End-of-topic test: Variations in Populations

Unit 7 · Topic 7.11 end-of-topic test

Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question, pick one option. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. 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. Then open the scoring guide and mark your own work against it. Every case that opens with ‘Suppose’ is imagined for the question.

Question 1

Four garganey populations: birds now, alleles at one gene, and the share of birds heterozygous at that gene.

Suppose garganey, small ducks, breed on four salt pans. The table below gives each population’s size, its number of alleles at one gene and the share of its birds heterozygous at that gene.

Which population is the most genetically diverse?

A. The Cinder Pan population B. The Sienna Pan population C. The Marl Pan population D. The Russet Pan population

Question 2

Two shanny populations: the fish sampled and, of those, the fish heterozygous at one gene.

Suppose shannies, small shore fish, live in the rock pools of two headlands. Biologists sample each population and count the fish heterozygous at one gene, as the table shows.

Which population is the more genetically diverse?

A. The Bluff Head population B. The Knoll Point population C. The two samples differ too much in size to be compared D. The alleles at the gene must be counted before the two can be compared

Question 3

Suppose pasqueflowers grow on two buttes. The Jasper Butte population carries seven alleles at a typical gene, and 46% of its plants are heterozygous at it. The Agate Butte population carries two alleles, and 3% are heterozygous. A new leaf disease of pasqueflowers reaches both buttes in the same spring.

Which of the following is the most likely outcome two years later?

A. Both buttes lose every plant B. Agate Butte keeps living plants, and Jasper Butte loses every plant C. Both buttes keep living plants D. Jasper Butte keeps living plants, and Agate Butte loses every plant

Question 4

Suppose biologists record four facts about a population of conger, a sea fish, before a new disease of fish reaches its coast.

Which fact best predicts whether the population will keep living fish after the disease?

A. How many fish the population holds B. How many other species of fish share the coast C. How old the population’s oldest fish are D. How many different alleles the fish carry at their genes

Question 5

Suppose a species of sifaka, a lemur, is down to 45 animals in one guarded valley, and no one hunts them there. A biologist says the species is still at risk.

Which of the following is the risk the biologist means?

A. Forty-five animals are too few for natural selection to act on B. The 45 animals’ alleles will change to match the valley C. None of the 45 animals is likely to carry an allele that resists a new disease D. Each of the 45 animals carries fewer genes than a sifaka of a large population

Question 6

Suppose a company raises one line of sole, a sea fish, in sea cages. The line was bred for many generations from a few chosen parents. A new skin disease of fish reaches the cages.

Which of the following is the most likely outcome?

A. Nearly every sole in the cages dies B. About half of the sole die, chosen by chance C. The sole the disease reaches first die, and the rest become resistant D. Few of the many sole in the cages die

Question 7

Suppose a herd of topi, antelope, lives in a fenced park. The whole herd descends from four animals released there a century ago. The herd has grown to 900, and no disease has troubled it.

When is the herd at risk of dying out?

A. Only if the herd falls to a few dozen animals B. When a new disease of antelope reaches the park C. Only while the herd was small, in its first years in the park D. At no time, after a century without a disease

Question 8

Suppose a population of shad, a river fish, carries a rare allele that lets a fish take up oxygen from water that holds little of it. In the cool, well-aired river the fish with the allele grow more slowly and lay fewer eggs than the other shad. Then the river warms, and warm water holds less oxygen: the fish with the allele now survive the summers and lay more eggs than the other shad.

Which of the following describes the allele in the two conditions?

A. The allele is adaptive in the cool river and deleterious in the warm river B. The allele is deleterious in the cool river and adaptive in the warm river C. The allele is deleterious in both rivers D. The allele changed into a different, adaptive allele when the river warmed

Question 9

Suppose a population of sea rocket, a beach plant, carries a rare allele for paler flowers. In the present conditions the paler plants set slightly fewer seeds than the other plants. Over many generations the allele’s frequency falls slowly, but the allele stays in the population.

Which of the following explains why the allele stays in the population for so many generations?

A. Plants with the allele set fewer seeds than the other plants, not none, so the allele passes to some seeds every generation B. Natural selection removes no allele from a population, however much it lowers fitness C. The allele raises the fitness of half the plants that carry it and lowers the fitness of the other half D. The allele changes a little each generation, so the present conditions never act on the same allele twice

Question 10

Suppose a population of bush-crickets carries an allele for a faint stripe along the back. For many generations striped and unstriped crickets leave the same number of young. Then a kind of lizard arrives that picks crickets off the grass by sight, and it misses the striped crickets.

