End-of-topic test: Introduction to Natural Selection

Unit 7 · Topic 7.1 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 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. Then open the scoring guide and mark your own work against it.

Question 1

Oaks of one kind grow through a wood, with ash and birch trees among them. A second wood 30 miles away holds oaks of the same kind.

Which of the following is one population?

A. All the oaks in the first wood this year B. All the trees of every kind in the first wood this year C. The oaks of both woods together this year D. One oak together with every seedling that grows from its acorns

Question 2

A zoo has kept two kinds of antelope that look alike in one paddock for twenty years and has recorded every pairing and every birth.

Which of the following records shows that the two kinds are two species?

A. The two kinds look alike B. A male of one kind and a female of the other produce a calf most years C. No calf of a mixed pair has ever produced a calf of its own D. The calves of mixed pairs grow up healthy

Question 3

A gardener grows a bed of runner beans from seed she saves each year. The plants differ in several ways.

Which of the following differences could natural selection make more common in the bed across generations?

A. Some plants have leaves full of holes where slugs have eaten them through the summer B. Some plants have red flowers and others white, and seed from a red-flowered plant grows red-flowered plants C. Some plants are shorter at the dry end of the bed than at the end near the water butt D. Some plants have pale leaves where the fence shades them for most of the day

Question 4

A farmer puts out a new poison for the rats in a barn. It is the first time this poison has been used on these rats. A few rats survive it, and the young of those rats survive it too.

When did the allele that lets these rats survive the poison most likely arise?

A. Before the poison was put out, by a chance mutation in one ancestor B. When the poison reached each rat, as its body changed the gene to match the poison C. After several generations of eating the poison, once the rats needed the allele D. In each surviving rat separately, at the moment it first ate the poison

Question 5

Mean number of adult katydids still alive after 8 weeks in enclosures given nothing, reed bundles, fresh grass leaves, or both.

Researchers put 20 adult katydids (grasshopper-like insects) into each of 24 mesh enclosures on one meadow. Every two days they added something to each enclosure: reed bundles to hide in, fresh grass leaves to eat, both, or nothing. The table shows the mean number of adults still alive after 8 weeks.

Which of the following was in short supply for the katydids before the additions?

A. Places to hide B. Food plants C. Both places to hide and food plants D. Neither places to hide nor food plants

Question 6

Salamander larvae from one pond kept 4 or 12 to an enclosure with the same daily prey: mass gain, chases and survival after 21 days.

Researchers collected salamander larvae from one pond and kept them in enclosures of one size, 4 larvae in some and 12 in others, adding the same 24 prey animals to every enclosure each day. The table shows the results after 21 days.

Which of the following best explains the results?

A. In the enclosures with 12 larvae, the larvae bumped into one another more often, but each still got enough food to grow B. In the enclosures with 12 larvae, the crowding itself slowed growth, without changing how much each larva ate C. Every enclosure got the same food, so the larvae in both kinds of enclosure grew and survived the same D. In the enclosures with 12 larvae, each larva got less food, so the larvae competed for it and grew less

Question 7

Freshwater snails of three shell colors on one lakeshore: the number present before a dry season and the number surviving to the next wet season.

Freshwater snails of one kind live on a lakeshore that dries into cracked brown mud each dry season. Birds hunt the snails by sight. Shell color is set by alleles. The table shows how many snails of each shell color were present before one dry season and how many survived to the next wet season.

Which statement best explains the pattern of survival?

A. Snails whose shells matched the brown mud were seen less often by the birds, so a larger share of them survived B. The dry mud darkened the light tan shells during the dry season, so many light tan snails became dark brown C. Dark brown snails were the most numerous at the start, so the most of them were bound to survive D. The three shell colors survived in equal shares, and the counts differ only because the starting numbers differ

Question 8

Striped and unstriped lizards of one kind through a drought: survival, hatchlings per survivor, and the share of hatchlings with their mother’s pattern when all were raised together.

Lizards of one kind are striped or unstriped. Researchers followed 240 adults through a drought, counted each survivor’s hatchlings, and raised all the hatchlings together in one enclosure. The table shows the results.

Which of the following conclusions do the results support?

A. Striped adults survived the drought more often, and their hatchlings acquired the pattern from the drought B. The two patterns survived the drought equally, and the pattern is heritable C. Striped adults survived the drought more often, but whether the pattern is heritable cannot be told from raising the hatchlings together D. Striped adults survived the drought more often, and the pattern is heritable

Question 9

Brown and green beetles of one kind over four generations: the share of each color and the share of each color eaten by birds.

Beetles of one kind in a wood are brown or green. Researchers recorded, for four generations, the share of each color in the population and the share of each color that birds ate. The table shows the results.

Which of the following best explains why the share of brown beetles stays at 50 % although birds eat far more green beetles?

