Practice questions · Topic 7.10

Unit 7 · Practice for the Topic 7.10 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: Speciation, summed up

Video coming soon

The biological species concept; speciation is reproductive isolation; barriers before and after the zygote; allopatric and sympatric; a barrier appears, a barrier goes; divergent, radiation, convergent; two tempos; when speciation is fast and when slow.

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

Biologists bring together members of four pairs of bird populations.

By the biological species concept, which pair of populations is two species?

A. Two skimmer populations that nest 900 km apart and, brought together, interbreed and raise chicks that breed B. Two noddy populations that differ in the color of the cap and, brought together, interbreed and raise chicks that breed C. Two golden plover populations whose hybrid chicks grow up but never raise chicks of their own D. Two booby populations that differ in the frequency of an allele at many genes and interbreed with chicks that breed

Question 2

Suppose two populations of tulip shells, a sea snail, have lived in two bays for 3,000 years. The two differ in the frequency of alleles at many genes and in the color of the shell. Brought together, tulip shells from the two populations mate, and their young grow up and breed.

Which of the following would biologists need to observe before they could say that speciation had occurred?

A. The two populations living farther apart than they do now B. A larger difference between the two populations in the frequency of an allele C. A larger difference between the two populations in the color of the shell D. Tulip shells from the two populations no longer mating, or their young failing to live or breed

Question 3

Suppose two populations of solenodons, shrew-like mammals that hunt insects at night, live in one region: one only in dry scrub, the other only in wet woodland. A solenodon keeps to its own ground, so the two populations seldom breed with each other.

Which kind of pre-zygotic mechanism keeps the two populations apart?

A. A different habitat B. A different breeding time C. A different courtship D. A mismatched anatomy

Question 4

Suppose two populations of galagos, small night-active primates, breed together. The hybrid young are born alive, and every one dies within its first month.

Which kind of reproductive isolating mechanism is this barrier?

A. A post-zygotic mechanism: the hybrid lives but is sterile B. A post-zygotic mechanism: the hybrid dies before it can breed C. A pre-zygotic mechanism: gametes that cannot fuse D. A pre-zygotic mechanism: a different courtship

Question 5

Suppose a kind of eyeless cave crustacean lives in the pools along the whole length of one cave. A roof fall blocks the passage in the middle of the cave, and no crustacean crosses the fallen rock.

Which of the following happens first after the roof fall?

A. The two sides come to differ in their allele frequencies B. The two sides can no longer interbreed C. The two sides come to look different D. Gene flow between the two sides stops

Question 6

Suppose biologists describe how the speciation began in each of four cases.

Which of the following is a case of sympatric speciation?

A. Flatfish of one bay, since some came to settle and spawn on sand and others on mud, each where it feeds B. Sand lizards of one plain, since a deep rift valley with sheer sides opened across the plain and no lizard crossed it C. Tower shells, sea snails, of two shores, since a stretch of deep, cold water that their larvae do not cross opened between the shores D. Sengis, small insect-eating mammals, of one plain, since a wide, fast torrent formed across it and no sengi swam it

Question 7

Suppose a kind of colugo, a mammal, glides between trees on a flap of skin stretched between its limbs, and a kind of lizard glides between trees on flaps of skin held out by its ribs. Both flaps catch the air the same way. The two animals are distant relatives, and their shared ancestor had no flap.

Which pattern of evolution do the two animals’ flaps show?

A. Divergent evolution B. An adaptive radiation C. Convergent evolution D. A likeness inherited from a recent shared ancestor

Question 8

A column of eight rock layers with a shell drawn in every layer; layer 1 is the bottom layer.

Suppose tun shells, sea snails with a thin rounded shell, left a shell in every layer of the undisturbed column below. Layer 1 is the bottom layer.

Which of the following does this record show?

A. Gradualism B. Stasis throughout C. Punctuated equilibrium D. Both tempos, one after the other

Question 9

Suppose one kind of shrub covers a whole plain, its pollen blown for kilometers, and the plain has changed little for thousands of years.

Which of the following changes would make speciation more likely among the shrubs?

A. Small patches of shrubs form, with no pollen passing between them B. The shrub population grows larger across the whole plain C. More new mutations arise in shrubs across the whole plain D. The plain stays unchanged for thousands more years, as it has been

Question 10

Biologists give a name to an offspring that can itself have offspring.

Which term names such an offspring?

A. A viable offspring B. A hybrid C. A fertile offspring D. A zygote

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 kind of wingless stick insect lives in one scrubland where two shrubs grow mixed together, gorse and broom. Gorse leaves are hard spines; broom leaves are soft. The young feed on the shrub they hatch on, and each adult mates on the shrub it grew on. Stick insects vary in the strength of their jaws. Over many generations the gorse insects come to have stouter jaws than the broom insects, and the two groups differ in many of their allele frequencies. A female now accepts only a male whose scent matches that of males from her own shrub. A male’s scent comes from his own body, so it stays the same whichever shrub he sits on.

(a) Describe what stopped gene flow between the gorse insects and the broom insects. (1 point)

Hint: Where does each adult mate, and which insects are there with it?

