Biology by Bradford
Unit 7: Natural Selection. Practice questions with scoring guidelines.
316 original AP-style questions on Unit 7, written to the CED learning objectives: 216 multiple-choice, 100 free-response. Below is a 20-mark practice set built from them, ready to assign as a unit check or homework, or to sit yourself and score against the guidelines. Print it, project it, or build a fresh one.
Topics in this unit
- 7.1 Introduction to Natural Selection 37 questions
- 7.2 Natural Selection 55 questions
- 7.3 Artificial Selection 24 questions
- 7.4 Population Genetics 27 questions
- 7.5 Hardy-Weinberg Equilibrium 32 questions
- 7.6 Evidence of Evolution 25 questions
- 7.7 Common Ancestry 26 questions
- 7.8 Continuing Evolution 25 questions
- 7.9 Phylogeny 30 questions
- 7.10 Speciation 37 questions
- 7.11 Variations in Populations 25 questions
- 7.12 Origins of Life on Earth 26 questions
In the bank for Unit 7
- 216 multiple-choice
- 100 free-response
Every question is original and tagged to a learning objective and science practice.
Make your own
The practice set
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A tree species grows on two islands. Its large fruits are swallowed by only one animal, a fruit-eating pigeon. Seeds that pass through the pigeon's gut and are dropped away from the parent tree sprout and grow well, while fruits that fall beneath the parent tree rot and few of their seeds survive. The trees take about 30 years to grow taller than 5 m and live for about 200 years. Hunting eliminated the pigeon from Island P 60 years ago; the pigeon is still common on Island Q. The table shows the number of trees of each size found in surveys of equal areas of forest on the two islands. Which prediction about the tree population on Island P is best supported?
| Island | Seedlings (under 1 m) | Saplings (1 to 5 m) | Adult trees (over 5 m) |
|---|---|---|---|
| Q (pigeon present) | 410 | 150 | 88 |
| P (pigeon eliminated 60 years ago) | 35 | 3 | 104 |
- It will persist, because Island P has more adult trees than Island Q, where the pigeon is present
- It will increase, because its seedlings now compete with far fewer seedlings of their own kind
- It will decline as the adult trees die, because few young trees are growing up to replace them
- Its trees will begin to produce seeds that no longer need the pigeon, since the pigeon is gone
A cladogram of five vertebrates places derived characters at nodes. Reading from the base: all five share vertebrae; jaws are shared by all except the lamprey; four walking limbs (tetrapod limbs) are shared by the salamander, lizard, and mouse; the amniotic egg is shared by the lizard and mouse; hair and mammary glands are unique to the mouse. The branching order of the tips is: lamprey, shark, salamander, lizard, mouse.
A student claims that the shark shares a more recent common ancestor with the mouse than the lamprey does. Using the same cladogram, which character best supports this claim?
- Jaws, which are present in the shark and mouse but absent in the lamprey
- Vertebrae, which are present in all three taxa
- The amniotic egg, which is present only in the mouse of the three
- Hair, which is present only in the mouse
A population of bacteria lives in a hot spring. A mutation produces a version of a key metabolic enzyme whose active site keeps its shape at 65 °C, whereas the ancestral enzyme denatures near 50 °C. How does this molecular variation most likely affect the population over time?
- The variation is neutral because enzyme structure does not affect survival.
- The heat-stable enzyme lowers survival because all proteins denature above 50 °C.
- The variation spreads only if the bacteria choose to express the new enzyme.
- Individuals with the heat-stable enzyme survive and reproduce more in the spring.
The maps show one population of lizards on a landmass at three times. At time 2 a river forms across the area; the two sides differ in rainfall. By time 3 the river has disappeared and the two types of lizard mingle but do not interbreed.
Which observation at time 3 provides the strongest evidence that speciation has occurred?
- The two types of lizard look different from each other
- The pink type is found only on the drier side of the area
- The two types live in the same area but do not interbreed
- The river that separated the populations has disappeared
The figure compares the bones of the forelimb of four mammals: a human, a bat, a whale and a horse. Corresponding bones are shown in the same color. The limbs are not drawn to the same scale.
What do the four forelimbs provide evidence for?
- Independent origins of the limb skeleton in each of the four mammal lineages
- Descent from a common ancestor, because the same bones occur in the same arrangement
- Analogous structures shaped by identical selection pressures in the four environments
- Convergent evolution, because the four limbs are used for very different functions
The figure shows the rock layers at two sites a few kilometers apart. None of the layers has been disturbed since it formed. Each volcanic ash layer contains isotope Z, which has a half-life of 10 million years, and the figure gives the percent of the isotope Z originally present in each ash layer that remains today. Shells of the marine snail species R are found only in rocks that formed during one short interval of time, wherever those rocks occur. Which age of the fossil teeth of mammal species K at site 2 is best supported by the data?
- Between 20 and 40 million years old
- Between 10 and 20 million years old
- Cannot be found, since no dated layer lies below the teeth at site 2
- Younger than 10 million years old
In a population of a small mammal, the length of the winter fur that an individual grows is heritable: some individuals grow long fur and others grow short fur. Over ten years, long-furred individuals raised more young after cold winters and short-furred individuals raised more young after mild winters, and cold and mild winters came in no regular order. Which statement best explains why both fur types remain common in the population?
