Biology • Year 11 • Module 3 • Lesson 11
Anatomical Evidence
Build HSC Band 5–6 extended-response technique: evaluate anatomical evidence for common ancestry, distinguish convergent from shared ancestry, and integrate multiple lines of anatomical evidence.
1. Extended response — evaluate anatomical evidence for evolution (Band 5–6)
7 marks Band 5–6
Q1. Compare and evaluate homologous structures, analogous structures and vestigial structures as anatomical evidence for evolution. In your response you must:
- Define each type of structure and give one named example of each.
- Explain what each type of evidence implies about evolutionary processes (common ancestry, convergent evolution, or modification of existing body plans).
- Explain when anatomical evidence alone may be insufficient and what additional evidence would be needed.
- Reach an explicit evaluative judgement about the overall value of anatomical evidence for establishing evolutionary relationships.
2. Scenario-based extended response — the dolphin classification problem (Band 5–6)
8 marks Band 5–6
Stimulus. A student says: “Dolphins and sharks look so similar — streamlined body, dorsal fin, pectoral fins, and both are fast ocean swimmers — that they must be more closely related to each other than dolphins are to other mammals such as hippos or cows. The physical similarity is just too strong to be a coincidence.”
Q2. Analyse and evaluate the student’s reasoning, using lesson content. In your answer:
- Identify what type of evolutionary process produced the physical similarity between dolphins and sharks.
- Explain why physical similarity in this case does not indicate close common ancestry.
- Identify what type of anatomical evidence would indicate common ancestry, and use the dolphin–mammal example to illustrate.
- Explain how molecular or embryological evidence could further test the student’s claim.
- Assess whether the student’s conclusion is valid and reach a justified conclusion.
3. Evaluate this claim (Band 5–6)
6 marks Band 5–6
“Vestigial structures prove that evolution is happening right now, and that organisms are actively losing features they no longer need in the current generation.”
Q3. Evaluate this claim. Identify which parts are defensible, which are flawed, and reformulate the claim into a biologically accurate statement.
Q1 — Sample Band 6 response (7 marks)
Homologous structures share the same underlying anatomical origin, even if they perform different functions today. The pentadactyl limb — the five-digit forelimb plan shared by humans, bats, whales and horses — is the classic example: all contain humerus, radius, ulna, carpals, metacarpals and phalanges in the same arrangement. This shared underlying plan implies descent from a common vertebrate ancestor; each lineage then modified the plan for different functions through divergent evolution. [1 — homologous defined + pentadactyl example + implication]
Analogous structures perform the same function but have entirely different evolutionary origins, produced by convergent evolution. For example, bird wings and insect wings both enable flight but have completely different internal structures — one evolved from a modified vertebrate forelimb, the other from arthropod appendages. Analogous structures indicate that similar selective pressures can independently drive similar solutions but do not indicate close ancestry. [1 — analogous defined + example + convergent evolution implication]
Vestigial structures are reduced remnants of once-functional ancestral features. The human coccyx is a vestigial remnant of an ancestral tail; whale pelvic bones are remnants of hind limbs from land-dwelling ancestors; python pelvic spurs are remnants of hind limbs retained in a mostly limbless lineage. These structures show that evolution modifies existing body plans rather than erasing ancestral structures completely — consistent with descent with modification. [1 — vestigial defined + named example + implication]
Anatomical evidence has limits. Analogous structures can mislead classification by grouping convergent organisms together, because physical appearance does not always reflect ancestry. When two organisms share physical features, there is no way from anatomy alone to distinguish inherited homology from convergent analogy. [1 — limitation of anatomical evidence identified]
Additional evidence is needed when anatomy is ambiguous. Molecular evidence (DNA or protein sequence comparison) reveals genetic relatedness independent of physical form, and can distinguish convergent similarity from true shared ancestry. Embryological comparisons can also help: organisms sharing conserved developmental stages likely share common ancestry. [1 — additional evidence needed + why]
