Unit 7 · Unit 7 end-of-unit test (also the test-out)
Suggested time: about 80 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Researchers follow every male cardinal in one wood through a breeding season and record several quantities for each male.
Which of the following quantities measures a male’s evolutionary fitness?
A magazine article says: “Over thousands of years, the oryx of the desert developed the ability to last for weeks on the water in its food so that it could survive the dry season.”
Which of the following rewrites the sentence so that it describes natural selection?
A saxifrage of one kind grows on a mountain. Its plants have either hairy leaves or smooth leaves, and the difference is heritable. Researchers sow 50 seeds of each form on a dry rock ledge and 50 of each in a damp gully, and count the plants of each form that set seed, as the table shows.
Seeds collected on the dry ledge, where most plants are hairy-leaved, are sown in a damp gully. Predict how the two forms’ shares change there over several generations.
Sea lavender grows on a saltmarsh. Its root cells pump salt out. Plants vary in how many copies of the pump protein each root cell makes, and the number is set by alleles. Rising sea levels bring saltier water farther up the marsh.
Which of the following best explains how natural selection can act on the number of pump proteins in a root cell?
In a petunia line, dwarf stems are set by a recessive allele d and tall stems by the dominant allele D. A breeder wants every plant in her line to be dwarf as soon as possible. About 25% of her plants are dwarf this year.
Which of the following breeding plans gives a line of dwarf plants only in the fewest generations?
Breeders start a line of teff, a grain, from a wild population. For ten generations they grow 200 plants and breed only from the 8 plants with the largest grains. Then researchers read five genes in 200 plants of the bred line and in 200 wild plants, and count the different alleles they find at each gene, as the table shows.
Which of the following explains the pattern in the table?
Biologists genotype 50 juncos, small birds, from one hillside at a plumage gene with two alleles, W and w. The table gives the counts.
Which of the following is the frequency of allele w in this sample?
A pond holds a large population of bluegill, a fish. During a flood, 25 bluegill are swept over a dam into a reservoir downstream that had no bluegill. Which fish were swept over did not depend on their alleles. Ten years later the reservoir holds 3,000 bluegill, and their allele frequencies at several genes differ from the pond’s.
Which of the following best describes the process that set the reservoir population’s allele frequencies?
A lake holds 80 walleye, a fish. At one gene with two alleles, G and g, the residents hold 120 copies of G. Then 20 walleye from a hatchery are released into the lake and breed with the residents. The table gives the copies each group holds.
Which of the following is the frequency of G in the lake population after the arrivals join?
Isopods, small crustaceans, live in the leaf litter on both sides of a four-lane highway. Researchers propose that the highway blocks gene flow between the two sides. They genotype 100 isopods from each side and compare the allele frequencies, and they do the same for two groups 100 m apart on the same side.
Which of the following is the null hypothesis for this study?
Suppose 4% of the 500 campion plants on a cliff have white flowers. White (w) is recessive to pink (W), the plants mate at random, and no force acts on this gene.
Which of the following is the expected number of Ww plants, the carriers?
Biologists follow the frequency of allele B in the damselfish of two reefs, J and K, for seven generations, as the graph shows.
Which reef’s allele frequency at this gene is consistent with Hardy–Weinberg equilibrium?
Biologists genotype 200 swordtails, a fish, from one stream at a fin gene with alleles S and s. The table shows the observed counts above the counts the Hardy–Weinberg model expects from the census’s own allele frequencies.
Which of the following is chi-square for the three genotype classes?
A lagoon holds 3,000 mullet, a fish. At one gene the frequency of allele T is 0.30. A storm drains most of the lagoon, and 40 mullet survive in one pool; which mullet survived did not depend on their alleles. Among the 40 survivors the frequency of T is 0.45. The survivors breed and the lagoon refills.
Which of the following Hardy–Weinberg conditions did the storm break?
A layer of volcanic rock holds a slow isotope with a half-life of 700 million years. 25% of the isotope the rock held when it formed is still left.
