Biology by Bradford
Unit 7: Natural Selection. Practice questions with scoring guidelines.
149 original AP-style questions on Unit 7, written to the CED learning objectives: 118 multiple-choice, 31 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 3 questions
- 7.2 Natural Selection 26 questions
- 7.3 Artificial Selection 13 questions
- 7.4 Population Genetics 13 questions
- 7.5 Hardy-Weinberg Equilibrium 24 questions
- 7.6 Evidence of Evolution 14 questions
- 7.7 Common Ancestry 5 questions
- 7.8 Continuing Evolution 6 questions
- 7.9 Phylogeny 19 questions
- 7.10 Speciation 15 questions
- 7.11 Extinction 6 questions
- 7.12 Variations in Populations 11 questions
- 7.13 Origins of Life on Earth 11 questions
In the bank for Unit 7
- 118 multiple-choice
- 31 free-response
Every question is original and tagged to a learning objective and science practice.
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The practice set
Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.
Individuals from a mainland population that has a high frequency of a particular allele repeatedly migrate to and interbreed with a small island population that originally lacked that allele. Over generations the island's allele frequency shifts toward the mainland value. Which mechanism most directly changed the island population's allele frequency?
- An increased mutation rate on the island
- Gene flow (migration between populations)
- Genetic drift within the island population
- Non-random mate choice on the island
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?
- The amniotic egg, which is present only in the mouse of the three
- Hair, which is present only in the mouse
- Vertebrae, which are present in all three taxa
- Jaws, which are present in the shark and mouse but absent in the lamprey
Radiometric dating places the oldest generally accepted microbial fossils at about 3.5 billion years old, and Earth's formation at about 4.6 billion years ago. What does the gap between these figures most reasonably indicate?
- Life must have arrived from elsewhere, because a billion years is too short for cells to arise chemically
- There was a period of roughly a billion years during which chemical processes could give rise to the first cells
- Cells existed from the moment the Earth formed but left no trace until rocks became able to preserve them
- Radiometric dating cannot be applied to rocks older than 3.5 billion years, so the gap is only apparent
A river changes course and divides a beetle population into two groups that no longer meet. Over many generations the two groups accumulate different mutations and are eventually unable to interbreed. This process is
- sympatric speciation
- allopatric speciation
- artificial selection
- convergent evolution
Two populations of the same bacterial species are exposed to an antibiotic. Population X contains several resistance alleles at low frequency; population Y contains none. Which outcome is most likely?
- X survives and recovers while Y is largely eliminated
- Both populations develop resistance during the exposure
- Y survives because it has not been weakened by carrying resistance alleles
- Neither population survives, because resistance must be acquired from other species
A single herbicide is applied across millions of hectares of farmland for many years. Weed populations increasingly consist of herbicide-resistant plants, while susceptible variants become rare. This outcome is best described as an example of how humans can
- directly cause new resistance mutations to appear in exposed plants on demand
- eliminate all evolutionary change in the affected populations
- increase the overall genetic diversity of the weed populations
- impose selective pressures that drive the evolution of resistant populations
After a mass extinction eliminated the large dinosaurs, the previously small and ecologically limited mammals rapidly diversified into many new forms occupying a wide range of habitats and diets. This burst of diversification following the extinction is best described as
- genetic drift eliminating all mammalian genetic variation
- convergent evolution of unrelated survivors into a single body form
- adaptive radiation into newly available ecological niches
- the inheritance of characteristics acquired during the survivors' lifetimes
A biologist compares the nucleotide sequences of the same gene in humans, chimpanzees, and mice and finds that humans and chimpanzees differ far less from each other than either differs from mice. Which category of evidence for common ancestry is the biologist using?
- Molecular evidence based on DNA sequence similarities
- Biogeographical evidence based on where the species live
- Evidence from directly observing speciation in the laboratory
- Fossil evidence from dated sedimentary rock layers
The maps show one population of lizards on a landmass at three times. At time 2 a river forms across the area; the western side is wetter and the eastern side drier. By time 3 the river has disappeared and the two types of lizard mingle but do not interbreed.
Individual plants of Brassica rapa differ in the number of hairs (trichomes) on their stems. A class grew a population of 100 plants from a commercial seed lot, counted the trichomes on each plant 14 days after germination, and calculated a population mean of 4.2 trichomes per plant. The students then cross-pollinated only the 10 plants with the highest trichome counts, collected their seed, and grew 100 offspring plants.
Cytochrome c is a protein of about 104 amino acids found in the mitochondria of all aerobic eukaryotes. The table shows the number of amino acid differences between the cytochrome c of five vertebrates, and the phylogenetic tree was constructed from these data.
| human | rhesus monkey | horse | chicken | tuna | |
|---|---|---|---|---|---|
| human | — | 1 | 12 | 13 | 21 |
| rhesus monkey | 1 | — | 11 | 12 | 21 |
| horse | 12 | 11 | — | 11 | 19 |
| chicken | 13 | 12 | 11 | — | 18 |
| tuna | 21 | 21 | 19 | 18 | — |
Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · AP® is a trademark registered by the College Board, which was not involved in and does not endorse this site.
