QUESTION 149 (5 marks)
Two groups descend from one flowering-plant population in the same field. A heritable flowering-time difference develops: group P flowers in March–April and group Q in September–October, even when grown under the same conditions. Pollen cannot persist between these periods. Later experimental crosses also produce sterile hybrids.
a) Identify the speciation pathway most consistent with the shared geographic area. [1 mark]
b) Explain how flowering time can reduce gene flow and contribute to speciation. [3 marks]
c) Explain what the sterile-hybrid result adds to the flowering-time evidence. [1 mark]
Practice marking scheme
Answer
Temporal isolation reduces pollen exchange without a geographic barrier, supporting a sympatric pathway. Reduced gene flow permits divergence; sterile hybrids supply further evidence of reproductive isolation.
Working
Non-overlapping flowering times prevent ordinary cross-fertilisation. The groups occupy the same geographic area, so the initial isolation is temporal rather than a physical barrier. Mutation, drift and selection can act separately as gene exchange falls. Later hybrid sterility indicates an additional reproductive barrier, beyond simply flowering at different times.
Marking criteria
- Identifies sympatric speciation/pathway. [1 mark]
- Links non-overlapping flowering to reduced cross-fertilisation. [1 mark]
- Links reduced cross-fertilisation to reduced gene flow. [1 mark]
- Explains subsequent genetic divergence through selection, drift or mutation. [1 mark]
- Identifies evidence of an additional reproductive barrier/hybrid infertility. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.