. The weedy pond C. The shaded pond D. The windy pond

Question 5

Suppose a nursery keeps one line of wild thyme, a small creeping herb, and carries its seed from year to year. The line carries a rare allele. On the nursery’s present ground, plants with the allele grow slightly fewer flowering shoots than the other plants. The grower plans to remove the allele by keeping seed only from plants free of it.

Which of the following does the line lose if the grower removes the allele?

A. One of its genes, so every plant carries fewer genes B. Some of its flowering shoots, which the line grows fewer of afterward C. Nothing that could ever raise the line’s fitness D. An allele that may raise fitness if the line’s conditions change

Question 6

Two tayra populations: animals now, alleles at a typical gene, and the share of animals heterozygous at it.

Suppose tayras, weasel-like mammals, live along two rivers. The table below gives each population’s size, its number of alleles at a typical gene and the share of its animals heterozygous at that gene. A new disease of tayras reaches both rivers.

Which claim do the data support?

A. The Dingle River population is more likely to survive the disease B. The Dell River population is more likely to survive the disease C. The two populations are equally likely to survive the disease D. The data allow no claim without each population’s history

Question 7

Suppose a pond holds 43 species of plant and animal.

Which of the following events raises the pond’s species diversity?

A. A species of water plant not seen there before settles in the pond B. The pond’s newt population gains a new allele by mutation C. The pond’s newt population doubles in size D. More of the pond’s newts become heterozygous at a typical gene

Question 8

Suppose colobus monkeys live in two river valleys. The Umber River population carries nine alleles at a typical gene; the Quartz River population carries two. A new disease of monkeys reaches both valleys. Four students justify the claim that the Umber River population is more likely to survive the disease.

Which of the following answers earns full credit?

A. Umber River monkeys carry nine alleles at a typical gene; Quartz River monkeys carry two. So the Umber River population is the more genetically diverse, and so it survives the disease. B. The Umber River population is far more likely than the Quartz River population to survive the disease. Some Umber River monkeys are likely to carry an allele that lets them survive, and those monkeys rebuild the population. C. Umber River monkeys carry nine alleles at a typical gene; Quartz River monkeys carry two. So some Umber River monkeys are likely to carry an allele that lets them survive the disease, and those monkeys rebuild the population. D. Umber River monkeys carry nine alleles at a typical gene; Quartz River monkeys carry two. So every Umber River monkey is better armed against the disease than every Quartz River monkey.

Question 9

Suppose a survey of a slough, a marshy backwater, reports four findings.

Which of the following findings is the slough’s species diversity?

A. In the slough’s water-violet population, 36% of the plants are heterozygous at a typical gene B. The slough’s water-violet population carries nine alleles at a typical gene C. The slough’s water-violet population holds 3,000 plants D. The slough holds 41 species of plant

Question 10

Suppose a species of dragonet, a small sea fish, has populations in three bays. A spill kills every dragonet in one bay, and the species lives on in the other two.

Which term names what happened to the dragonets of that bay?

A. A local extinction B. The extinction of the species C. A decline D. A bottleneck

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 · Conceptual Analysis · 5 points

Suppose a population of a kind of grass grows on a hillside downhill from an old mine. The grass carries a rare allele that lets a plant grow in soil that holds zinc. On ordinary soil, plants with the allele grow more slowly and set less seed than the other plants. Then rain washes zinc-rich mine tailings, the waste from the mine, down over the hillside, and zinc builds up in the soil. On zinc-rich soil, plants without the allele wither before they flower, while plants with the allele grow and set seed.

(a) Identify the kind of allele the zinc-tolerance allele is on ordinary soil. (1 point)

Hint: Read what the plants with the allele do on ordinary soil, seed for seed, against the other plants.

A full-credit answer: On ordinary soil the zinc-tolerance allele is a deleterious allele.

Check the box for each point your answer earns

a deleterious allele (on ordinary soil).

(b) Explain why the allele stayed rare while the soil was ordinary. (1 point)

Hint: Which plants set more seed on ordinary soil, and whose alleles does more seed carry forward?

A full-credit answer: The allele stayed rare because plants with the allele set less seed than the other plants on ordinary soil.
So each generation the allele passed to fewer seeds than the other alleles at the gene.
Its frequency fell slowly, and it stayed low.

