QUESTION 148 (5 marks)
A river separates two populations of one frog species for thousands of generations. Their allele frequencies diverge. After the river changes course, the frogs meet, interbreed readily under natural conditions and produce viable, fertile offspring.
a) Explain how the river could begin an allopatric speciation pathway. [3 marks]
b) Explain why allele-frequency divergence alone does not establish that speciation was completed in this case. [2 marks]
Practice marking scheme
Answer
The river could initiate an allopatric pathway by reducing gene flow. The later breeding evidence does not support completed reproductive isolation or classification as separate biological species.
Working
Geographic separation reduces allele exchange, allowing mutation, drift and different selection pressures to produce divergent gene pools. Such divergence can lead to reproductive isolation, but it does not guarantee it. The observed natural interbreeding with fertile offspring indicates that the tested populations remain reproductively compatible.
Marking criteria
- Identifies geographic isolation. [1 mark]
- Links isolation to reduced gene flow. [1 mark]
- Explains divergence through mutation, drift or different selection pressures. [1 mark]
- States that genetic divergence alone does not guarantee reproductive isolation. [1 mark]
- Uses viable fertile offspring from natural interbreeding as contrary evidence for completed isolation. [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.