Overall, anatomical evidence — particularly homologous structures and vestigial remnants — is valuable evidence for evolution because it directly shows the predicted pattern of modification from common ancestors. It is most reliable when combined with molecular and embryological evidence, especially in cases where convergent evolution might be misleading. [1 — evaluative judgement]
[1 quality mark]
Q2 — Sample Band 6 response (8 marks)
The student’s reasoning is flawed. The physical similarity between dolphins and sharks is not a coincidence — it is the predictable result of convergent evolution. Both lineages independently evolved streamlined bodies and fins because fast, efficient movement through water is strongly selected for in large aquatic predators. The similarity reflects shared selective pressure, not shared ancestry. [1 — convergent evolution identified as cause]
Physical similarity alone is not sufficient evidence of close common ancestry. When similar structures arise from different evolutionary origins — as analogous structures do — they tell us about environment and function, not about genealogical descent. [1 — why analogy ≠ ancestry]
The type of anatomical evidence that would indicate common ancestry is homologous structures — those sharing the same underlying anatomy and origin. Dolphins and other mammals share homologous structures that sharks do not: for example, dolphins have a vertebral column, mammary glands, and forelimbs with the pentadactyl bone arrangement, all inherited from a common mammalian ancestor. Sharks have no such mammalian features. [1 — homologous structures would show ancestry, with dolphin–mammal example]
Molecular evidence would further test the claim. DNA comparison would show that dolphins share far more sequence similarity with hippos, cows and other mammals than with sharks, because dolphins diverged from the mammal lineage only recently in geological terms, while the shark lineage diverged from the vertebrate lineage far earlier. [1 — molecular evidence would test the claim]
Embryological evidence could also be applied: dolphin embryos show mammalian developmental features — including the formation of limb buds that are later reduced — consistent with a mammalian developmental pathway, not a shark-like one. [1 — embryological evidence mentioned]
The student’s conclusion is not valid. They have committed a common error: assuming that physical similarity implies close ancestry, without distinguishing between analogous structures produced by convergent evolution and homologous structures produced by shared ancestry. The degree of physical similarity is not itself a measure of evolutionary closeness — the origin of the structure is what matters. [1 — conclusion evaluated as invalid + reason]
The correct conclusion is that dolphins and sharks are much less closely related than their appearance suggests. Their body plan similarity is an example of convergent evolution and constitutes analogous, not homologous, structures. Dolphins are more closely related to hippos than to sharks. [1 — correct conclusion stated]
[1 quality mark]
Q3 — Evaluate the vestigial structures claim (6 marks)
Overall judgement: The claim is largely flawed, though it contains a partially defensible element. [1]
What is defensible: Vestigial structures do provide evidence relevant to evolution — they are consistent with the prediction that evolution modifies existing body plans through descent with modification. [1]
Flawed — “happening right now”: Vestigial structures are evidence of evolutionary history (past processes), not proof that evolution is occurring in the current generation. A human being born with a coccyx today does not mean their coccyx is currently being reduced by natural selection — it means their lineage descended from ancestors with functional tails. [1]
Flawed — “actively losing features they no longer need”: This implies directed, immediate reduction. In reality, vestigial structures persist across many generations because selection pressure to remove them may be weak or absent. Evolution does not eliminate structures simply because they are not currently used — it only selects strongly against them if they impose a survival cost. [1]
Flawed — misrepresents the evidence: Vestigial structures demonstrate descent with modification from ancestors — the primary claim is about ancestry, not about current evolutionary change. [1]
Defensible reformulation: “Vestigial structures are reduced remnants of features that were functional in ancestral organisms. They are evidence for descent with modification — showing that organisms retain modified versions of structures inherited from their ancestors rather than being independently designed. They indicate the evolutionary history of a lineage but do not by themselves prove that a structure is being actively reduced in the current generation.” [1]