Which of the following describes the stripe allele before and after the lizards arrive?

A. Neither adaptive nor deleterious before the lizards, and neither after B. Adaptive before the lizards, and adaptive after C. Neither adaptive nor deleterious before the lizards, and adaptive after D. Deleterious before the lizards, and adaptive after

Question 11

Suppose a population of mining bees, small solitary bees, carries a rare allele that lets a bee fly in the cool air after sunrise. The flowers the bees visit open in mid-morning, so the early-flying bees find little nectar and raise slightly fewer young than the other bees. Then a plant whose flowers open at sunrise and close by mid-morning spreads over the meadow, and the early-flying bees feed on it and raise more young than the other bees.

Which of the following describes the frequency of the early-flight allele over the generations after the new plant arrives?

A. The frequency falls, generation after generation B. The frequency stays where it was before the new plant arrived C. The frequency reaches 100% in the first generation after the plant arrives D. The frequency rises, generation after generation

Question 12

Two storm-petrel populations: birds now, and each population’s history.

Suppose storm petrels, small seabirds, nest on two sea stacks. The table below gives how many birds each population holds now, and each population’s history. A new disease of seabirds reaches both stacks.

Which claim do the data support?

A. The Indigo Stack population is more likely to survive the disease B. The Ivory Stack 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 until the alleles at a gene are counted

Question 13

An exam question asks how a bayou’s species diversity has changed since fertilizer from nearby farms began to reach it. A student answers that the bayou’s bladderwort population now carries fewer alleles at a typical gene than it did.

Which measure did the student write about?

A. Genetic diversity B. Species diversity C. Population size D. Allele frequency

Question 14

Four statements describe living things.

Which of the following statements is about species diversity?

A. A population of titi monkeys carries fewer different alleles than it once did B. A population of monkshood carries thirteen alleles at a typical gene C. In a population of weevers, small sea fish, 34% are heterozygous at a typical gene D. A playa, a desert lake, holds 37 species of shorebird

Question 15

Suppose a millpond holds 120 species of plant and animal, and its population of frogbit, a floating plant, carries one allele at most of its genes.

Which of the following is true of the frogbit population?

A. The frogbit population’s genetic diversity is as high as the millpond’s species diversity B. The frogbit population’s genetic diversity is low C. The frogbit population’s species diversity has fallen D. The frogbit population carries about 120 alleles at a typical gene, one for each species

Question 16

Biologists count how many different species live in one place.

Which term names this count?

A. Species diversity B. Genetic diversity C. Genetic variation D. Population size

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 · 4 points

Suppose sandwort, a small cushion plant, grows on two mesas. The table below gives each population’s size now, its number of alleles at a typical gene, the share of its plants heterozygous at that gene, and its history. A new root disease of sandwort is spreading toward both mesas.

Two sandwort populations: plants now, alleles at a typical gene, the share of plants heterozygous at it, and each population’s history.

(a) Make a claim about which population is more likely to survive the disease. (1 point)

A full-credit answer: The Flint Mesa population is more likely to survive the disease.

Check the box for each point your answer earns

the Flint Mesa population.

(b) Support your claim with evidence from the table. (1 point)

A full-credit answer: Flint Mesa plants carry eight alleles at a typical gene, and 47% are heterozygous; Slate Mesa plants carry two alleles, and 3% are heterozygous.
The history says the same: Slate Mesa grew back from 19 plants sixty years ago, and Flint Mesa was never fewer than 200.

Check the box for each point your answer earns

Flint Mesa carries more alleles at a typical gene (eight against two) or has the greater share heterozygous (47% against 3%). Accept the history column as the evidence: Slate Mesa grew back from 19 plants sixty years ago.

Score part (b) against the student’s own claim in part (a): award the point for evidence from the table that bears on that claim’s population; the size of the populations (3,100 against 240) is not evidence of genetic diversity. A wrong claim loses its point in part (a) only.