How old is the rock?
A lab reads the same 10 amino acids of one protein in four birds. The table shows the four readings; the dipper is the reference species, and the protein is hypothetical.
Which bird shares the most recent common ancestor with the dipper?
Suppose fossils of one line of digging mammals lie in dated rock layers, and the living aardvark is the last member of the line. The table gives each form’s age and the length of the claws on its forelimbs.
Which of the following conclusions do the fossils support?
Two groups of single-celled organisms from two ponds both have rounded cells. A student says: “The two groups share a rounded cell shape, so they must share a recent common ancestor.”
Is the student correct?
Biologists describe four newly found single-celled organisms, as the table shows.
Which find, if confirmed, would challenge the claim that all eukaryotes descend from one ancestor?
A farmer sprays a field of dock, a weed, with a herbicide for the first time. Students find the share of dock plants carrying an allele for surviving the herbicide before the spray, 0.05, and among the plants alive two weeks after it, before any of them has set seed, 0.65. They conclude that the dock population has evolved resistance.
What is the flaw in the students’ conclusion?
A museum keeps goldenrod specimens collected from one coastal headland in 1964, 1994 and 2024. Researchers measure the flower heads of 80 specimens from each year, as the table shows, and conclude that the goldenrod population has stopped evolving.
Is the researchers’ conclusion supported by the data?
The table gives three characters of five plants. The clubmoss is the outgroup. Biologists build a cladogram from the table.
Which plant do biologists add to the cladogram first after the outgroup?
Biologists compare one stretch of DNA in two kinds of coati, a mammal of the Americas, and find 15 differences. From fossil-dated splits among the coatis’ relatives they set the rate at 2.5 differences per million years, and they treat the rate as roughly steady. The count and the rate are imagined.
About how long ago did the two coati lineages split?
The cladogram shows four primates.
Which primate is the most closely related to the langur?
Suppose four pairs of populations are kept apart by four different barriers, one in each case.
Which of the following barriers is a pre-zygotic mechanism?
Suppose a plant of one kind holds 22 chromosomes in each body cell. On one grassy riverbank a failure of meiosis gives rise to plants with 44 chromosomes. The 44-chromosome plants grow beside their 22-chromosome parents, flower in the same weeks, and are visited by the same bees. Crosses between the two kinds give healthy plants with 33 chromosomes that set no seed.
Which of the following best explains how the 44-chromosome plants became cut off from the 22-chromosome plants?
Suppose necklace shells, sea snails with a rounded shell, left shells in every layer of an undisturbed column of ten rock layers. Layer 1 is the bottom layer. Biologists measure about 50 shells from each layer; the table gives each layer’s mean shell height.
Which tempo does this record show?
Suppose choughs, crow-like birds, nest in three glens. The table gives each population’s size, its number of alleles at a typical gene and the share of its birds heterozygous at that gene. A new disease of crows reaches all three glens.
Which population is the most likely to lose every one of its birds to the disease?
Suppose geologists find a stromatolite in a layer of rock now on land, about 2,950 million years old. The iron minerals in the same layer are dark and unrusted. Free oxygen began to build up in the air and sea about 2,400 million years ago.
Which of the following conclusions do the two finds together support?
Under the RNA world hypothesis, RNA could have been the earliest genetic material, because an RNA molecule can carry a copyable sequence and, folded up, speed up a reaction.
Which of the following findings, if made, would be evidence against the hypothesis?
(a)(i) Describe the source of the variation in male color that natural selection acts on in the pupfish population. (1 point)
A full-credit answer: The alleles for bright and for dull color first arose by mutation.
Sexual reproduction shuffles them into new combinations, so males differ in color.
The difference is set by alleles, so it is heritable.
Check the box for each point your answer earns
Accept: ‘heritable variation in color that arose by mutation’.
Common slip: Naming the birds as the source. The birds sort the variation; they do not make it.
