Unit 7 · Topic 7.4 end-of-topic test
Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
The drawing shows every copy of a body-color gene carried by one population of woodlice. Each token is one copy of the gene.
How many woodlice are in the population?
Biologists catch 25 marsh beetles and record each beetle’s genotype at a wing-color gene with two alleles, R and r. The table gives the counts.
Which of the following is the frequency of allele R in this sample?
Biologists sample 100 adult toads from each of four isolated colonies in 2018 and again in 2024, and record each toad’s genotype at a gene with two alleles, T and t. The table gives the counts.
Which colony’s samples show that the colony evolved at this gene between 2018 and 2024?
Which of the following changes to a population is selective rather than random?
A meadow holds a large population of a wildflower. Allele R of a gene for a cell-surface protein protects a plant from a virus. The table gives the frequency of R over 20 years; the virus first reached the meadow in year 10.
Which of the following conclusions about how allele R arose do the data support?
A butterfly lives on two islands. Island A holds about 30 butterflies and island B about 2,500. The graph shows the frequency of an allele for orange wings on each island over eight generations.
Which of the following explains why the frequency changed more on island A than on island B?
A biologist keeps four colonies of flour beetles under the same conditions. The colonies differ only in how many beetles breed each generation. The table gives the frequency of allele R in each colony over ten generations.
Which colony most likely has the fewest breeding beetles?
A disease kills all but 40 of a marsh’s 5,000 terrapins. Which terrapin caught the disease did not depend on its alleles. Over the next 60 years the population grows back to 5,000 terrapins.
Which of the following is most likely true of the recovered population?
Which of the following distinguishes a founder effect from a bottleneck effect?
Migrating birds carry nine burr seeds in their feathers from a field of 20,000 burr plants to a mountain meadow 30 km away that had none. The seeds grow, and the meadow population is now 6,000 plants. Allele P is at 0.32 in the field.
Which of the following best predicts the frequency of allele P in the meadow population?
Which of the following is an example of gene flow?
Suppose 70 grass snakes live on a heath. Of their 140 copies of a stripe gene, 56 are allele B. Then 30 snakes arrive from a second heath and breed with the residents; the arrivals carry 48 copies of B among their 60 copies of the gene.
Which of the following is the frequency of B on the heath after the arrivals join?
A weir has kept a river’s upstream sticklebacks and downstream sticklebacks apart for a century. In generation 4 a fish ladder opens past the weir, and sticklebacks swim both ways and breed. The graph shows the frequency of allele S in each population.
Which of the following explains why the two frequencies move toward each other after generation 4?
A biologist stocks eight small ponds with 40 periwinkles each from one shore, where allele S, for a striped shell, is at 0.50. She adds a crab that eats periwinkles to four of the ponds and no crab to the other four. The table gives the frequency of S in each pond ten generations later.
Which of the following best predicts the result if every pond had been stocked with 4,000 periwinkles instead of 40?
The frequency of allele W, for wide wings, in a marsh’s dragonfly population fell from 0.42 to 0.27 over five years. A student says the change was natural selection.
Which of the following observations, if true, would show that the change was genetic drift instead?
A biologist keeps sixteen cages of flour moths, each cage divided into two halves by a screen. In eight cages the screen has holes that moths pass through; in the other eight the screen is solid. After ten generations she measures the difference between the two halves’ frequencies of a wing-spot allele in each cage.
Which of the following is the null hypothesis for this experiment?
The graph shows the frequency of allele A, one of the two alleles of a coat gene, in a population of harvest mice over five generations.
Which of the following is the frequency of allele a in generation 5?
Forty chipmunks live in a wood. At a stripe gene with two alleles, P and p, 4 chipmunks are PP, 24 are Pp and 12 are pp.
How many copies of allele p does the population’s gene pool hold?
(a) Describe the change in the frequency of allele R in the sprayed field from generation 3 to generation 10. (1 point)
A full-credit answer: The frequency of R in the sprayed field rose every generation from generation 4, from 0.12 in generation 3 to 0.90 in generation 10.
Check the box for each point your answer earns
Common slip: Describing the unsprayed field, or the generations before spraying began.
(b) Describe the pattern in the frequency of allele R in the unsprayed field over the ten generations. (1 point)
A full-credit answer: The frequency of R in the unsprayed field stayed close to 0.12 in every generation, moving only between 0.11 and 0.15, with no rise or fall.
Check the box for each point your answer earns
Common slip: Reading the small wobbles as a rise or a fall.
(c) A student claims that natural selection, rather than genetic drift, changed the frequency of R in the sprayed field. Support the claim with evidence from the graph. (1 point)
A full-credit answer: The rise began in generation 4, the first generation born after spraying began in generation 3, and continued in one direction every generation after.
The unsprayed field, the same size, stayed near 0.12 for ten generations.
Drift moves a frequency up and down by chance and would act in both fields alike, so a rise in one direction in the sprayed field only is natural selection by the insecticide.
Check the box for each point your answer earns
Common slip: Restating that the insecticide favored R without pointing at the graph.
(d) Explain why the change in the sprayed field counts as evolution. (1 point)
A full-credit answer: Evolution, measured, is a change in a population’s allele frequencies across generations.
In the sprayed field the frequency of R changed across generations.
So the population evolved at this gene.
Check the box for each point your answer earns
Common slip: Saying the grasshoppers ‘became resistant’ without naming the change in allele frequency.
(a) Describe how the 16 arrivals’ frequency of allele C came to differ from the mainland population’s. (1 point)
A full-credit answer: The ship carried whichever 16 geckos climbed aboard, and which gecko climbed aboard did not depend on its alleles.
So the 16 carried a chance sample of the mainland’s copies of the gene: 16 of their 32 copies were C, a frequency of 0.50, where the mainland’s is 0.30.
Check the box for each point your answer earns
Common slip: Saying geckos with the dark throat patch were more likely to board the ship.
(b) Explain why the island population is likely to hold fewer alleles at many of its genes than the mainland population does. (1 point)
A full-credit answer: Sixteen geckos carry 32 copies of each gene.
An allele carried by few mainland geckos was likely missing from those 32 copies.
The island population grew from those copies alone, and a missing allele arises again only by mutation, so the island holds fewer alleles.
Check the box for each point your answer earns
Common slip: Saying the island population lost alleles because it is far from the mainland.
(c) Predict how the frequency of allele C on the island would change over the following generations if geckos from the mainland port began arriving on cargo ships every year and breeding on the island. (1 point)
A full-credit answer: The island’s frequency of C would move toward the mainland’s 0.30.
Check the box for each point your answer earns
Common slip: Predicting a rise because more geckos carry C onto the island.
(d) Justify your prediction. (1 point)
A full-credit answer: Each arriving gecko that breeds adds its copies of the gene to the island’s gene pool: gene flow.
The arrivals carry C at the mainland’s 0.30, below the island’s frequency, which began at the founders’ 0.50.
So each year’s arrivals add copies with a lower share of C, and the pooled frequency moves toward 0.30.
Check the box for each point your answer earns
Common slip: Justifying with drift in the small island population.