A full-credit answer: Gene flow between the two groups stopped because each adult mates on the shrub it grew on.
So gorse insects mate with gorse insects, and broom insects with broom insects, and few alleles pass between the two groups.

Check the box for each point your answer earns

each adult mates on the shrub it grew on, so the two groups rarely mate with each other and few or no alleles pass between them.

(b) Explain why the gorse insects came to have stouter jaws than the broom insects. (1 point)

Hint: Name the variation, the pressure gorse applies, who survived and bred more and why, and the heritable change.

A full-credit answer: In the gorse group, the insects that survived and bred more were those with stouter jaws, because stouter jaws cut the hard spines that gorse has for leaves.
Their young inherited the alleles for stouter jaws, so the share of stout-jawed insects rose on gorse.
On broom the leaves are soft, so no pressure favored stouter jaws there.
With few alleles passing between the groups, nothing pulled the two back together.

Check the box for each point your answer earns

on gorse, insects with stouter jaws fed on the hard spines and so survived or bred more, and their young inherited the alleles for stouter jaws (the fitness difference, with no such pressure on broom). Accept a version that also names the lack of gene flow.

(c) A female’s acceptance of only a male whose scent matches her own shrub’s males is a barrier of its own. Identify the kind of reproductive isolating mechanism it is, and give the feature of the case that decides it. (1 point)

Hint: What does a female now respond to, and does any zygote form between the groups?

A full-credit answer: The mechanism is a different courtship, a pre-zygotic mechanism, because a female accepts only a male whose scent matches her own shrub’s males.
So the other group’s mating signal does not attract her, and no zygote forms between the groups.

Check the box for each point your answer earns

a different courtship (a pre-zygotic mechanism), because a female accepts only the scent of males from her own shrub, so the two groups never mate; the name alone does not earn the point.

(d) Predict whether the two groups will interbreed if the broom dies out and the broom insects move onto the gorse. (1 point)

Hint: Sharing a shrub puts the two side by side; ask what a female still accepts.

A full-credit answer: If the broom insects move onto the gorse, the two groups will not interbreed, and they stay two species on one shrub.

Check the box for each point your answer earns

the two groups do not interbreed (accept: stay separate / two species / do not merge). A prediction about survival or jaw strength alone does not answer the task and earns no point.

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

Hint: Does putting the two groups together change what a female accepts?

A full-credit answer: The prediction in part (d) holds because a female accepts only the scent of males from her own group, and a male’s scent stays the same on the gorse.
So gorse insects and broom insects do not mate, even side by side on one shrub.
So no alleles pass between the two groups, and nothing pulls their allele frequencies back together.

Check the box for each point your answer earns

reasoning consistent with the student’s own prediction in part (d): the courtship barrier means the two do not mate even when together (a male’s scent stays the same on the gorse), so no gene flow returns and the two stay apart.

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

Suppose one population of a kind of freshwater snail lived in two ponds joined by a stream, and snails moved along the stream between the ponds. Forty years ago the stream stopped flowing, and no snail has moved between the ponds since. The snails breed once a year. Biologists sample 200 snails from each pond every twenty years and record the frequency of two alleles, as the table shows. In year 40 they also bring snails from the two ponds together: the snails mate, and their young grow up and breed.

The frequency of allele R and allele T in the west pond and the east pond in years 0, 20 and 40.

(a) Using the data in the table, identify the pond in which the frequency of allele R fell between year 0 and year 40. (1 point)

A full-credit answer: The east pond, where allele R fell from 0.50 to 0.33 over the forty years.

Check the box for each point your answer earns

the east pond (0.50 to 0.33).

(b) Describe how the frequency of allele R changed in the west pond from year 0 to year 40. (1 point)

A full-credit answer: The frequency of allele R in the west pond rose from 0.50 in year 0 to 0.62 in year 20 and 0.71 in year 40: a rise of 0.21 over forty years.

Check the box for each point your answer earns

it rose (0.50 in year 0, 0.62 in year 20, 0.71 in year 40; a rise of about 0.2), across both twenty-year intervals.

(c) A student claims that the west-pond snails and the east-pond snails are now two species. Evaluate the claim using the information given. (1 point)

A full-credit answer: The claim is not supported.
Two populations are two species only when they can no longer interbreed with viable, fertile offspring.
In year 40, snails from the two ponds mated, and their young grew up and bred.
The two populations differ in the frequency of allele R, but differing allele frequencies alone do not make two species.

Check the box for each point your answer earns

not supported, because snails from the two ponds still interbreed and their young are viable and fertile (the difference in allele frequency is not the test). A verdict alone earns no point.

(d) Explain why the frequency of allele R came to differ between the two ponds after the stream stopped flowing. (1 point)

A full-credit answer: When the stream stopped flowing, no snail moved between the ponds, so no alleles passed between the two populations: gene flow stopped.
In each pond the frequency of R wandered by chance, and the two ponds wandered different ways.
Each pond may also have applied its own selective pressures.
With no gene flow pulling the two back together, the frequencies moved apart.

Check the box for each point your answer earns

no gene flow after the stream stopped flowing, so genetic drift (chance) and / or each pond’s own selective pressures changed the frequency separately in each pond, with nothing pulling the two together.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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