HSC Exam Practice

Sample response. Comparative embryology is the comparison of the early embryonic development of different species, used as evidence of relatedness. Closely related species show very similar early embryos, indicating descent from a common ancestor.

Marking notes. 1 mark for identifying it compares early embryonic development across species. 1 mark for linking embryonic similarity to relatedness / common ancestry.

Sample response. Any three of: the fossil record (ordered sequences and transitional forms showing change over time); comparative anatomy (homologous structures such as the pentadactyl limb showing shared ancestry); biochemical/molecular evidence (similar DNA or protein sequences in related species); biogeography (the distribution of species explained by ancestry and continental movement).

Marking notes. 1 mark for each correctly named and briefly described line of evidence, to a maximum of 3. Comparative embryology cannot be counted here.

Sample response. All three species are vertebrates that inherited these early embryonic features (pharyngeal arches and a post-anal tail) from a common vertebrate ancestor [1]. The developmental pathways producing these features are conserved and passed on to all descendants, so they appear in the early embryos even though they later develop into different structures or are lost [1].

Marking notes. 1 mark for inheritance from a common ancestor. 1 mark for explaining the features are conserved/inherited and later modified or lost.

Sample response. (i) Similar DNA sequences, biochemical/molecular evidence. (ii) Similar early embryos, comparative embryology. (iii) The pentadactyl limb, comparative anatomy (homologous structures).

Marking notes. 1 mark for two correct; 2 marks for all three correctly identified.

Sample response. Resistance comes from pre-existing genetic variation in the population, a few bacteria already carry a resistance gene before the antibiotic is used [1]. The antibiotic does not teach individual bacteria to resist; it simply kills susceptible bacteria while the resistant ones survive and reproduce, so resistance spreads through natural selection acting on existing heritable variation, not through individual learning [1].

Marking notes. 1 mark for identifying resistance as pre-existing genetic variation. 1 mark for explaining the antibiotic selects (kills susceptible, resistant survive and reproduce) rather than creating resistance.

Sample response. The table shows that all four species, fish, chick, human and snake, have both pharyngeal arches and a post-anal tail present in their early embryos [1]. In other words, despite being very different animals, they share the same two early embryonic features [1].

Marking notes. 1 mark for stating all four species share both features. 1 mark for noting this occurs despite the animals being very different.

Sample response. The fact that four very different vertebrates all share the same early embryonic features is best explained if they inherited those features from a shared common ancestor [1]. The developmental pathways producing pharyngeal arches and a post-anal tail were present in that ancestor and have been conserved and passed on to all of its descendants [1]. This common inheritance is direct evidence of descent with modification from a common ancestor, supporting evolution [1].

Marking notes. 1 mark for inheritance from a common ancestor. 1 mark for conserved/inherited developmental pathways. 1 mark for explicitly linking this to evidence for evolution / common ancestry.

Sample response. Comparative embryology is based on similarities in early development, not on the adult forms [1]. Each lineage modifies or loses the shared early features as development proceeds, so the adults look very different and none keeps external gills; but the embryos still start from the same inherited template, so the very different adults do not weaken the evidence, the shared early development is the point [1].

Marking notes. 1 mark for noting the evidence rests on early development, not adult form. 1 mark for explaining each lineage modifies/loses the features later, so different adults are expected and consistent with common ancestry.

Sample response. The evidence for evolution by natural selection is very strong because it rests on several independent lines that all converge on the same conclusion, common ancestry, and because we can now observe natural selection directly.

Comparative embryology shows that the early embryos of vertebrates such as fish, birds and humans share features like pharyngeal arches and a post-anal tail, which is best explained by inheritance from a common ancestor. Comparative anatomy shows homologous structures, such as the pentadactyl limb shared by humans, whales, bats and cats, indicating shared ancestry. Biochemical/molecular data show that closely related species have very similar DNA and protein sequences. The fossil record provides ordered sequences and transitional forms, and biogeography explains species distributions through ancestry and continental movement.

The real strength comes from the independence of these lines. Embryology and molecular data, for example, are measured in completely different ways, yet they independently produce the same evolutionary relationships. When independent methods agree, coincidence or error becomes extremely unlikely, so the combined evidence is far more convincing than any single line alone.

Finally, antibiotic resistance is a modern, observable example: within a bacterial population there is pre-existing genetic variation, antibiotics kill susceptible bacteria, and the resistant survivors reproduce, so resistance increases over generations. Because bacteria reproduce so quickly, this natural selection can be observed and measured within days, evolution happening in real time.

Overall, because so many independent lines of evidence converge on common ancestry and because natural selection can be directly observed, the evidence for evolution is very strong and well supported.

Marking criteria.

• 1 mark Explains comparative embryology as a line of evidence with a specific feature.
• 1 mark Identifies a second line of evidence with a brief explanation.
• 1 mark Identifies a third line of evidence with a brief explanation.
• 1 mark Explains that the lines are independent / measured in different ways.
• 1 mark Explains that convergence/agreement of independent lines makes coincidence or error unlikely.
• 1 mark Describes antibiotic resistance as a modern observable example (variation, selection, resistant survive and reproduce).
• 1 mark Links antibiotic resistance to natural selection observable in real time.
• 1 mark Reaches an explicit evaluative conclusion that the evidence is strong, using precise vocabulary (common ancestor, lines of evidence, natural selection).