AP Biology Unit 7: Natural Selection MCQs – 30 Practice Questions with Answers (College Board CED)
Free AP Biology practice for Unit 7: Natural Selection, covering fitness and selection, population genetics and Hardy-Weinberg calculations, evidence for evolution, phylogeny, speciation, and variation, with a short explanation after every question.
Natural Selection and Fitness
Q1. In evolutionary biology, fitness is best defined as what?
- (A) Body size relative to competitors
- (B) Physical strength of an individual, regardless of how many offspring it produces
- (C) Reproductive success in the next generation
- (D) Lifespan in years
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Answer: C – Fitness means how many viable offspring an organism contributes to the next generation, not strength or size alone.
Q2. A population of beetles shows green, brown, and speckled forms. Birds eat mainly the most visible form on light soil. Over generations the population shifts toward the camouflaged form. What is this an example of?
- (A) Natural selection on heritable variation
- (B) Artificial selection, in which humans deliberately choose breeding parents
- (C) Mutation alone
- (D) Genetic drift
Show Answer
Answer: A – Differential survival and reproduction of a heritable variant under predation pressure is natural selection.
Q3. A farmer repeatedly breeds only the cows that produce the most milk. Which process is this, and how does it differ from natural selection?
- (A) Natural selection, because humans are part of nature and any human action counts as natural
- (B) Artificial selection, because humans choose the breeding parents
- (C) Genetic drift, because the choice is random
- (D) Speciation, because a new species forms at once
Show Answer
Answer: B – Artificial selection occurs when humans deliberately choose which individuals breed, whereas in natural selection environmental pressures do the choosing.
Q4. Antibiotic use in a hospital rises, and within a few years the proportion of resistant bacteria in infections increases sharply. What best explains this pattern?
- (A) Resistant variants already present survive and reproduce
- (B) The antibiotic changes bacterial DNA in every cell identically
- (C) Resistant bacteria migrate from other hospitals only
- (D) The antibiotic causes individual bacteria to mutate on demand
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Answer: A – Antibiotics select for pre-existing resistant variants; they do not instruct bacteria to become resistant.
Q5. Which condition is required for natural selection to change a population over time?
- (A) The environment must stay constant
- (B) Mutations must stop occurring
- (C) Variation must be heritable
- (D) All individuals must reproduce equally
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Answer: C – Selection can only shift a population if the favoured variation is passed to offspring; non-heritable differences are not inherited.
Q6. A drought year favours finches with larger, stronger beaks that can crack hard seeds, while small-beaked birds survive less well. What type of selection is this?
- (A) Disruptive selection
- (B) Sexual selection
- (C) Stabilising selection
- (D) Directional selection
Show Answer
Answer: D – When one extreme phenotype is favoured and the population mean shifts in that direction, selection is directional.
Q7. Human babies with very low or very high birth weights survive less well than babies of intermediate weight. What type of selection does this illustrate?
- (A) Disruptive selection
- (B) Stabilising selection
- (C) Artificial selection
- (D) Directional selection
Show Answer
Answer: B – Selection against both extremes and in favour of the intermediate phenotype is stabilising selection.
Q8. A species of bird has two feeding forms: small-beaked birds eat small seeds and large-beaked birds eat large seeds, while intermediate birds compete poorly for both. Over time both extremes increase. What is this?
- (A) Genetic drift
- (B) Stabilising selection
- (C) Directional selection
- (D) Disruptive selection
Show Answer
Answer: D – When both extremes are favoured over intermediates, selection is disruptive and can increase variation in the population.
Population Genetics and Hardy-Weinberg
Q9. A population is in Hardy-Weinberg equilibrium for a gene with alleles A and a. Which statement must be true?
- (A) Natural selection is strong, so allele frequencies change every generation
- (B) Mutation rates are high
- (C) No evolution is occurring at that locus
- (D) The population is very small
Show Answer
Answer: C – Hardy-Weinberg equilibrium describes a non-evolving population: allele frequencies stay constant when its assumptions hold.
Q10. In a population, 4 percent of individuals show a recessive phenotype (aa). Assuming Hardy-Weinberg equilibrium, what is the frequency of the recessive allele a?
