Biology • Year 11 • Module 3 • Lesson 12
Comparative Embryology & Evidence for Evolution
Apply comparative embryology to data, reason about converging lines of evidence, and explain antibiotic resistance using the language of natural selection.
1. Interpret the embryo comparison table
The table shows how many early embryonic features four species share with species W. Use it to answer the questions. 8 marks
| Species | Early embryonic features shared with species W (out of 10) |
|---|---|
| Species W | 10 (reference species) |
| Species X | 9 |
| Species Y | 5 |
| Species Z | 1 |
1.1 Which species is most likely the most closely related to species W? Justify using comparative embryology. 2 marks
1.2 Which species is most likely the least closely related to species W, and why? 2 marks
1.3 Human embryos briefly develop a post-anal tail and pharyngeal arches. Explain what this suggests about human ancestry. 2 marks
1.4 Explain why comparative embryology is stronger evidence when it agrees with molecular (DNA) data. 2 marks
2. Analyse an antibiotic resistance scenario
Use the information below to answer the questions. 8 marks
Scenario. A patient with a bacterial infection begins a course of antibiotics. After three days they feel better and stop taking the medication, leaving some bacteria alive. A week later the infection returns, and this time the same antibiotic has little effect. A laboratory test confirms the new bacterial population is largely resistant to the antibiotic.
2.1 Before treatment, where did the resistance in the bacterial population come from? 2 marks
2.2 Explain how stopping the course early made the population more resistant, using the term selection pressure. 2 marks
2.3 A friend says “the antibiotic made the surviving bacteria become resistant.” Identify and correct the misconception. 2 marks
2.4 Explain why antibiotic resistance is described as evolution that can be observed in real time. 2 marks
3. Cause-and-effect chain, how antibiotic resistance spreads
Complete the cause-and-effect chain by filling in each empty box. The first box is done for you. 5 marks
Step 1: An antibiotic is used. It acts as a _________________________ that kills the susceptible (non-resistant) bacteria.
Step 2: The few _________________________ bacteria survive the treatment.
Step 3: The survivors _________________________, passing the resistance gene to their offspring.
Outcome: Over _________________________ the proportion of resistant bacteria increases, this change in the population by differential survival is _________________________.
4. Applied scenario, weighing the evidence
Read each scenario and decide whether the conclusion is well supported by the evidence. Explain your reasoning. 4 marks
4.1 A student concludes two species are closely related based on a single shared anatomical feature alone. 2 marks
4.2 A scientist concludes two species are closely related because their early embryos are very similar AND their DNA sequences are very similar AND the fossil record links them. 2 marks
Q1.1, Most closely related
Species X is most likely the most closely related to species W [1], because it shares the most early embryonic features (9 out of 10) with W; in comparative embryology, the more similar two species' early development, the more closely related they tend to be [1].
Q1.2, Least closely related
Species Z is most likely the least closely related to species W [1], because it shares the fewest early embryonic features (only 1 out of 10), indicating the most distant common ancestor and the least conserved shared development [1].
Q1.3, Human tail and arches
The brief appearance of a post-anal tail and pharyngeal arches in human embryos suggests that humans inherited these features from a common vertebrate ancestor that possessed them [1]. They are present because the developmental pathways are conserved and inherited, even though they are later lost or transformed in humans, this is evidence of common ancestry with other vertebrates [1].
Q1.4, Agreement with molecular data
Comparative embryology and molecular (DNA) data are measured in completely different and independent ways [1]. When two independent methods produce the same relationships between species, the chance of coincidence or error becomes very small, so the combined evidence is far stronger than either line alone [1].
Q2.1, Origin of resistance
The resistance was already present in the population before treatment, as pre-existing genetic variation [1]. By chance, a few bacteria carried a resistance gene (e.g. from a mutation), so the antibiotic did not create the resistance, it was there beforehand [1].
Q2.2, Stopping early
The antibiotic acts as a selection pressure that kills the susceptible bacteria but not the resistant ones [1]. Stopping the course early leaves resistant bacteria alive; with the susceptible competitors removed, these resistant survivors reproduce and become a larger proportion of the population, increasing overall resistance [1].
Q2.3, Misconception
The misconception is that the antibiotic caused or created resistance in the bacteria [1]. Correctly: the antibiotic does not make bacteria resistant; it selects for bacteria that were already resistant due to pre-existing variation. The resistant individuals survive and reproduce, so resistance spreads through natural selection, the antibiotic selects, it does not create [1].
Q2.4, Evolution in real time
Bacteria reproduce very rapidly (e.g. about every 20 minutes), so the change in the frequency of resistance genes through differential survival and reproduction (natural selection) can be directly observed and measured within days [1]. This makes antibiotic resistance an example of evolution by natural selection happening in real time, which is strong modern evidence for evolution [1].
Q3, Cause-and-effect chain (marking criteria)
- Step 1: selection pressure [1]
- Step 2: resistant [1]
- Step 3: survive and reproduce [1]
- Outcome: (successive) generations; natural selection [1]
Q4.1, Single feature conclusion (weak)
This conclusion is not well supported [1]. A single shared anatomical feature is weak evidence because similar features can arise through convergent evolution in unrelated species (analogous structures). Relatedness should be based on multiple independent lines of evidence, not one feature alone [1].
Q4.2, Converging lines conclusion (strong)
This conclusion is well supported [1]. Embryology, molecular data and the fossil record are independent lines of evidence measured in different ways; when they all converge on the same conclusion, coincidence or error becomes extremely unlikely, so the combined evidence is strong [1].