End-of-topic test: Speciation

Unit 7 · Topic 7.10 end-of-topic test

Suggested time: about 46 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 pairs of populations and what happened when members of each pair were brought together.

Biologists bring together members of four pairs of populations and record what happens, as the table shows. The scoters of the two populations look alike.

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

A. The two ruff populations B. The two gunnel populations C. The two greenshank populations D. The two scoter populations

Question 2

A biologist wants to decide whether each of four pairs is one species or two.

To which pair can the biological species concept be applied?

A. Two living populations of teal, a small waterbird, that breed by mating B. Two kinds of trilobite, sea animals known only from fossils C. Two kinds of single-celled alga, each of which divides in two D. A living population of paddlefish and a fossil paddlefish from a rock layer

Question 3

Suppose two populations of a kind of gall wasp live in one wood. Biologists make four observations about the two populations.

Which observation shows that speciation has occurred?

A. The two populations differ in the frequency of an allele at twenty genes B. Wasps of the two populations never mate with each other, even when brought together C. Wasps of one population are a third larger than wasps of the other D. The two populations live in different parts of the wood

Question 4

Suppose two populations of a kind of tiger moth live on one hillside. A female moth releases a scent, and males fly toward the scent. The two populations never mate. Males of each population fly only toward the scent of females of their own population.

Which kind of pre-zygotic mechanism is this?

A. Gametes that cannot fuse B. A different breeding time C. A mismatched anatomy D. A different courtship

Question 5

Suppose biologists describe the barrier between two populations in each of four cases.

Which of the following barriers is a post-zygotic mechanism?

A. Female swallowtails of one population fly from males of the other population, and the two never mate B. Medaka, a small fish, of one population spawn in spring and of the other in autumn, and the two never spawn together C. Hybrid young of two populations of rockling, a small fish, hatch and all die long before they can spawn D. Sperm of one jingle shell population reach the eggs of the other, and no sperm fuses with an egg

Question 6

Suppose a kind of evening primrose holds 16 chromosomes in each body cell. In one plant meiosis fails, and its seeds grow into plants with 32 chromosomes. Crosses between the 32-chromosome plants and the 16-chromosome plants give plants with 24 chromosomes, and those plants set no seed.

Which of the following explains why the 24-chromosome plants are sterile?

A. A cell with 24 chromosomes holds too many chromosomes to divide by mitosis B. Three copies of each kind of chromosome cannot be shared evenly in meiosis C. The 24-chromosome plants’ pollen never reaches a flower of any other plant D. The 24 chromosomes are too few to pair up when the plant makes gametes

Question 7

Suppose a kind of small fly lays its eggs in the flower heads of two wildflowers, selfheal and woundwort, which grow mixed in one meadow, and the larvae grow inside the flower heads. Adult flies fly freely between the two plants, and flies from the two plants, paired in a cage, have fertile young. Yet in the meadow few alleles pass between the selfheal flies and the woundwort flies.

Which of the following explains why few alleles pass between the two groups of flies?

A. The two groups differ in allele frequency, so flies of the two groups cannot mate B. The two kinds of plant change the flies’ alleles directly C. Flies from the two plants have sterile young when they mate with each other D. Each adult mates on the kind of plant it grew in

Question 8

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

Which of the following is a case of allopatric speciation?

A. Antechinus, a marsupial mouse, of two woods, since an eruption buried the strip of woodland between them under bare ash B. Rockroses of one hillside, since a plant with twice the chromosome count appeared among them and set seed only with plants like itself C. Globeflowers of one meadow, since some came to flower in May and others in July D. Mollies, small fish, of one lake, since some came to spawn in the shallows and others on the lake floor, each where it feeds

Question 9

Suppose a railroad embankment, a long bank of packed earth and stone, stood across a dry plain for 150 years and divided a population of amphisbaenians, legless burrowing reptiles, in two, and no amphisbaenian crossed it. The railroad closes, and the embankment is dug away. Amphisbaenians from the two sides now mate with each other, and their young grow up and breed.

Predict what happens to the two populations over the coming generations.

A. The two populations stay two species B. The two populations become two species now that they live side by side C. The two populations merge back into one population D. The two populations cannot be judged until they have been together for 150 years

Question 10

Suppose one kind of sea holly, a coastal plant, grows on a sandy shore. Some sea hollies spread inland onto dry grassland. Over many generations the shore plants grow thicker, waxier leaves and the grassland plants grow deeper roots.