- The fitness of each fur type depends on the conditions, which vary from year to year
- Each animal changes its fur length to match the winter of the year that it lives through
- Selection cannot act on fur length, because it does not affect the number of young raised
- Long-furred and short-furred animals have exactly the same fitness after both kinds of winter
A small fish lives in three mountain streams, P, Q, and R, which flow down separate valleys at different heights; no fish can move from one stream to another. One gene affects how well the fish tolerate warm water: fish that carry allele H survive summer water temperatures above 22 °C much better than fish that do not. The model shows the mean summer water temperature of each stream, the frequency of allele H in the population of each stream, and a plan by wildlife managers. The mean summer temperature of stream R is predicted to rise from 16 °C to 22 °C over the next 50 years, so the managers plan to move 200 fish from stream P into stream R.
The following information applies to parts C and D.
About 1,800 fish now live in stream R.
In a species of songbird, the number of eggs a female lays in one nest (the clutch size) is a heritable trait that varies among females from 3 to 7. Both parents feed the nestlings, and a nestling that does not receive enough food dies before it can leave the nest (fledge). Researchers studied a colony of these birds over five breeding seasons. For 40 nests of each clutch size they recorded the number of nestlings that fledged. The results are shown in Table 1.
| Clutch size (eggs per nest) | Mean number of nestlings fledged per nest (±SE) |
|---|---|
| 3 | 2.6 (±0.12) |
| 4 | 3.4 (±0.14) |
| 5 | 4.1 (±0.15) |
| 6 | 3.3 (±0.16) |
| 7 | 2.4 (±0.19) |
The following information applies to parts C and D.
In a second study, the researchers compared two colonies of the same species that were similar at the start of the study. In one colony, feeders stocked with insect larvae were placed near every nest during each of ten breeding seasons (supplemented colony); the other colony received no extra food (unsupplemented colony). In the tenth season the researchers recorded the mean number of nestlings fledged from nests with a clutch size of 7 and the mean clutch size of all nests in each colony. The results are shown in Table 2.
The following information applies to parts C and D.
| Colony | Mean nestlings fledged per 7-egg nest (±SE) | Mean clutch size of all nests (±SE) |
|---|---|---|
| Unsupplemented | 2.5 (±0.18) | 5.0 (±0.06) |
| Supplemented | 5.9 (±0.17) | 5.7 (±0.07) |
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Each point is worth 1 point and is credited independently. Accept any one of the listed alternatives per point; ( ) marks optional wording and / separates interchangeable wording.
- C: on Island P only 3 saplings are growing up against 150 on Island Q, so almost no trees have been recruited since the pigeon was lost; because the trees live about 200 years, the adults alive now are mostly trees that were established while the pigeon was still present, and as they die there will be too few young trees to replace them, putting the population at risk of extinction. A reads the current adults as a sign of safety and ignores the long lag before a long-lived population declines. C describes a change arising because it is needed; a new trait cannot appear in response to need, and any heritable change would take many generations of trees. D ignores the data: the seedlings on Island P are few and almost none survive to become saplings.
- A: jaws mark a node uniting the shark and mouse (and other jawed vertebrates) that excludes the jawless lamprey, so the shark and mouse share a more recent ancestor with each other than with the lamprey.
- D: Molecular variation that keeps an enzyme functional in the population's environment raises the fitness of individuals carrying it, so it tends to increase in frequency.
- C: Under the biological species concept, populations are separate species when they no longer interbreed successfully where they meet; appearance alone does not settle it.
- B: Homologous structures share an underlying anatomy inherited from a common ancestor, even when natural selection has modified them for different functions.
- B: Ash 1 keeps 25 percent of its isotope Z, two half-lives (100 to 50 to 25 percent), so it is 2 × 10 = 20 million years old; ash 2 keeps 50 percent, one half-life, so it is 10 million years old. At site 1 the R shells lie above ash 1, so the rocks with R shells formed less than 20 million years ago, and because R lived during one short interval, the R layer at site 2 is of the same age. At site 2 the teeth lie above the R shells and below ash 2, so they are younger than about 20 million years and older than 10 million years. B: the teeth lie above the R layer, which formed after ash 1, so they cannot be older than 20 million years; C: the teeth lie below ash 2, so they are older than 10 million years; D: the R shells tie the site 2 layers to ash 1 at site 1.
- A: each fur type has higher fitness after some winters and lower fitness after others, so as conditions change, selection favors first one type and then the other and neither is eliminated; the variation lets the population respond to whichever conditions occur. B contradicts the data, C contradicts the heritable difference between individuals, and D ignores that the two types raised different numbers of young.