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Markscheme BbB-IABAAAAAABQAGLVc
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.
- B — the movement of alleles into a population through the migration and interbreeding of individuals is gene flow.
- D — 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.
- The gap sets a window, not a mechanism. It is long enough for prebiotic chemistry, which is why the fossil record is treated as consistent with an origin on Earth rather than as evidence against it.
- A geographic barrier splits the population first and reproductive isolation follows, which is the definition of allopatric. Nothing here involves human choice or unrelated lineages converging.
- Selection acts on variation already present; it does not create it on demand. X has something for the antibiotic to spare, Y does not.
- D — sustained herbicide use is a human-imposed selective pressure that favors resistant genotypes, reducing diversity and shifting the population toward resistance.
- C — extinctions open ecological niches, and surviving lineages diversifying to fill them is adaptive radiation.
- A — comparing DNA (or protein) sequences among species is molecular/biochemical evidence, one of several independent categories supporting common ancestry.
- Allopatric speciation: the river is a geographic barrier that stops gene flow between the two halves of the population.
- Allopatric; the river physically separates the populations so alleles can no longer be exchanged between them.
Total for part (A): 1 point
- The two environments favor different traits, so natural selection acts in different directions on each side; over generations allele frequencies diverge, producing the pink type on the drier side.
- Different selection pressures on either side of the river drive the isolated populations to adapt differently, accumulating genetic differences.
Total for part (B): 1 point
- The populations would probably have interbred and merged back into one, because too little genetic difference and no reproductive isolation would have accumulated in so short a time.
- Gene flow would resume and the slight differences would be swamped; no speciation.
Total for part (C): 1 point
- Species are defined by reproductive isolation; populations can differ in appearance yet still interbreed (one species), or look alike yet be isolated (two species), so failure to interbreed on contact is the decisive test.
- Appearance can vary within one species, but a persistent lack of gene flow where the populations overlap shows that separate gene pools now exist.
Total for part (D): 1 point
Total for question 9: 4 points
- The mean trichome number of the 100 offspring, counted at the same age, is greater than the parent population mean of 4.2
- The offspring distribution is shifted toward higher trichome numbers than the original population
Total for part (A): 1 point
- Offspring grown from seed of randomly chosen (unselected) parents from the same original population, raised alongside the selected line under identical conditions
- An unselected control line from the same seed lot grown at the same time in the same chamber
Total for part (B): 1 point
- Light intensity and photoperiod, kept the same for both generations so that any difference in trichome number is not caused by the light environment
- Water and nutrient supply, soil type, planting density, or temperature, held the same across both generations
- The age at which trichomes are counted (14 days after germination in both generations)
Total for part (C): 1 point
- Selection acts by allowing certain phenotypes to reproduce, but only alleles are passed to offspring; if the variation among parents came from the environment rather than from differences in alleles, the selected parents would transmit no advantage and the offspring mean would not shift
- A change in allele frequencies across generations requires that the phenotypic differences being selected are heritable; purely environmental variation is not transmitted through gametes
Total for part (D): 1 point
Total for question 10: 4 points
- Human and rhesus monkey; node 2
- Human and rhesus monkey share the most recent common ancestor (node 4); horse and chicken last shared an ancestor at node 2
Total for part (A): 1 point
- Tuna has the largest number of differences from every other species (18–21), so its lineage has been accumulating changes independently for the longest time, meaning it shares the oldest common ancestor with the others.
- The greater the number of amino acid differences, the longer since two lineages diverged; tuna differs most from all four others, so it diverged first.
Total for part (B): 1 point
- As a branch joining the horse lineage: the dog is most similar to horse (6 differences), so the two share a common ancestor more recent than node 3 but after the split from the human–rhesus lineage.
- On the horse branch, splitting from horse between node 3 and the horse tip.
Total for part (C): 1 point
- Body form can be misleading because unrelated species may evolve similar features through convergent evolution, and homologous structures can be modified beyond recognition; sequence changes accumulate at a roughly steady rate and can be compared objectively across all species that share the molecule.
- Molecular sequences provide many independent, quantifiable characters that are not affected by convergence in the way anatomy is, and cytochrome c is present in all the species compared.
Total for part (D): 1 point
Total for question 11: 4 points
Other AP Biology units
- Unit 1 Chemistry of Life 96
- Unit 2 Cell Structure and Function 105
- Unit 3 Cellular Energetics 112
- Unit 4 Cell Communication and Cell Cycle 104
- Unit 5 Heredity 130
- Unit 6 Gene Expression and Regulation 116
- Unit 8 Ecology 131
All eight units → · IB Biology subtopics → · Open the exam maker →