Biology • Year 11 • Module 3 • Lesson 11
Anatomical Evidence
Apply the distinctions between homologous and analogous structures to real examples, evaluate claims about evolutionary relationships, and assess vestigial and embryological evidence.
1. Compare anatomical evidence types
The table below shows three pairs of organisms. For each pair, decide whether the structures described are homologous or analogous, then answer the questions. 8 marks
| Pair | Structures compared | Homologous or analogous? |
|---|---|---|
| Human arm & horse foreleg | Both contain humerus, radius, ulna, carpals, metacarpals and phalanges | |
| Bat wing & butterfly wing | Both used for flight; bat wing has bones and membrane; butterfly wing has chitin veins | |
| Cactus stem & euphorb stem | Both are succulent water-storing structures in desert plants but evolved in different plant lineages |
1.1 For the pair you identified as homologous, explain what the shared underlying anatomy implies about the evolutionary relationship of the two organisms. 2 marks
1.2 For one pair you identified as analogous, explain why visual similarity in this case is not evidence of close common ancestry. 3 marks
1.3 Explain what additional evidence would help resolve whether two similar-looking organisms are closely related or only convergent. 3 marks
2. Cause-and-effect chain — how convergent evolution can mislead classification
Complete the chain below. Each cause is given; fill in the effect. 5 marks
Cause 1: Two unrelated lineages (e.g. sharks and dolphins) are subjected to the same selective pressure: fast, efficient swimming in open water.
Effect 1 / Cause 2:
Cause 3: A biologist classifies sharks and dolphins as close relatives because of their physical similarity.
Effect 3 / Cause 4:
Cause 5: Molecular comparison reveals that dolphins share far more DNA with other mammals such as whales and hippos than with sharks.
Effect 5 (Overall outcome):
3. Case study — python pelvic spurs as vestigial structures
Read the scenario, then answer the questions. 6 marks
Scenario. Pythons and boa constrictors possess small, claw-like structures called pelvic spurs, located near the base of the tail. These spurs are remnants of hind limbs present in the limbless ancestors of modern snakes. Unlike most modern snakes, which have no trace of hind limbs, pythons and boas retain these small reduced remnants. The structures have no locomotor function; they may be used in courtship behaviour in some species.
3.1 Explain why pelvic spurs are classified as vestigial structures using the lesson definition. 2 marks
3.2 Explain what the pelvic spurs imply about the evolutionary history of pythons and boas. 2 marks
3.3 A student argues that vestigial structures prove evolution is imperfect, not that it happened. Evaluate this argument. 2 marks
Q1 — Comparison table classifications
Human arm & horse foreleg: homologous. Bat wing & butterfly wing: analogous. Cactus stem & euphorb stem: analogous.
Q1.1 — Homologous pair implication (2 marks)
The shared underlying bone arrangement (humerus, radius, ulna, carpals, metacarpals and phalanges) implies that the human and horse share a common vertebrate ancestor from which both inherited this basic forelimb plan [1]. Although the limbs perform different functions today (manipulation vs running), the common structural origin is best explained by common ancestry followed by divergent evolution [1].
Q1.2 — Analogous pair: why similarity ≠ close ancestry (3 marks)
Sample answer for bat wing & butterfly wing: The wings of bats and butterflies both enable flight, but they evolved independently in entirely different lineages — one in vertebrates from a modified forelimb, and the other in arthropods from a different developmental origin [1]. This is convergent evolution: similar selective pressure (airborne locomotion) produced a similar outcome (wings) without any close common ancestor being responsible [1]. Basing classification on this visual similarity would group bats and butterflies together as close relatives, which would be incorrect [1].
Q1.3 — Additional evidence to resolve convergence (3 marks)
Molecular evidence (DNA or protein sequence comparison) would help by revealing the actual degree of genetic relatedness between the organisms [1]. Closely related organisms will share far more similar DNA sequences than convergent ones, regardless of physical appearance [1]. Comparative embryology could also be used: if the organisms share similar early developmental stages, this would support common ancestry; if they do not, it supports convergent evolution rather than shared ancestry [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1 / Cause 2: Both lineages independently evolved streamlined body shapes, dorsal fins and similar aquatic locomotion structures through convergent evolution — each solving the same problem in a similar way despite having completely different ancestors.
Effect 3 / Cause 4: The classification is incorrect. Dolphins are far more closely related to other mammals (such as whales and hippos) than to sharks. Their physical resemblance is due to convergent adaptation, not shared ancestry.
Effect 5 (Overall outcome): Morphological classification based on appearance alone is overturned by molecular evidence. The correct classification places dolphins with mammals and sharks with cartilaginous fish, confirming that appearance can be highly misleading and that structural origin matters more than superficial similarity for inferring common ancestry.
Q3.1 — Why pelvic spurs are vestigial (2 marks)
Vestigial structures are reduced remnants of structures that were functional in ancestral organisms [1]. Pelvic spurs are the reduced remnants of hind limb bones that were functional in the land-dwelling ancestors of pythons and boas — they have no current locomotor function but retain their structural presence as a reduced remnant, fitting the lesson definition exactly [1].
Q3.2 — What the spurs imply about evolutionary history (2 marks)
The pelvic spurs imply that pythons and boas descended from ancestors that had functional hind limbs [1]. This is consistent with the evolutionary model of descent with modification — as the lineage adapted to a limbless body plan, the hind limbs were gradually reduced rather than immediately disappearing, leaving vestigial remnants in species such as pythons and boas [1].
Q3.3 — Evaluate the student’s argument (2 marks)
The argument is flawed. Vestigial structures are actually strong positive evidence for evolution, not merely evidence of imperfection [1]. They are exactly what the theory of descent with modification predicts — that ancestral structures are retained in modified form rather than perfectly redesigned for each new lineage. The presence of pelvic spurs in a limbless snake is far more consistent with descent from a limbed ancestor (as predicted by evolution) than with independent creation from scratch [1].