Compare allopatric and sympatric speciation
Explain how geographical, ecological or behavioural isolation can reduce gene flow and lead to reproductive isolation.
Section 17.3 · Evolution
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What begins allopatric speciation in the standard model?
What you will learn
- Explain allopatric speciation following geographical separation
- Explain sympatric speciation through ecological or behavioural separation
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Recorded group | Olive 18; bronze 12 |
| Sample total | 30 beetles |
| Olive proportion | 18/30 = 60% |
| Evidence limit | Sample phenotype count; not a genotype census |
Build the evolutionary explanation
Allopatric speciation begins when geographical separation restricts gene flow between populations. Different mutations, selection pressures and genetic drift can cause them to diverge. Sympatric speciation occurs without geographical separation when ecological or behavioural differences reduce interbreeding. In either case, divergence can ultimately result in reproductive isolation, so populations no longer exchange alleles successfully.
Interpret data with care
Geographical separation can initiate allopatric divergence but does not automatically produce a new species. Sympatric speciation requires reduced gene flow within the same geographic area; simply living together does not rule it out.
Pause and explain
A barrier is present but individuals continue to interbreed frequently across it. Has allopatric speciation necessarily begun? Explain.
Worked example
A river separates two populations and they diverge until they cannot interbreed. Name the speciation route.
Show the worked solution
- A geographical barrier restricts gene flow.
- The populations become reproductively isolated.
Answer Allopatric speciation.
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Explain how geographical separation can lead to allopatric speciation.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- The barrier reduces gene flow.
- Different evolutionary changes accumulate.
Geographical separation restricts gene flow between populations. They can accumulate different mutations and experience different selection or drift; if reproductive isolation develops, speciation has occurred.
Look for these in your work
- Population process identified
- Cause and consequence linked
Two insect populations on the same island use different host plants and mate at different times. Which route could this represent?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- They occupy the same broad geographical area.
- Ecological or behavioural separation reduces gene flow.
It could represent sympatric speciation if host preference and mating time reduce gene flow enough for reproductive isolation to develop.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A barrier is present but individuals continue to interbreed frequently across it. Has allopatric speciation necessarily begun? Explain.
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- A physical barrier matters through its effect on gene flow.
- Frequent interbreeding maintains gene exchange.
Not necessarily. If interbreeding continues, gene flow may remain high; geographical separation promotes allopatric divergence only when it restricts gene flow sufficiently.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Saying speciation happens immediately after separation
- Defining sympatric speciation as any two species living together
What can you explain now?
What begins allopatric speciation in the standard model?
Compare with the explanation
Geographical separation that restricts gene flow
The separated populations can then diverge.
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Make it stick
What differs between the initiating isolation in allopatric and sympatric speciation?
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