Biology • Year 11 • Module 3 • Lesson 7
Speciation
Apply allopatric speciation to new scenarios, classify reproductive barriers, reason about Australian marsupials, and interpret a speciation data scenario.
1. Apply allopatric speciation — frog population
Read the scenario and answer the questions below. 8 marks
Scenario. A river in eastern Australia changes course over thousands of years, splitting a tree frog population into two groups: one on the northern bank and one on the southern bank. Over many generations, the northern population evolves a slightly different call frequency and the two groups now rarely respond to each other’s calls. When researchers bring individuals from both populations together, they attempt to mate only with members of their own group and rarely produce offspring with the other group. When hybrid tadpoles are produced, they develop more slowly and most fail to metamorphose.
1.1 Identify the type of speciation occurring and justify your choice. 2 marks
1.2 Identify one pre-zygotic and one post-zygotic reproductive barrier present in the scenario. Justify each. 4 marks
1.3 Would removing the river barrier at this point necessarily mean the two frog populations merge back into one species? Explain your reasoning. 2 marks
2. Allopatric vs sympatric speciation — comparison table
Complete the empty cells. 6 marks — 1 per cell
| Feature | Allopatric speciation | Sympatric speciation |
|---|---|---|
| Geographic barrier required? | Yes | |
| How gene flow is blocked | Chromosome changes (e.g. polyploidy) or ecological differences | |
| Common in which organisms? | Many animal groups, including marsupials | |
| Australian example | Bread wheat (hexaploid formed through hybridisation) | |
| Key defining feature | Physical separation precedes reproductive isolation |
3. Cause-and-effect chain — marsupial speciation in Australia
Complete the cause-and-effect chain. 5 marks
Step 1: _________________________ between Australian mammal lineages and those on other landmasses stopped or became very limited.
Step 2: Marsupial lineages in Australia faced _________________________ selection pressures than placental mammals elsewhere and accumulated distinct _________________________ over millions of years.
Step 3: This is an example of _________________________ speciation, driven by the geographic barrier of oceanic isolation.
Outcome: Australia today has many _________________________ marsupial species — species found only in Australia — reflecting millions of years of independent evolution.
Q1.1 — Speciation type and justification
Allopatric speciation [1]. The river created a geographic barrier that physically separated one population into two isolated groups, reducing gene flow between them. Both groups then diverged independently (the northern population evolved a different call frequency), consistent with the definition of allopatric speciation as speciation driven by geographic isolation [1].
Q1.2 — Pre-zygotic and post-zygotic barriers
Pre-zygotic: The different call frequencies act as a pre-zygotic barrier [1] because the two groups rarely respond to each other’s calls and rarely attempt to mate — this prevents fertilisation from occurring in the first place (behaviour-based mating isolation before zygote forms) [1].
Post-zygotic: The failure of hybrid tadpoles to metamorphose is a post-zygotic barrier [1] because fertilisation does occur (hybrid offspring are produced) but those hybrids develop abnormally and most fail to survive to adulthood, blocking successful gene flow between the lineages [1].
Q1.3 — Does removing the barrier reverse speciation?
No, removing the river barrier would not necessarily cause the populations to merge back into one species [1]. Both pre-zygotic (behavioural) and post-zygotic (hybrid failure) reproductive isolation mechanisms have already developed. Even without the physical barrier, the two populations would rarely interbreed successfully and would not restore significant gene flow. The lesson notes that “even if contact returns, fertile interbreeding no longer occurs” — speciation is already complete or substantially advanced [1].
Q2 — Comparison table
- Geographic barrier (sympatric): No — occurs in the same geographic area without physical separation.
- How gene flow is blocked (allopatric): Physical geographic barrier (mountain, river, ocean, etc.) prevents contact and reduces gene flow.
- Common in (sympatric): Plants (especially through polyploidy); also some animal examples through ecological specialisation.
- Australian example (allopatric): Marsupial diversification after Australia’s separation from Gondwana.
- Key defining feature (sympatric): Reproductive isolation arises within the same geographic area, usually through chromosome changes or strong ecological differences, without prior physical separation.
Award 1 mark per cell (6 cells in empty column). Accept valid answers consistent with lesson content.
Q3 — Cause-and-effect chain
- Step 1: Gene flow [1]
- Step 2: different; adaptations / mutations [1 for both; accept equivalent terms]
- Step 3: allopatric [1]
- Outcome: endemic [1]