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Quiz: Evidence for Evolution and Mechanisms of Change

Test your understanding of the evidence for evolution, natural selection, and the endosymbiotic origin of eukaryotic cells with these review questions.

1. Which of the following best defines a vestigial structure?

  1. An anatomical feature that serves the same function in two unrelated species
  2. An anatomical feature that appears only during early embryonic development
  3. An anatomical feature that had a function in an ancestral species but has reduced or lost function in the current species
  4. An anatomical feature that is identical in structure and function across all vertebrates
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The correct answer is C. Vestigial structures, such as the human appendix or whale hip bones, are remnants of organs that served a clear purpose in an ancestral lineage but have lost most or all of their original function in the descendant species. Option A describes an analogous structure, and option D misrepresents homology, since homologous structures often differ in appearance despite sharing an evolutionary origin.

Concept Tested: Vestigial Structures

2. How do relative dating and absolute dating differ in the information they provide about a fossil?

  1. Relative dating determines the fossil's position and thus relative age within rock strata, while absolute dating uses radioactive isotope decay to estimate the fossil's actual age in years
  2. Relative dating provides the fossil's dietary habits, while absolute dating provides its size
  3. Relative dating uses radiometric decay, while absolute dating uses the law of superposition
  4. Relative and absolute dating both measure the exact same quantity and always must agree precisely
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The correct answer is A. Relative dating relies on stratigraphy and index fossils to establish the sequence in which fossils were deposited, indicating only whether one fossil is older or younger than another. Absolute dating uses the known decay rates of radioactive isotopes, such as carbon-14 or potassium-40, to calculate a fossil's actual numerical age in years, providing a complementary and more precise timescale.

Concept Tested: Relative and Absolute Dating

3. Which statement most accurately reflects how natural selection operates on a population?

  1. Individual organisms change their traits during their lifetime in response to environmental pressure and pass on the changed traits
  2. Natural selection creates new genetic variation whenever the environment changes
  3. Natural selection ensures that every generation becomes a "perfect," flawlessly adapted version of the species
  4. Natural selection acts on existing heritable variation, favoring individuals whose traits increase survival and reproduction, thereby shifting allele frequencies in the population over generations
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The correct answer is D. Natural selection requires pre-existing heritable variation; it does not generate new variation or act with foresight. Individuals with traits that improve survival and reproductive success leave more offspring, and because those traits are heritable, allele frequencies in the population shift over successive generations. Option A restates the discredited Lamarckian mechanism, and options B and C attribute intention or perfection to a process that has neither.

Concept Tested: Natural Selection

4. In a population, genotype BB produces an average of 12 offspring, Bb produces 9, and bb produces 3. What is the relative fitness of the bb genotype?

  1. 3.0
  2. 0.25
  3. 1.0
  4. 0.75
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The correct answer is B. Relative fitness is calculated by dividing each genotype's average reproductive output by the output of the most successful genotype. Here, BB has the highest output (12), so it is assigned a fitness of 1.0. The bb genotype's relative fitness is 3 ÷ 12 = 0.25, meaning bb individuals contribute only one-quarter as many offspring, on average, as the fittest genotype in the population.

Concept Tested: Relative Fitness

5. A dolphin's flipper and a shark's fin both allow the animal to swim, but the dolphin flipper contains the same bones as a mammalian forelimb while the shark fin has an entirely different internal structure with no corresponding bones. How should this pair of structures be classified?

  1. Analogous structures, because they serve a similar function (swimming) but evolved independently from different ancestral structures
  2. Homologous structures, because dolphins and sharks share a very recent common ancestor
  3. Both are vestigial structures because swimming is not an efficient use of the appendage
  4. Homologous structures, because they perform the same function
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The correct answer is A. Because the dolphin flipper and shark fin share no common internal skeletal plan despite both enabling swimming, they arose independently under similar selective pressures — the hallmark of convergent evolution and analogous structures. Homology instead requires a shared underlying structure inherited from a common ancestor, as seen among mammalian forelimbs, which is absent here since sharks (fish) and dolphins (mammals) are only distantly related.

Concept Tested: Analogous Structures

6. Cytochrome c amino acid sequences differ by 1 residue between humans and chimpanzees, by 13 residues between humans and dogs, and by 44 residues between humans and yeast. What does this pattern most strongly suggest?