Which pattern of evolution do the shore plants and the grassland plants show?

A. Convergent evolution B. Divergent evolution C. Sympatric speciation D. A likeness inherited from a recent shared ancestor

Question 11

Suppose a chain of newly risen islands holds thirteen species of a kind of ichneumon wasp, each on its own island or food, and the nearest mainland holds many more species of the same kind of wasp. A student says the thirteen species came from thirteen separate arrivals from the mainland.

Which finding would show instead that the thirteen species arose on the islands from one founder species?

A. The thirteen island species differ from each other in size and color B. Each of the thirteen island species feeds on a different kind of insect on its own island C. The islands lie far from the mainland, across a wide stretch of open sea D. The island species are more closely related to each other than to any mainland species

Question 12

Suppose a kind of lumpsucker, a fish, clings to wave-washed rocks with a sucker on its belly, and a kind of volute, a sea snail, clings to the same rocks with its broad foot.

Which observation would show that the two animals’ grip on the rock is convergent evolution rather than a likeness inherited from a shared ancestor?

A. The sucker is built on fin bones, and the volute’s foot is a sheet of muscle with no bones B. Both animals cling to rocks that the same waves wash over every day C. Both animals hold on more tightly when the waves grow stronger D. The two animals are the same size, and both are the same color

Question 13

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

Suppose ark shells, sea animals with a two-part shell, left a shell in every layer of the undisturbed column below. Layer 1 is the bottom layer.

Which tempo does this record show?

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

Question 14

Three lakes that hold one kind of small fish: the number of fish, whether fish from every part breed together, and the habitats.

One kind of small fish lives in three lakes. The table gives each lake’s fish.

In which lake is speciation expected to be fastest?

A. The north lake B. The middle lake C. The south lake D. Equally fast in all three lakes

Question 15

Suppose a kind of harp shell, a sea snail, lives on two rocky shores 35 km apart, with sand between them that harp shells do not cross. The two populations descend from one ancestral population, known from fossils, and have been apart for many thousands of generations. Biologists find that the two current populations differ from each other at more genes than either differs from the ancestral population.

Which of the following explains that finding?

A. The ancestral population held every allele that either current population holds B. Each population gained its own changes since the split, so the two differ from each other by both sets of changes C. The sand changed the alleles on each side, so the two populations moved apart D. The ancestral population went on changing after the split, so it now differs from both current populations

Question 16

Suppose a volcano’s lava spreads across a wooded valley and cools into a field of bare black rock 4 km wide. Four kinds of living thing have populations on both sides of the field.

For which population is the field of lava a geographic barrier?

A. A kind of moth that flies across the field at night B. A kind of hawk that hunts over both sides of the field C. A kind of grass whose pollen the wind carries across the field D. A kind of caecilian, a legless amphibian that dries out on bare rock

Question 17

Biologists describe a pattern: one ancestral species gives rise to many species in a short time, as new habitats and new foods open, each new species adapted to a different one.

Which term names this pattern?

A. Divergent evolution B. An adaptive radiation C. Convergent evolution D. Sympatric speciation

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 Model or Visual Representation · 4 points

Suppose verdins, small birds of desert scrub, live in two scrublands, west and east, with a broad belt of open grassland between them. Verdins keep to scrub, and no verdin crosses the grassland. Biologists bring verdins from the two scrublands together in large aviaries, pair them, and record the crosses in the table. The biologists then keep the grown birds from each cross together, with the other grown birds from the same cross, for three breeding seasons.

Four crosses within and between the two verdin populations: eggs laid, eggs that hatched, chicks that grew up, and grown birds that later raised chicks.

(a) Describe the post-zygotic mechanism that prevents gene flow between the west verdins and the east verdins. (1 point)

A full-credit answer: Hybrid chicks of a west verdin and an east verdin hatch and grow up, but as adults they never raise chicks: the hybrids live but are sterile.
A sterile hybrid passes no alleles on, so no alleles pass between the two populations.

Check the box for each point your answer earns

the hybrids grow up but never breed (are sterile), so no alleles pass on between the two populations. Accept: the hybrids live but cannot reproduce; the hybrids are sterile.