Check the box for each point your answer earns

plants with the allele set less seed on ordinary soil, so the allele passed to fewer offspring each generation and its frequency stayed low (or fell slowly).

(c) Predict what happens to the frequency of the zinc-tolerance allele once zinc builds up in the hillside’s soil. (1 point)

Hint: Which plants set seed on zinc-rich soil?

A full-credit answer: Once zinc builds up in the soil, the frequency of the zinc-tolerance allele rises.

Check the box for each point your answer earns

it rises (increases).

(d) Justify your prediction in part (c). (1 point)

Hint: Compare the seed set of plants with and without the allele on zinc-rich soil.

A full-credit answer: The prediction in part (c) holds because on zinc-rich soil the plants without the allele wither before they flower, so they set no seed.
The plants with the allele grow and set seed.
So the allele passes to more of the next generation’s seeds than the other alleles, and its frequency rises.

Check the box for each point your answer earns

reasoning consistent with the student’s own prediction in part (c): on zinc-rich soil only plants with the allele set seed (the others wither before flowering), so the allele passes to a greater share of the next generation.

(e) A second population of the same grass, on a nearby hillside, lost the zinc-tolerance allele by chance many generations ago. Zinc-rich tailings now wash over that hillside too. Explain why the second population is worse placed than the first. (1 point)

Hint: Ask which plants survive on zinc-rich soil, and whether the second population has any.

A full-credit answer: The second population is worse placed because none of its plants carries the zinc-tolerance allele.
On zinc-rich soil every plant without the allele withers before it flowers.
So no plant in the second population sets seed, and there are no survivors to rebuild it.
The first population kept the allele, so some of its plants survive and rebuild it.

Check the box for each point your answer earns

no plant in the second population carries the allele, so none survives the zinc to set seed and rebuild the population (the first population, which kept the allele, has plants that do).

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

Suppose powan, a lake fish, live in four lakes. For each population, biologists read the share of fish heterozygous at each of ten genes; the bar chart below shows each population’s mean share across the ten genes, with error bars of ±2SE. The Playa Lake population was founded forty-five years ago from sixteen powan carried there from another lake.

Four powan populations: the mean share of fish heterozygous across ten genes, with ±2SE error bars.

(a) Based on the bar chart, identify the population with the greatest mean share of fish heterozygous. (1 point)

A full-credit answer: The Coulee Lake population, with a mean of 44%.

Check the box for each point your answer earns

Coulee Lake (44%).

(b) Based on the bar chart, determine which of the other populations have a mean share heterozygous that is statistically the same as the Playa Lake population’s. (1 point)

A full-credit answer: The Gulch Lake population only.
Its error bar, 9% to 19%, overlaps Playa Lake’s, 5% to 13%.
The error bars of Coulee Lake and Arroyo Lake lie far above Playa Lake’s and do not overlap it.

Check the box for each point your answer earns

Gulch Lake only, because its error bar overlaps Playa Lake’s and the other two do not. Both the decision and the overlap it rests on are needed.

(c) Explain how the founding of the Playa Lake population from sixteen fish accounts for its mean share heterozygous. (1 point)

A full-credit answer: Only sixteen fish founded the Playa Lake population.
So the population began with only the alleles those sixteen fish happened to carry: few different alleles at each gene.
A fish is heterozygous at a gene only if two different alleles are there to inherit.
With few alleles at each gene, few Playa Lake fish are heterozygous at any gene: on average only 9% of them.

Check the box for each point your answer earns

the sixteen founders carried only some of the source lake’s alleles, so the population holds few different alleles at each gene and few fish can be heterozygous. Accept: a founder effect or a bottleneck named with the loss of alleles.

(d) A new disease of fish reaches all four lakes. Explain why the population with the greatest mean share heterozygous is the most likely to keep living fish. (1 point)

A full-credit answer: A greater share heterozygous means the population carries more different alleles at its genes.
A population with more alleles is more likely to contain some fish whose alleles let them survive the disease.
Those fish breed and rebuild the population.
So the population with the greatest share heterozygous is the most likely to keep living fish.

Check the box for each point your answer earns

more alleles (the greater share heterozygous) make it more likely that some fish carry an allele that lets them survive the disease, and those survivors rebuild the population.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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