QUESTION 150 (5 marks)
One plant species occupies adjoining metal-rich and metal-poor soils without a physical barrier. Plants disperse pollen mainly to nearby plants. Metal-tolerance alleles are common in the metal-rich soil and rare in the metal-poor soil. A narrow boundary zone contains hybrids, many with reduced fertility.
a) Identify the speciation pathway most consistent with the adjoining ranges. [1 mark]
b) Explain how spatial mating patterns and soil conditions could contribute to divergence. [3 marks]
c) Explain why the boundary-zone hybrids prevent a conclusion that there is no genetic exchange. [1 mark]
Practice marking scheme
Answer
The pattern is consistent with a parapatric pathway. Spatially biased mating and different selection pressures reduce effective gene exchange, while hybrids show that exchange is not necessarily zero.
Working
Adjacent ranges can diverge when local mating limits exchange over a gradient and different soil conditions favour different alleles. Reduced hybrid fertility can further restrict gene flow at the boundary. The evidence supports ongoing divergence rather than proving that all gene flow has stopped or that complete isolation is already established.
Marking criteria
- Identifies parapatric speciation/pathway. [1 mark]
- Links local pollen dispersal/proximity mating to restricted gene flow. [1 mark]
- Identifies different selection pressures across the soil gradient. [1 mark]
- Links reduced gene exchange and selection to genetic divergence; accepts reduced hybrid fertility as an additional barrier. [1 mark]
- States that hybrids indicate crosses occur and reduced fertility does not necessarily mean zero transmission. [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.