(a)(ii) Explain why a male’s color affects his evolutionary fitness in a pool where wading birds hunt. (1 point)
A full-credit answer: Wading birds hunt by sight in shallow water.
A bright blue male is easier to see than a dull one, so the birds take bright males more often.
A male that is eaten before he breeds leaves fewer offspring.
So bright color lowers a male’s fitness where birds hunt.
Check the box for each point your answer earns
Scoring note: ‘bright males die more’ with no link to reproduction does not earn the point.
(b)(i) Identify an independent variable in the experiment. (1 point)
A full-credit answer: Whether wading birds were present or excluded by a net.
Check the box for each point your answer earns
Accept one of the following: the presence or absence of wading birds (open or netted); the presence or absence of plant cover in the pool (bare or planted).
Common slip: Naming the share of bright males. That is what the researchers measured, the dependent variable.
(b)(ii) Identify a negative control in the experiment. (1 point)
A full-credit answer: The netted pools, where no birds could hunt.
Check the box for each point your answer earns
Accept one of the following: the netted pools; the netted bare pool; the netted planted pool.
(b)(iii) Justify the researchers’ inclusion of a netted pool of each kind. (1 point)
A full-credit answer: A netted pool shows how bright and dull males survive when no bird hunts in a pool of that kind.
So a change in the open pool of the same kind can be attributed to the birds, not to the bare or planted pool itself.
Check the box for each point your answer earns
Scoring note: ‘it is the control’ with no purpose stated does not earn the point.
Common slip: Writing ‘it is the control’ and stopping. The point needs what the netted pool shows: the share of bright males with no birds hunting.
(c)(i) Based on the data, describe the effect of the wading birds on the share of bright males. (1 point)
A full-credit answer: In the bare pools the birds cut the share of bright males from 0.50 to 0.20.
In the planted pools the birds cut it only from 0.49 to 0.45.
Check the box for each point your answer earns
Accept one of the following: the birds lower the share of bright males (0.50 to 0.20 in the bare pools); the birds lower the share far more in the bare pools than in the planted pools (0.20 against 0.45).
Scoring note: a direction is required; ‘the birds change the share’ does not earn the point.
(c)(ii) Calculate the percent change in the share of bright males in the open bare pool compared with the netted bare pool, giving a fall as a negative value. (1 point)
Write down the values in the question:
share in the open bare pool (new) = 0.20
share in the netted bare pool (old) = 0.50
Write down the equation:
Substitute the values into the equation:
A full-credit answer: The share is 0.20 in the open bare pool and 0.50 in the netted bare pool.
(0.20 − 0.50) ÷ 0.50 × 100 = −60%.
(d)(i) The survivors in the open bare pool breed, and their offspring grow up in the same pool with the same birds. Predict how the share of bright males changes over the next several generations. (1 point)
A full-credit answer: The share of bright males keeps falling, toward zero.
Check the box for each point your answer earns
Scoring note: accept ‘falls, then levels off at a low share’ where the student argues that bright color also helps a male win a mate.
Common slip: Predicting a return to 0.50. Nothing restores the alleles of the males the birds took; the birds remove bright males every generation.
(d)(ii) Justify your prediction. (1 point)
A full-credit answer: Color is set by alleles, so the mostly dull survivors pass on mostly alleles for dull color.
Each generation the birds take the bright males before they breed.
So fewer alleles for bright color are passed on each generation, and the share of bright males keeps falling.
Check the box for each point your answer earns
(a)(i) Estimate, to the nearest million years, the age of the most recent common ancestor of the West Indian manatee and the Amazonian manatee. Answers within 1 million years are accepted. (1 point)
A full-credit answer: The node joining the two manatees sits a little short of the 5 mark on the scale.
So their most recent common ancestor lived about 4 million years ago.
(a)(ii) Identify the sea cow whose lineage split from the others first. (1 point)
A full-credit answer: The dugong.
