Isolating mechanisms
CIE A-Level BiologyΒ· 30 min read
1. Prezygotic Isolating Mechanismsβ β ββββ± 15 min
Prezygotic Isolation
Reproductive barriers that prevent fertilisation from occurring, so no zygote is formed. These barriers avoid wasting gametes, making them more energy-efficient than postzygotic barriers.
Geographical (ecological) isolation: Populations are separated by physical barriers or occupy different habitats, so never meet to breed.
Seasonal (temporal) isolation: Populations have different breeding seasons or active periods, so do not meet when sexually receptive.
Behavioural isolation: Unique courtship displays, mating calls or pheromones only attract members of the same species.
Mechanical isolation: Morphological differences in reproductive organs prevent successful mating.
Gametic isolation: Gametes are incompatible, so sperm cannot fertilise eggs of another species.
Two species of wildflower grow in the same meadow. Species A flowers in April, while Species B flowers in June. No hybrids are observed. What type of prezygotic isolation is this? Explain why no hybrids form.
- 1
Step 1: Identify the key difference between the two populations: their breeding (flowering) timing.
- 2
Step 2: Match this to the correct prezygotic category: temporal (seasonal) isolation.
- 3
Step 3: Explain why no hybrids form: The species are not reproductively active at the same time, so pollen cannot transfer between them. Fertilisation never occurs, so no hybrid zygotes form.
2. Postzygotic Isolating Mechanismsβ β β βββ± 15 min
Postzygotic Isolation
Reproductive barriers that act after a hybrid zygote has formed, reducing the viability or fertility of hybrid offspring, preventing gene flow between species.
Hybrid inviability: Hybrid zygotes fail to develop normally and die before reaching sexual maturity.
Hybrid sterility: Hybrids develop into healthy adults but are sterile, unable to produce functional gametes.
Hybrid breakdown: First-generation (F1) hybrids are viable and fertile, but their second-generation (F2) offspring are inviable or sterile.
A horse (2n = 64) and a donkey (2n = 62) mate to produce a mule with 63 chromosomes. Mules are almost always unable to produce offspring. What type of postzygotic isolation does this show? Explain your answer.
- 1
Step 1: Confirm a hybrid zygote formed and developed into a viable mule, so this must be a postzygotic barrier.
- 2
Step 2: Since the mule is viable but cannot reproduce, this matches the definition of hybrid sterility.
- 3
Step 3: Explain why sterility occurs: The mule has an odd number of chromosomes, so homologous pairing cannot occur during meiosis. No functional gametes are produced, so the mule is sterile.
Exam tip:
CIE frequently uses the horse/donkey/mule example in exams, so always link the odd chromosome number to sterility.
3. Role of Isolating Mechanisms in Speciationβ β β βββ± 20 min
Isolating mechanisms are critical for speciation because they stop gene flow between populations. When there is no gene flow, different selective pressures act on each isolated population, leading to accumulated genetic differences over generations. Eventually, populations become so genetically distinct that even if the barrier is removed, they can no longer interbreed to produce fertile offspring, making them separate species.
Test your understanding of prezygotic vs postzygotic isolation:
Which of the following is an example of a postzygotic isolating mechanism?
A. Behavioural isolation
B. Hybrid sterility
C. Temporal isolation
D. Gametic isolation
Reveal answer
B βCorrect! Hybrid sterility acts after zygote formation. All other options are prezygotic barriers that prevent fertilisation.
Explain how geographical isolation of tree snail populations on two separate islands leads to new species formation.
- 1
Step 1: The sea between the islands acts as a prezygotic geographical barrier that completely stops gene flow between the two snail populations.
- 2
Step 2: The two populations experience different environmental conditions: different food sources, predators, and abiotic factors. Different alleles are selected for in each population.
- 3
Step 3: Genetic differences accumulate between the populations over many generations, leading to reproductive incompatibility.
- 4
Step 4: Even if sea levels fall and the islands reconnect, the snails can no longer interbreed to produce fertile offspring. Isolating mechanisms maintain the separation between the two new species.
4. Common Pitfalls
Wrong move:
Classifying geographical isolation as a postzygotic barrier
Why:
Geographical isolation prevents mating and fertilisation from occurring at all
Correct move:
All barriers that act before zygote formation, including geographical isolation, are prezygotic.
Wrong move:
Claiming mules are an example of hybrid inviability
Why:
Mules develop into healthy, viable adults, they are only unable to reproduce
Correct move:
Mules are a classic example of hybrid sterility, not hybrid inviability.
Wrong move:
Confusing geographical isolation itself with speciation
Why:
Isolation is just the barrier that stops gene flow; speciation requires genetic divergence
Correct move:
Always link isolation β reduced gene flow β genetic divergence β reproductive incompatibility β new species in long answers.
Wrong move:
Stating postzygotic barriers are more common than prezygotic barriers
Why:
Prezygotic barriers avoid wasting gametes, so they are far more common in most sexually reproducing species
Correct move:
Remember prezygotic barriers are generally more energy-efficient and widespread than postzygotic barriers.
5. Quick Reference Cheatsheet
Isolation Type | Category | Key Description | Common Example |
|---|---|---|---|
Geographical | Prezygotic | Separated by physical barrier | Snails on separate islands |
Temporal | Prezygotic | Different breeding timing | Flowers flowering different seasons |
Behavioural | Prezygotic | Different courtship behaviour | Unique bird mating calls |
Mechanical | Prezygotic | Incompatible reproductive structures | Different insect species |
Gametic | Prezygotic | Incompatible gametes | Sea urchin cross-species fertilisation |
Hybrid inviability | Postzygotic | Hybrid zygote dies early | Mouse interspecies hybrid embryos |
Hybrid sterility | Postzygotic | Viable but sterile hybrid | Horse + donkey = sterile mule |
Hybrid breakdown | Postzygotic | F1 fertile, F2 inviable | Cotton interspecies hybrids |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Compare pre vs postzygotic isolation
- 2021 Β· 1
MCQ identify isolation type
- 2023 Β· 4
Role of isolation in speciation
What's Next
Isolating mechanisms are a core foundation for understanding how speciation occurs, connecting the definition of a species to the evolutionary processes that generate biodiversity. This topic is regularly assessed alongside natural selection and speciation in long answer questions for CIE Paper 2 and Paper 4, so linking these concepts is key to scoring full marks. Mastering isolating mechanisms will also help you tackle questions about evolution and biodiversity in extended response questions.