- (A) 0.80
- (B) 0.04
- (C) 0.20
- (D) 0.40
Show Answer
Answer: C – The recessive phenotype frequency equals q-squared, so q = square root of 0.04 = 0.20.
Q11. Using the same population, where q = 0.20 and p = 0.80, what proportion of individuals are expected to be heterozygous carriers (Aa)?
- (A) 0.16
- (B) 0.32
- (C) 0.64
- (D) 0.04
Show Answer
Answer: B – Carrier frequency is 2pq = 2 x 0.80 x 0.20 = 0.32.
Q12. A small island population is founded by just ten individuals. By chance, a rare allele becomes common in the new population. What is this an example of?
- (A) Gene flow
- (B) Stabilising selection
- (C) Natural selection
- (D) The founder effect
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Answer: D – The founder effect occurs when a small founding group carries allele frequencies that differ by chance from the source population.
Q13. A catastrophic flood randomly kills most of a frog population, and the survivors happen to carry a different mix of alleles. Which process best describes the change?
- (A) Speciation
- (B) A population bottleneck
- (C) Directional selection, favouring one extreme phenotype over several generations
- (D) Artificial selection
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Answer: B – A bottleneck is a sharp, random reduction in population size that changes allele frequencies by chance rather than by fitness.
Q14. Gene flow between two neighbouring populations of a plant species is most likely to have which effect?
- (A) Increase genetic differences between them, by keeping each gene pool isolated
- (B) Reduce genetic differences by mixing alleles
- (C) Cause immediate speciation
- (D) Stop all mutation
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Answer: B – Movement of individuals and gametes between populations transfers alleles and tends to make their gene pools more similar.
Q15. A researcher compares observed genotype frequencies with Hardy-Weinberg expected frequencies and finds a large excess of homozygotes. Which explanation is plausible?
- (A) Inbreeding or selection against heterozygotes
- (B) No mutation and no migration
- (C) Infinite population size
- (D) Random mating in a large population, which keeps genotype frequencies at expected values
Show Answer
Answer: A – Inbreeding increases homozygosity above Hardy-Weinberg expectations, as can selection that disfavours heterozygotes.
Q16. In a population, allele B has frequency 0.6. If the population is in equilibrium, what fraction of individuals are expected to show the recessive phenotype bb, where b has frequency 0.4?
- (A) 0.36
- (B) 0.60
- (C) 0.16
- (D) 0.24
Show Answer
Answer: C – The bb frequency is q-squared = 0.40 x 0.40 = 0.16.
Evidence, Phylogeny and Common Ancestry
Q17. The forelimbs of humans, bats, and whales share the same basic bone pattern but serve different functions. What does this best support?
- (A) Artificial selection
- (B) Convergent evolution only, with no shared ancestor behind the similar bone pattern
- (C) Common ancestry with divergent evolution
- (D) Independent origins of each limb
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Answer: C – Homologous structures with a shared underlying pattern point to descent from a common ancestor, modified for different functions.
Q18. Two unrelated desert plants both have thick, water-storing stems and spines. Their similar traits evolved independently in similar environments. What is this called?
- (A) Common ancestry
- (B) Genetic drift
- (C) Homology
- (D) Convergent evolution
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Answer: D – Analogous structures arise by convergent evolution when different lineages adapt to similar environments, without a recent common ancestor.
Q19. A cladogram shows species X and Y sharing a more recent common ancestor with each other than either shares with species Z. Which statement is correct?
- (A) All three are equally related
- (B) X and Y are most closely related
- (C) X and Z are sister species
- (D) Z is the direct ancestor of X
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Answer: B – On a cladogram, species sharing the most recent node are the most closely related pair.
Q20. Which character is most useful for grouping species on a cladogram?
- (A) A trait present in all organisms, including the outgroup
- (B) A shared derived character found in the group but not in the outgroup
- (C) A trait that varies randomly within every species, with no pattern shared by related groups
- (D) Body size alone
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Answer: B – Shared derived characters, inherited from a common ancestor of the group, are the evidence used to define clades.