(c) Provide reasoning to justify your claim. (1 point)

A full-credit answer: A genetically diverse population is more likely to contain some individuals whose alleles let them survive a new pressure, and those individuals rebuild it.
So some Flint Mesa plants are likely to carry an allele that lets them survive the root disease.
Those plants set seed and rebuild the population.
Slate Mesa’s plants carry two alleles, so nearly all of them live or die together.

Check the box for each point your answer earns

the more diverse population is more likely to contain plants whose alleles let them survive the disease, AND those plants rebuild the population (both halves needed: survive AND rebuild). ‘Because it is more genetically diverse’ names the label, not the mechanism, and earns no point.

Score part (c) against the student’s own claim and evidence: award the point for reasoning that links that evidence to that claim through the mechanism.

(d) Explain why the Slate Mesa population carries few alleles at a typical gene today. (1 point)

A full-credit answer: Sixty years ago only 19 Slate Mesa plants were alive.
Only those 19 plants set seed, so only the alleles those plants happened to carry passed into the plants that grew back.
An allele carried only by plants that had died was gone.
So the population of 3,100 today carries few alleles at a typical gene: two.

Check the box for each point your answer earns

the mechanism: only the 19 plants alive sixty years ago bred, so only the alleles those plants carried passed into the regrown population (alleles carried only by the plants that died were lost), and the growth since then added plants, not alleles. Accept: a bottleneck, with the loss of alleles stated. The history read back from the table (‘it grew back from 19 plants’) without the mechanism earns no point.

Free-response score: 0 of 4

Free response 2 · Analyze Data · 4 points

Suppose biologists study 24 populations of lousewort, a meadow plant, across one region. From old counts they sort the populations into two groups: twelve that were never fewer than 1,000 plants, and twelve that were once fewer than 50 plants and have since grown back. In each population they read the share of plants heterozygous at a typical gene. Then a new disease of lousewort sweeps the region, and two years later they count the share of each population still alive. The box-and-whisker plots below show the two readings for the two groups.

Upper panel: the share of plants heterozygous at a typical gene in each group of populations, before the disease. Lower panel: the share of each population still alive two years after the disease.

(a) Based on the box-and-whisker plots, identify the median share of plants heterozygous at a typical gene among the populations that were once fewer than 50 plants. (1 point)

A full-credit answer: The median is 10% of plants heterozygous.

Check the box for each point your answer earns

10% (accept 9% to 11%).

(b) Based on the box-and-whisker plots, identify the group in which the share of plants heterozygous varied more from population to population. (1 point)

A full-credit answer: The populations that were never fewer than 1,000 plants: their shares lie between 24% and 58%, a wider spread than the 3% to 19% of the other group.

Check the box for each point your answer earns

the populations never fewer than 1,000 plants. Accept either comparison, with the values a reader would quote: the whiskers (about 24% to 58%, a span of about 34 points, against about 3% to 19%, about 16 points) or the boxes (about 32% to 48%, about 16 points, against about 6% to 14%, about 8 points).

(c) A student claims that every population that was once fewer than 50 plants lost more than half of its plants to the disease. Based on the data in the box-and-whisker plots, evaluate the student’s claim. (1 point)

A full-credit answer: The claim is not supported.
The median share alive in that group is 30%, so most of those populations did lose more than half.
But the upper whisker reaches 70% alive, so at least one population that was once fewer than 50 plants kept more than half of its plants.
So the claim that every one of them lost more than half goes beyond the data.

Check the box for each point your answer earns

not supported, because the top of the range for that group is about 70% alive, so at least one such population kept more than half its plants (the median, about 30%, shows only that most lost more than half). A verdict alone earns no point.

(d) Explain how the difference in the share of plants heterozygous between the two groups accounts for the difference in the share of plants alive two years after the disease. (1 point)

A full-credit answer: The populations never fewer than 1,000 plants had the greater share heterozygous, so they carried more different alleles.
A population with more alleles is more likely to contain some plants whose alleles let them survive the disease.
So a greater share of those populations was still alive two years later.
The populations once fewer than 50 plants carried few alleles, so few of their plants were likely to carry a surviving allele, and more of those populations were lost.

Check the box for each point your answer earns

the group with the greater share heterozygous carried more different alleles, so it was more likely to contain plants whose alleles let them survive the disease, and so kept a greater share alive (accept the turned-round form for the low-diversity group: few alleles, so few or no plants carried a surviving allele).

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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