- The warmer the stream, the higher the frequency of allele H: 0.04 at 16 °C, 0.30 at 20 °C, and 0.75 at 24 °C
- Allele H frequency increases with mean summer water temperature
Total for part (A): 1 point
- Because no fish move between the streams, there is no gene flow among the populations, so the differences produced by selection in streams of different temperature are not evened out by migration
- Each population is isolated, so its allele frequency is shaped only by what happens in its own stream; without gene flow, alleles common in the warm stream do not spread into the cold streams
Total for part (B): 1 point
- (1,800 × 0.04 + 200 × 0.75) ÷ 2,000 = (72 + 150) ÷ 2,000 = 0.111 (accept 0.11)
- Counting alleles: (3,600 × 0.04 + 400 × 0.75) ÷ 4,000 = (144 + 300) ÷ 4,000 = 0.111
Total for part (C): 1 point
- It will increase, because once summer temperatures rise above 22 °C, fish that carry allele H survive better and leave more offspring than fish without it; the transfer has made H carriers nearly three times as common, so more fish can survive the warm summers while the frequency of H rises
- Allele H will become more common, since warming water will select for heat-tolerant fish; with the added genetic variation from stream P, the population is more likely to persist through the warming instead of declining sharply while H is still rare
Total for part (D): 1 point
Total for question 8: 4 points
- Clutch size on the x-axis and mean number of nestlings fledged per nest on the y-axis, both labeled, a scale that uses most of the grid, all five means (2.6, 3.4, 4.1, 3.3, 2.4) plotted correctly and ±SE error bars drawn on each point or bar
- Correctly labeled and scaled graph (bar or scatter) with the five means plotted accurately and error bars of ±SE shown
- A curve rising from 2.6 at clutch size 3 to a peak of 4.1 at clutch size 5 and falling to 2.4 at clutch size 7
- A smooth hump-shaped trend line with its maximum at clutch size 5
Total for part (A): 2 points
- The number of nestlings fledged increases with clutch size from 2.6 (clutch of 3) to a maximum of 4.1 (clutch of 5) and then decreases for larger clutches, to 3.3 at 6 eggs and 2.4 at 7 eggs
- Fledging success is highest for the intermediate clutch size of 5 and lower for both smaller and larger clutches; nests with 7 eggs fledge fewer nestlings (2.4) than nests with only 3 eggs (2.6)
- Yes: the ±2 SE intervals do not overlap (5 eggs: 4.1 ± 0.30 = 3.80 to 4.40; 6 eggs: 3.3 ± 0.32 = 2.98 to 3.62), so the difference is unlikely to be due to chance
- The two means are significantly different because the error bars (±SE, and even ±2 SE) of 4.1 and 3.3 do not overlap
- Parental age or experience / territory quality (food availability near the nest) / the date the eggs were laid / weather in a given season / predation at the nest: any of these also affects how many nestlings survive, so if it differed systematically among clutch-size groups it could confound the effect of clutch size
- Nests should be compared within the same seasons and habitat (or the researchers should record laying date and parental age), because food supply and weather vary and could cause differences in fledging success that are not due to clutch size
Total for part (B): 3 points
- Extra food removed the disadvantage of large clutches, so selection now favors larger clutch sizes (selection became directional toward larger clutches) instead of favoring the intermediate clutch size of 5
- In the supplemented colony, large clutches are no longer selected against, and the mean clutch size of the population has increased
- In the supplemented colony 7-egg nests fledged 5.9 nestlings compared with 2.5 in the unsupplemented colony (and only 2.4 in Table 1), and the mean clutch size rose to 5.7 versus 5.0 in the unsupplemented colony; the ±2 SE intervals (5.56 to 5.84 versus 4.88 to 5.12) do not overlap
- 7-egg clutches now fledge more nestlings (5.9) than 5-egg clutches did in Table 1 (4.1), and the population mean clutch size is 0.7 eggs larger in the supplemented colony (5.7 versus 5.0), a difference far larger than the standard errors
Total for part (C): 2 points
- Parents of large clutches cannot bring enough food for all their nestlings, so many nestlings starve and only 2.4 fledge from a 7-egg nest, fewer than the 4.1 that fledge from a 5-egg nest; because clutch size is heritable, females laying 5 eggs leave the most surviving offspring, so alleles for an intermediate clutch size remain the most common and the mean stays near 5
- Fitness (nestlings fledged) is highest for the intermediate clutch size, so individuals at both extremes (3 and 7 eggs) contribute fewer offspring to the next generation; this stabilizing selection keeps the population mean at the clutch size with the highest fledging success
- The mean clutch size would decrease from 5.7 back toward about 5, because without extra food 7-egg nests would again fledge only about 2.4 nestlings, so females carrying alleles for large clutches would leave fewer offspring than females laying 5 eggs and the frequency of those alleles would fall over generations
- Mean clutch size would return toward 5.0 (the value in the unsupplemented colony) as the original selection against large clutches, which starve when food is limited, is restored; the change requires several generations because it depends on differential reproduction of heritable clutch-size variants
Total for part (D): 2 points
Total for question 9: 9 points
Other AP Biology units
- Unit 1 Chemistry of Life 196
- Unit 2 Cell Structure and Function 260
- Unit 3 Cellular Energetics 195
- Unit 4 Cell Communication and Cell Cycle 187
- Unit 5 Heredity 203
- Unit 6 Gene Expression and Regulation 219
- Unit 8 Ecology 215
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