Q21. DNA sequencing shows that the haemoglobin genes of humans and chimpanzees differ at far fewer positions than those of humans and fish. What does this support?
- (A) Fish are not vertebrates
- (B) Haemoglobin is not heritable
- (C) Humans evolved from chimpanzees, with modern chimpanzees as direct ancestors
- (D) Humans and chimpanzees share a more recent common ancestor
Show Answer
Answer: D – Smaller molecular differences indicate a more recent common ancestor; the result does not mean one living species evolved from another.
Q22. Fossils of marine organisms are found in rock layers on a mountaintop. What is the most reasonable inference?
- (A) Marine organisms once lived on mountains
- (B) Fossils do not provide evidence about past environments
- (C) The fossils were planted there recently
- (D) The area was once under water and later uplifted
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Answer: D – Fossil distribution, combined with geological evidence, shows how environments and life have changed over time.
Q23. Embryos of fish, birds, and mammals all show pharyngeal pouches early in development. What does this similarity best support?
- (A) Convergent evolution in adults
- (B) They are the same species
- (C) Common ancestry
- (D) Identical adult forms
Show Answer
Answer: C – Shared early developmental features are evidence of common ancestry, even when adult forms diverge greatly.
Q24. A student claims a cladogram shows that species A evolved into species B. What is wrong with this claim?
- (A) Cladograms show relationships, not ancestor-descendant chains
- (B) Cladograms cannot show any relationships
- (C) Species A is always older than species B
- (D) Cladograms show only fossil species
Show Answer
Answer: A – Branching diagrams show relative relatedness through common ancestors; living species at the tips are cousins, not ancestors of one another.
Speciation, Variation and Origins of Life
Q25. Two populations of the same frog species live on opposite sides of a river that formed 10,000 years ago. Their mating calls have diverged so much that they no longer recognise each other as mates. Which isolation mechanism is this?
- (A) Hybrid sterility
- (B) Postzygotic isolation
- (C) Prezygotic behavioural isolation
- (D) Temporal isolation only
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Answer: C – Differences in mating behaviour that prevent mating in the first place are prezygotic, behavioural isolation.
Q26. A mule is the offspring of a horse and a donkey but is sterile. Which type of reproductive isolation does this illustrate?
- (A) Postzygotic isolation (hybrid sterility)
- (B) Temporal isolation
- (C) Geographic isolation
- (D) Behavioural isolation
Show Answer
Answer: A – When hybrid offspring form but cannot reproduce, isolation acts after fertilisation, so it is postzygotic.
Q27. A plant population doubles its chromosome number, producing tetraploid plants that can no longer breed with the original diploid plants. What has occurred?
- (A) Sympatric speciation by polyploidy
- (B) Allopatric speciation by migration
- (C) Genetic drift only
- (D) Artificial selection
Show Answer
Answer: A – Polyploidy can create instant reproductive isolation within the same area, which is sympatric speciation.
Q28. Which source of genetic variation creates entirely new alleles in a population?
- (A) Mutation
- (B) Random fertilisation
- (C) Independent assortment
- (D) Crossing over
Show Answer
Answer: A – Mutation is the original source of new alleles; assortment, crossing over, and fertilisation only reshuffle existing alleles.
Q29. A population shows a wide range of beak sizes. A researcher argues this variation is entirely genetic. What evidence would challenge that claim?
- (A) Beak size varies continuously
- (B) Beak size differs between genetically identical individuals
- (C) Beak size is heritable
- (D) Beak size affects survival
Show Answer
Answer: B – If genetically identical individuals differ, part of the variation must be environmental, so variation is not entirely genetic.
Q30. Which statement best describes the scientific account of the origin of life on Earth?
- (A) Life arrived with no chemical precursors
- (B) The first organisms were multicellular animals
- (C) Complex cells appeared fully formed at once
- (D) Simple organic molecules formed first, then self-replicating systems
Show Answer
Answer: D – The origin-of-life model proposes a gradual chemical sequence from simple molecules to replicating systems enclosed in membranes.
Work through any questions you missed again, then revise those areas before moving on to ecology.