(b) Based on the data in the table, explain how the biologists know that zygotes formed in the crosses between the two populations. (1 point)

A full-credit answer: In the two between-population crosses, 53 and 52 of 60 eggs hatched, and 47 and 46 chicks grew up.
An egg hatches only after a sperm has fused with it.
So a zygote formed in every egg that hatched.

Check the box for each point your answer earns

eggs from the between-population crosses hatched (or chicks grew up), and an egg hatches only after fertilization, so zygotes formed.

(c) Identify the kind of speciation that produced the two verdin populations. (1 point)

A full-credit answer: Allopatric speciation.

Check the box for each point your answer earns

allopatric speciation. Accept: speciation that began with a geographic barrier.

(d) Explain how the belt of grassland led to the west verdins and the east verdins becoming two species. (1 point)

A full-credit answer: No verdin crosses the grassland, so no alleles passed between the two scrublands: gene flow stopped.
In each scrubland the allele frequencies wandered by chance, and each scrubland applied its own selective pressures.
So the two populations changed separately, over many generations.
In time their hybrids were sterile: the two could no longer interbreed, and they were two species.

Check the box for each point your answer earns

the grassland stopped gene flow (no verdin crossed, so no alleles passed), so drift and each side’s own selection changed the two populations separately until they could no longer interbreed (their hybrids sterile).

Free-response score: 0 of 4

Free response 2 · Analyze Data · 4 points

Suppose bonnet shells, sea snails with a thick rounded shell, left shells in every layer of an undisturbed column of eleven rock layers. Layer 1 is the bottom layer. Biologists measure the height of about 60 shells from each layer; the table gives each layer’s mean shell height and ±2SE. The rock of layer 6 holds the first remains of a crab that crushes snail shells, and the crab’s remains are found in every layer above.

The mean shell height of bonnet shells in each of eleven layers, with ±2SE; layer 1 is the bottom layer.

(a) Using the data in the table, identify the two layers between which the mean shell height changed. (1 point)

A full-credit answer: Between layer 5 and layer 6: the mean rises from 14.0 mm to 19.6 mm.

Check the box for each point your answer earns

between layer 5 and layer 6.

(b) Using the data in the table, describe the difference between the mean shell height in layer 11 and the mean shell height in layer 1. (1 point)

A full-credit answer: The mean in layer 11 is 19.9 mm and the mean in layer 1 is 14.1 mm.
So the layer-11 shells are about 5.8 mm taller, about 40% taller.
The two ±2SE ranges, 19.5 to 20.3 mm and 13.7 to 14.5 mm, do not overlap.

Check the box for each point your answer earns

the layer-11 mean is greater than the layer-1 mean by about 5.8 mm (accept: by about 40%; accept: the two ±2SE ranges do not overlap). Accept a correct numerical comparison of the two layers.

(c) A student claims that the bonnet shells’ record shows one short burst of change between two long stretches in which the shell stayed the same. Support the claim using the data in the table. (1 point)

A full-credit answer: From layer 1 to layer 5 every mean lies within ±2SE of every other, so the shell did not change: a stasis.
The whole change came between two neighboring layers, 5 and 6: a short burst.
From layer 6 to layer 11 every mean again lies within ±2SE of every other: a stasis after the burst.
So the record shows one short burst between two stretches in which the shell stayed the same.

Check the box for each point your answer earns

the means within layers 1 to 5 (and within layers 6 to 11) lie within each other’s ±2SE, so the shell did not change over those stretches, and the whole change came between two neighboring layers. Restating the claim alone earns no point.

(d) Explain why the mean shell height stayed the same over the layers that hold the crab’s remains, although natural selection acted on the bonnet shells in every generation. (1 point)

A full-credit answer: From layer 6 on, the shell-crushing crab was present in every layer, so the bonnet shells’ conditions held steady.
In every generation the taller shells survived the crab best, and natural selection kept favoring the same shell.
A population under the same pressure keeps the same shell, generation after generation: a stasis.

Check the box for each point your answer earns

the conditions (the crab) stayed the same after layer 6, so selection favored the same shell height in every generation and the trait did not change. Accept: selection removed shells shorter than the favored height each generation, holding the mean where it was.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 17 correct.

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