๐ Common ancestry and evolutionary relationships (12 MCQs)
๐ From Campbell Biology โข 1. Evolution and the theme of Biology and Scientific Inquiry โข 12 questions available
What is Common ancestry and evolutionary relationships?
Definition:
Common ancestry is the principle that all living organisms share a common ancestor from which they have diverged over billions of years, and evolutionary relationships describe the degrees of relatedness between species, often represented by phylogenetic trees, with closer relatives having a more recent common ancestor, and these relationships are inferred from shared features, DNA sequences, and fossil evidence.
Working:
Common ancestry works by linking all life through descent with modification, where species are related through a branching tree, and the degree of relatedness is quantified by genetic divergence, with the time since divergence estimated using molecular clocks, given by , and traits that are shared due to common ancestry (homologies) versus convergent evolution (analogies) are distinguished to infer true relationships.
Example:
A simple example is the shared characteristic of having hair among mammals, where all mammals inherited hair from a common ancestor, and within mammals, humans and chimpanzees share a more recent common ancestor (about 6 million years ago) than humans and dogs (about 80 million years ago), illustrating common ancestry and degrees of relatedness.
Reason:
Common ancestry is the foundation of all evolutionary biology, explaining the unity of life, and understanding evolutionary relationships is essential for classification, comparative biology, and predicting properties of newly discovered organisms, as well as for tracing the origins of diseases and developing conservation strategies.
๐ All Common ancestry and evolutionary relationships MCQs
Q1. Which observation provides the strongest evidence that two species inherited characteristics from a shared ancestral population rather than independently developing identical traits?
๐ Explanation: Corresponding structural organization is stronger evidence because inherited developmental patterns can persist while functions change. Similar habitat, body size, or seasonal responses can arise independently through environmental pressures and therefore do not necessarily demonstrate shared ancestry.
Q2. A researcher finds that Species A and Species B share several unusual DNA sequences that are absent from closely related species. What is the most reasonable evolutionary interpretation?
๐ Explanation: Shared unusual DNA sequences are more informative than common traits because rare inherited similarities are less likely to arise independently. Their presence in both species is therefore consistent with inheritance from a common ancestor, especially when other evidence agrees.
Q3. Two organisms have similar limb bones arranged in a comparable pattern, but one uses its limb for swimming and the other for walking. Which conclusion best follows?
๐ Explanation: A shared structural pattern can be inherited from an ancestor and subsequently modified as descendants adapt to different environments. Functional differences therefore do not automatically contradict common ancestry; they may instead demonstrate evolutionary modification.
Q4. A biologist compares three species. Species X and Y share five derived molecular features, while X and Z share only one. Species Y and Z share two. Assuming the features were inherited rather than independently acquired, which relationship is most strongly supported?
๐ Explanation: The greater number of shared derived features between X and Y provides stronger evidence that they share a more recent common ancestor than either does with Z. However, this does not mean X directly transformed into Y; both may descend from a shared ancestor.
Q5. A population contains individuals with a particular inherited DNA marker. After several generations, the marker appears in two geographically separated populations but not in a nearby third population. Which explanation is most consistent with common ancestry?
๐ Explanation: Geographic separation today does not erase historical relationships. If two populations share an unusual inherited marker, they may have descended from an ancestral population carrying it, whereas the nearby population may have diverged earlier or lost the marker.
Q6. A student argues: "Humans and other primates cannot share an ancestor because modern humans did not exist at the same time as the ancestral species." What is the key error in this reasoning?
๐ Explanation: The student incorrectly assumes that a common ancestor must coexist with its modern descendants. Evolutionary lineages branch through time, so an ancestral population can exist long before descendant species appear and can possess characteristics different from both modern groups.
Q7. A fossil species has a combination of features found separately in two modern groups. Researchers also find that its age fits the predicted time when those lineages diverged. What inference is most justified?
๐ Explanation: A combination of characteristics shared across two groups, together with an appropriate geological age, can support a close evolutionary relationship near a divergence event. However, the evidence does not prove that the fossil is the direct ancestor.
Q8. A researcher constructs a relationship diagram using shared inherited characteristics. Species P and Q form one branch, while Species R branches earlier. A student concludes that R is less evolved than P and Q. Why is this conclusion incorrect?
๐ Explanation: Branching diagrams represent relationships rather than rankings of evolutionary progress. Once lineages diverge, each continues evolving independently. An earlier-branching lineage is not necessarily less adapted, less complex, or more primitive than later-branching lineages.
Q9. A graph shows the percentage of shared genetic markers between species as their estimated evolutionary separation increases. The plotted points show a general decrease in shared markers with increasing separation. Which interpretation best matches the pattern?
๐ Explanation: The declining pattern is consistent with descendants accumulating genetic differences after their lineages diverge. Species that separated more recently generally retain more inherited similarities, whereas longer independent evolutionary histories provide more opportunity for differences to accumulate.
Q10. Species A and B share a distinctive protein sequence. Species C lacks that sequence but shares another distinctive sequence with A. If both sequences are inherited traits, which additional evidence would best help determine whether A is more closely related to B or C?
๐ Explanation: One shared characteristic can be misleading because traits may change, disappear, or arise independently. Comparing many independent inherited characteristics allows researchers to identify consistent patterns and reduces the chance that one unusual trait determines the inferred relationship incorrectly.
Q11. Two species have very different appearances, yet their embryos show a similar early developmental arrangement and their genomes contain several corresponding genes. Which conclusion best integrates these observations?
๐ Explanation: Different adult appearances can result from evolutionary modification of inherited developmental programs. When developmental patterns and corresponding genes independently support similarity, the combined evidence provides a stronger basis for inferring shared ancestry than appearance alone.
Q12. Three species have genetic differences represented by distances , , and . If smaller genetic distance generally indicates a more recent common ancestor, which inference is most defensible?
๐ Explanation: Because the smallest genetic distance is between A and B, those species are inferred to have diverged most recently under the stated model. The distances do not imply that A directly became B; they indicate relative recency of shared ancestry.