  1. Humans are equally related to all three species since cytochrome c is a universal protein
  2. Yeast diverged from the human lineage more recently than dogs did
  3. The degree of sequence divergence correlates with the time since each species shared a common ancestor with humans, with chimpanzees being most closely related and yeast most distantly related
  4. The number of amino acid differences is unrelated to evolutionary relationships
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The correct answer is C. Molecular evolution accumulates roughly over time, so species that diverged from humans more recently, such as chimpanzees, tend to share more similar sequences of highly conserved proteins like cytochrome c, while species that diverged much earlier, such as yeast, accumulate far more differences. The progressive increase in amino acid differences from chimpanzee to dog to yeast mirrors the known evolutionary distances among these lineages.

Concept Tested: Molecular Evidence for Evolution

7. Which single piece of evidence for the endosymbiotic origin of mitochondria most directly rules out the alternative explanation that mitochondria simply budded off from the host cell's own endomembrane system?

  1. Mitochondria are similar in size to bacteria
  2. Mitochondria divide by binary fission
  3. Mitochondria are surrounded by a membrane
  4. Mitochondrial DNA is circular, lacks histones, and its sequence closely matches that of α-proteobacteria rather than the host nuclear genome
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The correct answer is D. A membrane budded from the host's own endomembrane system would not carry its own independent genetic material. The fact that mitochondria possess circular, histone-free DNA whose sequence closely resembles α-proteobacteria — rather than resembling the host's nuclear genome — is direct genetic evidence of a separate bacterial origin. Size, binary fission, and the presence of a membrane are each consistent with endosymbiosis but do not, by themselves, exclude a purely host-derived origin.

Concept Tested: Endosymbiotic Theory

8. Marsupial mammals are highly diverse in Australia but comprise only a small fraction of mammal species elsewhere in the world, even in environments ecologically similar to Australia. What does this pattern best illustrate?

  1. Marsupials are incapable of surviving outside of Australia's climate
  2. Australia's isolation by continental drift before placental mammals diversified allowed marsupials to undergo adaptive radiation into ecological niches that placental mammals fill elsewhere
  3. Convergent evolution, because marsupials and placental mammals are the same species evolving independently
  4. Australia has a fossil record that is too incomplete to draw any biogeographic conclusions
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The correct answer is B. Continental drift separated Australia before placental mammals became widespread, isolating marsupial ancestors from competition. With that ecological space open, marsupials underwent adaptive radiation, diversifying to fill niches that placental mammals occupy on other continents. This biogeographic pattern — related species clustering where they evolved rather than where their ecological niche exists elsewhere — is strong evidence for evolution shaped by geographic history.

Concept Tested: Biogeography

9. A skeptic argues that mitochondria could simply be a specialized organelle that evolved gradually from folds of the host cell's own plasma membrane, rather than an engulfed bacterium. Which body of evidence best supports the endosymbiotic hypothesis over this alternative?

  1. Mitochondria are found in nearly all eukaryotic cells, so they must be original organelles
  2. Mitochondria are roughly the same size as lysosomes, supporting a shared host origin
  3. Mitochondria's possession of their own circular, histone-free DNA, 70S ribosomes, binary fission division, and DNA sequence similarity to α-proteobacteria are best explained by a bacterial origin, since a membrane derived purely from the host would not carry independent bacterial-type genetic machinery
  4. Mitochondria lack a double membrane, which supports a host-derived origin
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The correct answer is C. A membrane infolding hypothesis cannot account for mitochondria having their own independent genome, bacterial-type ribosomes, and a division mechanism identical to bacteria, nor for that genome's close sequence similarity to α-proteobacteria rather than to the host's own DNA. Taken together, these independent molecular and structural features are best explained by a genuinely separate bacterial ancestor that was engulfed and retained, not by internal membrane remodeling.

Concept Tested: Endosymbiotic Theory

10. A student wants to construct the strongest possible written argument that two beetle species share a recent common ancestor. Which approach best integrates multiple independent lines of evidence, as evolutionary biologists actually do?

  1. Combine evidence from the fossil record, comparative anatomy (homologous structures), and DNA sequence similarity, showing that independent data sets converge on the same evolutionary relationship
  2. Rely solely on the fact that both beetles live in similar habitats
  3. Compare only the outward color pattern of the two species' exoskeletons
  4. Assume common ancestry based on the shared common name "beetle" alone
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The correct answer is A. No single line of evidence is fully conclusive on its own, but when independently derived evidence from the fossil record, shared skeletal or structural homologies, and DNA sequence comparisons all point to the same evolutionary relationship, the convergence of multiple independent data sets makes the case for common ancestry far stronger than any one type of evidence alone. Shared habitat, superficial color, or common names do not indicate relatedness.

Concept Tested: Evidence for Evolution (Convergence of Multiple Lines)