A. Meiosis and fertilization reshuffle the alleles each generation, so the two colors return to 50 % each B. The population is so large that the birds cannot change the share of each color C. Beetle color is set by the leaves the larvae eat, not by the alleles a beetle carries D. Green beetles that escape the birds change to brown, keeping the two shares even

Question 10

The share of lightly spotted fish in one lake, year by year; the dashed line marks the year the mussels arrived.

A lake holds a large population of fish of one kind, some heavily spotted and some lightly spotted; spotting is set by alleles. Birds hunt the fish from above by sight. In year 2 a mussel that strains tiny particles from the water arrived, and the water became much clearer. The graph shows the share of lightly spotted fish each year.

Which of the following best describes what happened in the fish population?

A. The clearer water changed each fish’s alleles, so heavily spotted fish became lightly spotted B. Each heavily spotted fish lost its spots during its life so that the birds could not see it in the clear water C. The birds ate fish of both kinds at random, so the rise in the share of lightly spotted fish is chance alone D. Lightly spotted fish survived and bred more than heavily spotted fish once the water was clear, so their share rose

Question 11

Four female jays of one kind: years lived, eggs laid in her life, and offspring that survived to breed.

Researchers followed four female jays of one kind in one wood for their whole lives. The table shows how long each lived, how many eggs she laid, and how many of her offspring survived to breed.

Which jay had the highest evolutionary fitness?

A. Jay W B. Jay X C. Jay Y D. Jay Z

Question 12

A biologist tags one female swift at birth and measures her wing length every summer for eight years. She also measures the average wing length of the swift population nesting in the same town every summer for thirty years.

Which of the two records could show evolution?

A. Only the record of the population B. Only the record of the one swift C. Both records D. Neither record

Question 13

Average jaw width of a beetle population over five generations, with the seeds each generation was fed.

Researchers kept a population of seed-eating beetles for five generations. From generation 2 they fed the beetles hard seeds only, and the average jaw width rose, as the table shows. To test whether the population had evolved, they collected eggs from generation 5 and raised generation 6 on soft seeds only. Generation 6 had an average jaw width of 2.8 mm.

Which of the following conclusions does the generation 6 result support?

A. The wide jaws were acquired by chewing hard seeds during each beetle’s life B. The beetles of generation 6 widened their own jaws because their parents had needed wide jaws C. The population evolved: the beetles inherited wider jaws from parents that had them D. The population did not evolve, because generation 6 was never fed hard seeds

Question 14

Beetles of one kind live on a windy island. Over many generations, the population’s average wing length has become shorter.

Which of the following best explains the change in wing length by natural selection?

A. The beetles shortened their own wings over the generations because long wings kept getting them blown out to sea B. Each beetle’s wings wore down in the constant wind, and its offspring inherited the worn-down wings C. The beetles needed shorter wings to stay on the island, so a mutation for short wings arose in each of them D. Beetles that already had shorter wings were blown away less often and left more offspring, which inherited short wings

Question 15

Trout of one kind live in a mountain lake. Over one year, the lake changes in several ways.

Which of the following changes is a change in a biotic factor for the trout?

A. The water grows 2 °C warmer over the summer B. A new kind of fish arrives that eats trout eggs C. The stream feeding the lake shrinks in a dry summer D. Silt washes over the gravel beds where the trout lay their eggs

Question 16

An annual grass over four years: the rainfall each year and the share of shallow-rooted and deep-rooted adults that survived to set seed.

An annual grass of one kind grows, sets seed and dies within a year. Root depth is set by alleles: plants are shallow-rooted or deep-rooted. Over four years, researchers recorded the rainfall and the share of each root type that survived to set seed. The table shows the results.

Which of the following claims about the grass population fits the results?

A. The population is steadily moving toward deep roots, because deep roots help in every year B. Mutations arise each year in response to the rainfall, producing the root type the year needs C. Natural selection changes direction from year to year, because the rainfall that decides survival changes D. Shallow roots are being lost from the population, because they are the less advanced root type

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 · 4 points

A researcher sets up two artificial ponds of equal size, Pond A and Pond B, with the same water, light and temperature. She puts the same amount of algae, the insects’ only food, into each pond and adds no more. She then puts 100 insect larvae of one kind into Pond A and 1,000 into Pond B. No predators live in either pond. After four weeks she counts the surviving insects and weighs them. The table shows the results.

Two artificial ponds given the same amount of algae: larvae added at the start, insects alive at week 4, and the mean mass of a survivor.

(a) Identify the one thing the researcher set differently between Pond A and Pond B. (1 point)

A full-credit answer: The number of larvae added at the start: 100 in Pond A and 1,000 in Pond B.

Check the box for each point your answer earns

the number of larvae added (100 against 1,000).

Common slip: Naming the algae or the pond size. Both ponds got the same algae and were the same size; only the number of larvae differed.

