Dolphins and sharks both have streamlined torpedo-shaped bodies with dorsal fins and powerful tail propulsion. A student concludes they must share a recent common ancestor that first evolved this body plan. What is the flaw in this reasoning?
ASharks have cartilaginous skeletons while dolphins have bony skeletons, so they cannot share a body plan
BThe similar body shapes are the result of convergent evolution — independent adaptation to the same aquatic pressures — not shared ancestry; the similarity is analogous, not homologous, and cannot be used as evidence of close relationship
CThe student is correct; similar structures always indicate shared ancestry, regardless of how different the lineages otherwise appear
DDolphins are actually more closely related to sharks than to land mammals, based on their aquatic lifestyle
Dolphins are mammals (their closest relatives include hippos); sharks are cartilaginous fish. They last shared a common ancestor hundreds of millions of years ago, long before either entered the water. The similar body plan evolved independently in each lineage under the same physical constraint: moving efficiently through water requires minimizing drag and maximizing thrust. This is convergent evolution — analogous, not homologous. Homologous structures (like the forelimb bones shared by all tetrapods) reflect shared ancestry; analogous structures reflect shared selective pressures. Conflating them leads to incorrect phylogenies.
Question 2 Multiple Choice
Researchers found that bats and dolphins, which both use echolocation, show convergent amino acid substitutions in the protein prestin, which is critical for high-frequency hearing. What is the most significant implication of this molecular convergence?
ABats and dolphins share an echolocating common ancestor that scientists have not yet discovered
BMolecular evolution is random, so convergent changes in prestin are simply a coincidence with no biological significance
CThe number of functional genetic paths to certain adaptive solutions may be surprisingly limited, meaning natural selection channels evolution through specific molecular routes when facing the same problem
DConvergent evolution at the molecular level proves that DNA sequences are not reliable for reconstructing evolutionary relationships
The prestin finding suggests that when two lineages independently evolve the same complex function, they may arrive at the same molecular solution because the solution space is constrained. Not all amino acid substitutions produce a functional high-frequency hearing protein — only certain changes work, and natural selection drives both lineages toward those same changes. This reveals that evolution is not aimlessly random even at the molecular level: the structure of the problem constrains the space of viable solutions. It also complicates phylogenetics, because molecular convergence can mislead sequence-based analyses.
Question 3 True / False
Convergent evolution provides evidence that natural selection is not a random process — certain adaptive solutions are so strongly favored by physical or ecological constraints that they emerge repeatedly in unrelated lineages.
TTrue
FFalse
Answer: True
This is exactly the argument convergent evolution makes. Eyes have evolved independently over 40 times; flight evolved in insects, pterosaurs, birds, and bats; streamlined body forms evolved in sharks, ichthyosaurs, and dolphins. This repetition reveals that when organisms face the same physical or ecological problem, natural selection channels evolution toward the same solutions. The physics of moving through water, the optics of focusing light, and the aerodynamics of flight constrain what solutions work — and selection reliably finds them. Convergence shows that evolution has structure, not just randomness.
Question 4 True / False
When two distantly related species share a similar trait, that trait is expected to be the result of convergent evolution rather than inheritance from a common ancestor.
TTrue
FFalse
Answer: False
Distant relationship alone does not establish convergence. Even distantly related species can share traits inherited from a common ancestor if that trait is ancient and conserved. The key distinction is between homology (shared ancestry) and analogy (convergent evolution), and this must be determined by careful analysis — comparing the developmental origins, underlying structures, and molecular basis of the trait, and examining the phylogenetic history. A trait shared by distantly related species might be homologous (evolved once in a deep ancestor) or analogous (evolved independently). You cannot infer convergence from taxonomic distance alone.
Question 5 Short Answer
What is the difference between homology and analogy in evolutionary biology, and why does correctly distinguishing them matter for reconstructing evolutionary relationships?
Think about your answer, then reveal below.
Model answer: Homologous traits are shared because of common ancestry — they derive from the same structure in a shared ancestor, even if they now serve different functions (like the forelimbs of humans, bats, and whales, which are all modified versions of the same tetrapod limb). Analogous traits are similar because of convergent evolution — they evolved independently under similar selective pressures (like the wings of birds and insects, which evolved from completely different structures). The distinction matters because phylogenetics uses shared traits to infer common ancestry. Using analogous (convergent) traits as evidence of relationship leads to wrong family trees. Only homologous traits reflect shared history and can legitimately be used to reconstruct evolutionary relationships.
The practical challenge is that convergence can be very convincing — the camera eyes of vertebrates and cephalopods are structurally almost identical, yet evolved completely independently. Distinguishing homology from analogy requires examining developmental pathways, genetic bases, and comparative anatomy in detail. In dolphins vs. fish, the 'tail' is actually different: dolphins use horizontal flukes (derived from mammalian tail vertebrae) while fish use vertical tails — a subtle but crucial developmental difference that reveals independent origin despite surface similarity.