Practice questions · Topic 7.8

Unit 7 · Practice for the Topic 7.8 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: Continuing evolution, summed up

Video coming soon

Nothing has stopped evolving; four kinds of evidence that it is still happening; resistance is selection we can watch; pathogens escaping immunity and new diseases emerging; predicting what a changed regime does; why counts go on a log scale, reading one and plotting one; finding the flaw in a study and proposing the next one.

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 count and every table in these questions is imagined for the question.

Question 1

The frequency of allele R in a hoverfly population sampled every four years after it reached a river island; the frequencies are imagined.

Biologists genotype 150 hoverflies in each sampling year from a population that reached a newly formed river island in 2005. The table gives the frequency of allele R.

Which statement do the data support?

A. The frequency changed because each hoverfly’s alleles changed during its life B. The population stopped evolving in 2010, once the allele’s frequency had risen C. The population kept evolving after it reached the island, because the allele’s frequency kept changing D. The population evolved little, because the frequency in 2022 is close to the frequency in 2010

Question 2

Biologists group the evidence that evolution continues today into four kinds. A fly that breeds in manure heaps on farms now survives an insecticide that killed nearly all of the flies twelve years ago.

Which kind of evidence is this case?

A. Genomic change over time B. The evolution of resistance C. Continuous change in the fossil record D. Pathogens evolving into emergent diseases

Question 3

Frozen samples of a fungus of stored apples: how many of 1,000 cells carried an allele for surviving the fungicide, before and after its first use in 2012.

A fungicide was first used in 2012 against a fungus that grows on stored apples. Researchers read the fungus’s DNA in frozen samples from three years, as the table shows.

What do the data show?

A. The allele’s share rose by chance, because 1,000 cells is a small sample B. The fungicide made the allele in 2012, and the allele then spread through the whole population year by year C. The fungus got used to the fungicide, so more cells carried the allele each year D. The allele was in the population before the fungicide, and its share rose while the fungicide was used

Question 4

A dog shelter treats every dog for fleas every month. It proposes instead to treat only the dogs on which fleas are seen.

Compared with treating every dog every month, how would resistance to the treatment change among the shelter’s fleas, and why?

A. Resistance would rise more slowly, because fewer fleas are exposed, so there are fewer rounds of selection B. Resistance would rise faster, because the untreated dogs let the resistant fleas breed without any check C. Resistance would rise faster, because fleas on untreated dogs get used to the treatment’s smell D. Resistance would stay the same, because each monthly treatment still kills the same share of the fleas it reaches

Question 5

Health officers describe four diseases of people.

Which of the following is an emergent disease?

A. A disease of farm animals that began infecting people in one coastal district two years ago; its human cases double each year B. A disease that has caused about 300 cases a year in one inland province for as long as records go back C. A disease whose usual medicine now fails in one patient in ten, while its cases and the places where it occurs have not changed D. A disease found across a whole continent for centuries, whose cases fall a little each decade

Question 6

To test whether a gull population on a cliff is evolving, students genotype 30 gulls: 25 chicks from five nests and their 5 mothers. They compare the allele frequencies with those of 30 gulls genotyped on the cliff twenty years ago.

What is the flaw in the new sample?

A. Chicks should have been left out, because a chick’s alleles are still changing as it grows B. Thirty gulls is too small a sample to estimate any allele frequency in a whole cliff’s population C. The sample should have come from a different cliff, so that the students’ own cliff did not bias it D. Most of the 30 gulls are siblings, so the sample counts a few families’ alleles many times over

Question 7

A biologist plots bacterial counts on a log-scale y-axis.

Which two counts sit exactly one step apart on the axis?

A. 3,000 and 4,000 cells per mL B. 3,000 and 30,000 cells per mL C. 3,000 and 6,000 cells per mL D. 30,000 and 60,000 cells per mL

Question 8

Bacteria in an opened bottle of juice, counted every two hours and plotted on a log scale; no values are printed.

A student leaves a bottle of juice open and counts the bacteria in it every two hours. The graph plots the counts on a log scale; its minor gridlines sit at 2, 3 and 5 times each mark.

What is the count at 4 hours?

A. 500,000 cells per mL B. 3,000,000 cells per mL C. 5,000,000 cells per mL D. 5,500,000 cells per mL

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 laboratory counts the copies of a virus in a culture of cells once a day for four days. The table gives the counts. A student plots them on a log-scale y-axis whose marks run from 10 to 1,000,000 copies per mL; its minor gridlines sit at 2, 3 and 5 times each mark.

Copies of a virus per mL of a cell culture on five days; the counts are imagined.

(a) Identify the two marks between which the day-2 count sits, and the two minor gridlines between which its point goes. (1 point)

Hint: Find the two marks 4,000 lies between, then name the minor gridlines of that step (2, 3 and 5 times the lower mark) that 4,000 lies between.

