Comparative Embryology & Evidence for Evolution

Q1, Sample Band 6 response (7 marks), annotated

Comparative embryology compares the early embryonic development of different species and uses the similarities as evidence of how closely related they are. [1, defines comparative embryology]

The early embryos of fish, reptiles, birds and mammals (including humans) all develop pharyngeal arches (gill-like structures) and a post-anal tail. [1, names specific shared embryonic features] In the adults, these features develop into very different structures, or are lost entirely: a human, for example, does not retain external gills or a visible tail, yet both appear in the early embryo. [1, describes what happens to the features in the adult]

The presence of the same early features in animals that become so different as adults is best explained by inheritance from a common vertebrate ancestor. The developmental pathways producing pharyngeal arches and a post-anal tail were present in the shared ancestor and are conserved and inherited by all descendants, each lineage then modifies them differently. The more similar two species' early development, the more closely related they tend to be. [1, explains shared features via common ancestry] [1, links degree of embryonic similarity to degree of relatedness]

Comparative embryology is one of several independent lines of evidence for evolution, alongside the fossil record, comparative anatomy (homologous structures), biochemical/molecular data and biogeography. [1, places embryology among the other lines of evidence] Its real power emerges when it agrees with these other lines. Because embryology and molecular data are measured in completely different ways, when they independently produce the same evolutionary relationships, coincidence or error becomes extremely unlikely, which strengthens the overall case for common ancestry. [1, explains why convergence of independent lines strengthens the evidence]

Marking criteria.

• 1 mark Defines comparative embryology.
• 1 mark Names specific shared embryonic features (pharyngeal arches, post-anal tail).
• 1 mark Describes what happens to the features in the adult (lost or transformed).
• 1 mark Explains shared features as inherited from a common ancestor.
• 1 mark Links the degree of embryonic similarity to the degree of relatedness.
• 1 mark Places embryology among the other independent lines of evidence.
• 1 mark Explains why agreement of independent lines strengthens the evidence.

Q2, Sample Band 6 response (8 marks), annotated

The change from 2 percent to 85 percent resistant is best explained by natural selection acting on pre-existing variation, not by the antibiotic creating resistance. [1, explicit judgement]

Within the original bacterial population there was genetic variation: the genetic testing confirms the resistance gene was already present at low frequency before the antibiotic was introduced. By chance, a small minority of bacteria carried this resistance gene. [1, role of pre-existing genetic variation, supported by the stimulus]

Step by step: when the antibiotic was used widely on the ward, it acted as a selection pressure that killed the susceptible (non-resistant) bacteria. The few resistant bacteria survived. [1, antibiotic as selection pressure killing susceptible bacteria] These survivors reproduced, and because bacteria reproduce very rapidly, the resistance gene was passed on to many offspring. Over successive generations the proportion of resistant bacteria in the population rose. [1, differential survival and reproduction over generations]

The data support this directly: resistance rose from 2 percent to 85 percent over six weeks of antibiotic use. This is exactly the pattern expected if the antibiotic selectively removed susceptible bacteria, allowing the small resistant fraction to expand and dominate the population. [1, uses the 2 percent to 85 percent data to support the explanation] Because this change in gene frequency by differential survival and reproduction was observed and measured within weeks, it is an example of evolution by natural selection occurring in real time. [1, identifies this as observable evolution by natural selection]

The student who says “the antibiotic taught the bacteria to resist it” is incorrect. The antibiotic did not teach or create resistance; the resistance gene was already present before the antibiotic was introduced. The antibiotic only selected for the bacteria that already had it. [1, evaluates and rejects the "taught/created" misconception with reference to the stimulus] In conclusion, the rise in resistance is a clear case of natural selection acting on existing heritable variation, the antibiotic selects, it does not create, and the data show this evolution happening in real time. [1, justified conclusion using precise vocabulary]

Marking criteria (8 marks).

• 1 mark Explicit judgement: change is natural selection on pre-existing variation, not creation.
• 1 mark Explains pre-existing genetic variation, supported by the stimulus.
• 1 mark Identifies the antibiotic as a selection pressure that kills susceptible bacteria.
• 1 mark Explains differential survival and reproduction over generations.
• 1 mark Uses the 2 percent to 85 percent data to support the explanation.
• 1 mark Identifies this as observable evolution by natural selection in real time.
• 1 mark Evaluates and rejects the "antibiotic taught/created resistance" misconception.
• 1 mark Reaches a justified conclusion using precise vocabulary (variation, selection pressure, natural selection).

Q3, Sample Band 6 response (6 marks)

The claim is partly defensible but largely flawed. [1, overall evaluative judgement]

What is defensible: comparative embryology is a genuine and valuable line of evidence for evolution. Shared early embryonic features such as pharyngeal arches and a post-anal tail in vertebrates are real and are well explained by common ancestry, so it is reasonable to treat embryology as supporting evolution. [1, concedes the defensible element with example]

What is flawed: the claim that it is the “single most important and reliable” line, and that the other lines are unnecessary, is incorrect. The strength of the evidence for evolution does not come from any one line on its own, it comes from the convergence of several independent lines (fossils, comparative anatomy, embryology, molecular data and biogeography) on the same conclusion. [1, refutes "single most important" by explaining strength comes from convergence] Because these lines are gathered using completely different and independent methods, when they all agree, coincidence or error becomes extremely unlikely. Relying on embryology alone would discard this powerful independent agreement. [1, explains why independent agreement is what makes the case strong]

Furthermore, embryonic similarity alone can be misleading or limited; a single line of evidence is always weaker than multiple lines, and conclusions about relatedness are far more secure when embryology agrees with molecular (DNA) data and the fossil record. [1, notes the limitation of relying on a single line]

Defensible reformulation: “Comparative embryology is one valuable, independent line of evidence for evolution, but it is not inherently the single most important one. The case for evolution is strongest because several independent lines of evidence, fossils, comparative anatomy, embryology, molecular data and biogeography, all converge on the same conclusion of common ancestry. No single line replaces the others.” [1, biologically defensible reformulation]

Marking criteria.

• 1 mark States an overall evaluative judgement (e.g. “partly defensible but largely flawed”).
• 1 mark Identifies the defensible element: embryology is a genuine line of evidence with a valid example.
• 1 mark Refutes “single most important” by explaining that strength comes from convergence of lines.
• 1 mark Explains why independent agreement makes coincidence or error unlikely.
• 1 mark Notes the limitation of relying on a single line of evidence.
• 1 mark Reformulates the claim into a biologically defensible statement that values converging lines.