(b) Explain how the survivor counts and the survivors’ mean masses together show that the larvae competed for a limited resource. (1 point)

A full-credit answer: Pond B started with ten times as many larvae as Pond A, yet at week 4 it held about the same number of insects, 92 against 85: the food kept about 90 alive whatever the starting number.
Pond B’s survivors had a mean mass of 1.1 mg against 2.4 mg: each larva got less of the same food, so each grew less.
The larvae were competing for the same limited food.

Check the box for each point your answer earns

about the same number survived (85 and 92) whatever the starting number, AND Pond B’s survivors were lighter (1.1 mg against 2.4 mg) because each larva got less of the same food: competition for the limited food.

Accept also, for the counts: the survival share fell from 85 % in Pond A to about 9 % in Pond B, so most of Pond B’s larvae died.

Common slip: Saying the Pond B insects were smaller because more of them died. The masses are of the survivors; the survivors were lighter because each had less to eat.

(c) In a later experiment, a few larvae in a pond set up like Pond B, with 1,000 larvae and the same algae, carry an allele that lets them digest the plentiful plant debris on the pond floor. Predict how the share of insects carrying that allele changes over several generations. (1 point)

A full-credit answer: The share of insects carrying the allele rises over the generations.

Check the box for each point your answer earns

the share of insects with the allele rises (the allele becomes more common).

Common slip: Predicting that every insect gains the allele. An allele spreads through the offspring of those that carry it; it does not appear in the others.

(d) Justify your prediction, using the idea of a limited resource. (1 point)

A full-credit answer: The algae is limited, so most larvae get too little food and die before they breed.
Larvae with the allele can also eat the plentiful debris, so they get enough food, survive and breed more than the others.
Their offspring inherit the allele.
So insects with the allele make up a larger share of each new generation.

Check the box for each point your answer earns

insects with the allele get food the others cannot, so they survive and breed more; their offspring inherit the allele, so its share rises.

Common slip: Stopping at “they have more food”. The point needs the chain to offspring: they breed more, and the offspring inherit the allele.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

A marsh grass of one kind varies in how much silica (a hard, glassy mineral) it lays down in its leaves, and the amount is set by alleles. Marsh crabs eat the grass, but leaves with a lot of silica are too tough for them. Laying down silica costs the plant: high-silica plants grow more slowly and set fewer seeds than low-silica plants. For many years the crab population was large. In year 1 a virus that infects only the crabs reached the marsh. The table shows the crab population over ten years.

Marsh crabs per 100 m² of marsh, from the year before the virus arrived to ten years after.

(a) Describe the advantage that high-silica plants had over low-silica plants before year 1. (1 point)

A full-credit answer: Before year 1 the marsh held many crabs.
Crabs could not eat the tough, high-silica leaves, so high-silica plants kept their leaves while low-silica plants were grazed.
So high-silica plants survived and set seed more often than low-silica plants.

Check the box for each point your answer earns

with many crabs present, high-silica plants were grazed less, so they survived and set seed more than low-silica plants.

Common slip: Saying only that silica makes the leaves tough. The advantage is what the toughness did: fewer leaves eaten, so more survival and seed.

(b) Explain how the change in the crab population changes the balance between the benefit and the cost of silica to a plant. (1 point)

A full-credit answer: The virus killed most of the crabs: from 48 to 2 per 100 m² of marsh.
The crabs, a biotic factor, were the reason tough leaves paid: crabs could not eat high-silica leaves.
With few crabs, low-silica leaves are hardly grazed either, so silica now protects a plant from almost nothing.
Laying down silica still costs the plant slower growth and fewer seeds.
So silica now costs a high-silica plant more than it gains.

Check the box for each point your answer earns

the crabs fell from 48 to 2 per 100 m² (a biotic factor removed), so the benefit of silica (protection from grazing) is almost gone while its cost (slower growth, fewer seeds) remains.

Common slip: Saying the virus changed the grass. The virus infects only crabs; it changed the grass’s environment, and that changed what silica is worth to a plant.

(c) Predict how the share of high-silica plants in the population changes between year 1 and year 10. (1 point)

A full-credit answer: The share of high-silica plants falls between year 1 and year 10.

Check the box for each point your answer earns

the share of high-silica plants falls.

Common slip: Predicting that the share stays put because silica is still useful. It is useful only against crabs, and the crabs are almost gone.

(d) A student claims that natural selection has been making the grass population better every year since year 1. Evaluate this claim, using the data. (1 point)

A full-credit answer: The claim is not supported.
Natural selection favors whichever variation does best in this year’s conditions; it does not make a population better in any fixed sense.
With many crabs, high-silica plants did best; with the crabs gone (2 per 100 m² from year 7), low-silica plants do best.
If the crabs returned, high-silica plants would be favored again.

Check the box for each point your answer earns

the judgement (the claim is not supported) AND the ground (selection favors whatever does best in the current conditions, so the favored type changed when the crabs disappeared).

Accept with or without: that high-silica plants would be favored again if the crabs returned.

Common slip: Agreeing that low-silica plants are “better”. They do better only while crabs are scarce.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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