A full-credit answer: The day-2 count, 4,000 copies per mL, sits between the 1,000 mark and the 10,000 mark.
Within that step the minor gridlines are 2,000, 3,000 and 5,000.
4,000 is more than 3,000 and less than 5,000, so its point goes between the 3,000 gridline and the 5,000 gridline.

Check the box for each point your answer earns

between the 1,000 and 10,000 marks, between the 3,000 and 5,000 gridlines.

(b) Explain why the student chose a log scale rather than a linear y-axis reaching 600,000 copies per mL. (1 point)

Hint: Work out what share of a 600,000-high axis the counts 70 and 600 would take up, then say what a log scale does differently.

A full-credit answer: The student chose a log scale because on a linear y-axis each equal step adds the same number of copies.
70 and 600 copies per mL are both less than 1% of 600,000.
So on the linear axis both points would sit on the x-axis, on top of each other.
On a log scale each tenfold change takes one equal step, so every count can be read.

Check the box for each point your answer earns

on a linear axis to 600,000 the counts 70 and 600 would lie against the x-axis (each under 1% of the axis), while a log scale gives each tenfold step the same space, so every count can be read.

(c) Calculate the count that sits exactly one step above the day-2 count on the log scale. (1 point)

Hint: One step up a log scale multiplies the count by ten.

copies per mL

Write down the values in the question:

day-2 count = 4,000 copies per mL

one step up a log scale = a tenfold change

Write down the equation:

count one step up = count \times 10

Substitute the values into the equation:

count one step up = 4, 000 \times 10 = 40, 000 copies per mL

A full-credit answer: One step up a log scale multiplies the count by ten.
4,000 × 10 = 40,000 copies per mL.

(d) The day-2 point sits between two marks. Determine which of those two marks it is nearer, and give the reason. (1 point)

Hint: Halfway between two marks on a log scale is about 3 times the lower mark, not the value halfway by adding.

A full-credit answer: Nearer the 10,000 mark.
Halfway between two marks on a log scale is about 3 times the lower mark: about 3,160 copies per mL here.
4,000 is more than 3,160, so its point sits in the upper half of the step, nearer the 10,000 mark.

Check the box for each point your answer earns

nearer the 10,000 mark, because halfway up the step is about 3,160 (3 times the lower mark) and 4,000 is above it.

(e) Describe what the finished graph’s y-axis label must give. (1 point)

Hint: Three things: what is counted, in what unit, and what kind of scale the axis is.

A full-credit answer: The label must give the quantity, its unit and the kind of scale: count (copies per mL, log scale).

Check the box for each point your answer earns

the quantity counted, its unit (copies per mL) and the words log scale.

Free-response score: 0 of 5

Free response 2 · Scientific Investigation · 4 points

A student on a fruit farm wants to know whether ten years of one insecticide raised the share of mealybugs carrying an allele for surviving it. This spring, before the year’s first spraying, she genotypes 40 mealybugs collected across the farm: 36 carry the allele. From the 36 of 40, she claims that the insecticide raised the share, and that it made the allele in the first place.

(a) Identify a flaw in the student’s claim. (1 point)

A full-credit answer: The student has only this spring’s share.
She has no share from the years before the farm began spraying.
So she cannot show that the share rose at all.

Check the box for each point your answer earns

either: she has only one sample, taken after ten years of spraying, and no share from before the insecticide, so she cannot show a rise; or a rise in the allele’s share would not show that the insecticide made the allele (a mutation makes an allele; the insecticide can only raise its share).

(b) The student’s teacher suggests genotyping 40 mealybugs that were frozen from the farm before the insecticide was first used. Justify the suggestion. (1 point)

A full-credit answer: The frozen mealybugs give the allele’s share on the farm before the insecticide was used.
Comparing that share with this spring’s 36 of 40 shows whether the share rose.
It also shows whether any mealybug carried the allele before the insecticide, which tests the claim that the insecticide made it.

Check the box for each point your answer earns

the frozen sample gives the share before the insecticide, so the before-and-after comparison shows whether the share rose (and whether the allele existed before the insecticide).

(c) Predict the share of carriers the frozen sample would show if the student’s claim that the insecticide made the allele were right, and give the reason. (1 point)

A full-credit answer: Zero carriers.
If the insecticide had made the allele, no mealybug carried it before the first spraying.
So a sample frozen before the spraying would hold no carriers.

Check the box for each point your answer earns

no carriers (zero) — if the insecticide made the allele, none existed before the spraying.

(d) The frozen sample’s result comes back: 3 of the 40 mealybugs carry the allele. Evaluate the student’s claim that the insecticide made the allele. (1 point)

A full-credit answer: The claim is not supported.
Three mealybugs in 40 carried the allele before any spraying, so a mutation made it earlier.
The insecticide killed the mealybugs without the allele and left the carriers to breed, so it raised the allele’s share.
It selected the allele; it did not make it.

Check the box for each point your answer earns

not supported — carriers were present before the spraying, so a mutation made the allele earlier and the insecticide raised its share (selected, not created).

Free-response score: 0 of 4

Multiple choice checked: 0 of 8 correct.

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