Check the box for each point your answer earns
Common slip: Naming the African manatee. Its line joins the other manatees at 12 million years; the dugong’s joins only at the root, 25 million years ago.
(b) Determine which two sea cows a tree built from the protein counts in Table 1 alone would place as the most closely related, and state what in the table your decision rests on. (1 point)
A full-credit answer: The West Indian manatee and the African manatee.
Their count, 1, is the smallest in the table, and the pair with the fewest differences is joined at the node nearest the tips.
Check the box for each point your answer earns
(c) Justify the claim that the tree built from whole mitochondrial DNA sequences is more reliable than a tree built from the protein counts in Table 1. (1 point)
A full-credit answer: Whole mitochondrial DNA sequences compare thousands of positions across many genes: many characters.
One protein compares a few positions in one gene.
A likeness at a few positions can arise by chance, or because the protein is shaped by selection for a shared way of life.
Thousands of positions almost never come to agree by chance, so the DNA tree is the more reliable.
Check the box for each point your answer earns
Accept one of the following: the DNA comparison uses many more positions (many genes) than one protein does, so it rests on many characters; a single protein can be shaped alike by selection (a shared way of life), so its likeness need not reflect ancestry; DNA sequences vary at more positions than the protein they code for, so they carry more information.
Scoring note: ‘DNA is more accurate’ with no reason does not earn the point.
(a) Describe the barrier that stopped gene flow between the west snakes and the east snakes. (1 point)
A full-credit answer: The stretch of farmland is a geographic barrier.
Smooth snakes never cross open fields, so no snake from one heath reaches the other to mate.
So no alleles pass between the two populations.
Check the box for each point your answer earns
Accept: a pre-zygotic mechanism, with the farmland named as what keeps the two apart (the course’s framing of a geographic barrier as a barrier of the kind a different habitat).
Scoring note: ‘they live far apart’ with no barrier named does not earn the point.
Common slip: Saying the farmland changed the snakes’ alleles. The barrier changes no alleles; it stops alleles passing between the two heaths.
(b) Smooth snakes with a wider head can swallow larger lizards. For the snakes of the east heath, explain the difference in fitness between wider-headed snakes and narrower-headed snakes. (1 point)
A full-credit answer: On the east heath the small lizard has been replaced by a larger lizard.
A wider-headed snake can swallow the larger lizard, so it eats more often.
A snake that eats more often survives and leaves more offspring.
So wider-headed snakes have the higher fitness on the east heath.
Check the box for each point your answer earns
Scoring note: ‘wider heads are better’ with no link to eating and reproducing does not earn the point.
(c) Suppose the farmland is abandoned and the heath’s shrubs grow back across the cleared stretch, so snakes can cross it again. Predict whether the west snakes and the east snakes will merge back into one population or stay two separate populations. (1 point)
A full-credit answer: The two will merge back into one population.
Check the box for each point your answer earns
Scoring note: ‘the two stay separate populations’ does not earn the point. Brought together, west snakes and east snakes mate and their young grow up and breed, so the two still interbreed.
Common slip: Predicting that the two stay separate because their allele frequencies differ. The frequencies show divergence; whether the two merge depends on whether they still interbreed.
(d) Justify your prediction in part (c). (1 point)
A full-credit answer: West snakes and east snakes still mate, and their young grow up and breed, so no reproductive isolating mechanism has formed.
Once the shrubs return, snakes cross the stretch and mate with snakes of the other heath.
Their fertile young carry alleles from both heaths, so alleles pass between the two again.
Over generations the differences in Table 1 shrink, and the two are one population.
Check the box for each point your answer earns
Scoring note: a justification of ‘stay separate’ earns nothing, however it is reasoned — from the allele frequencies that differ, or from the larger lizard still on the east heath. The enclosure result shows the two interbreed with fertile young, so no isolating mechanism has formed, and alleles pass again once snakes can cross. Differing frequencies are divergence; the species test is whether the two interbreed with fertile young.