When Australia broke away from Antarctica about 45 million years ago, the mammals isolated on that drifting landmass followed their own evolutionary path. Speciation is how one ancestral population can split into new species when isolation, mutation, selection and time push populations so far apart that they can no longer interbreed.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Take a position before we formalise the process.
1. If two populations are separated by a mountain range for thousands of generations, what would have to change before you would call them different species?
2. If two organisms can mate but their offspring are sterile, are they the same species?
Write your starting answer now. We will revisit it once the reproductive-isolation examples are clear.
Write your initial answer in your book, then return later to compare it with your final explanation.
Core Content
Connect this concept to the broader biology framework. Understanding how systems interact is essential for HSC success.
What has to happen before one lineage becomes two
Speciation happens when one ancestral species diverges into two or more populations that become reproductively isolated, meaning they can no longer exchange genes by producing fertile offspring together.
The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. That is why a mule matters in this topic. A horse and a donkey can mate and produce a mule, but the mule is sterile, so gene flow does not continue between the parent lineages. That makes the horse and donkey separate species even though mating and fertilisation can occur.
Population split -> barrier -> divergence -> reproductive isolation
Allopatric speciation begins when a geographic barrier physically divides one population into two isolated groups.
Once separated by an ocean, river, glacier, mountain range or disappearing land bridge, the populations stop exchanging genes easily. They then experience different selection pressures, accumulate different mutations, and evolve different adaptations. If enough divergence builds up, even removing the barrier later will not restore successful interbreeding. At that point, speciation has occurred.
| Stage | What Happens | Why It Matters |
|---|---|---|
| Population split | A barrier divides one ancestral population | Gene flow drops sharply |
| Independent evolution | Each population faces different mutations and selection pressures | Allele frequencies diverge |
| Accumulated differences | Adaptations, mating signals or chromosomes become less compatible | Isolation grows stronger |
| Speciation | Even if contact returns, fertile interbreeding no longer occurs | Two species now exist |
How barriers block gene flow before or after fertilisation, and when new species can arise without geographic separation
Reproductive isolation can act either before fertilisation happens or after fertilisation has already occurred.
| Barrier Type | Meaning | Examples |
|---|---|---|
| Pre-zygotic | Prevents mating or gamete fusion from happening at all | Different mating seasons, courtship behaviours, mating calls, mechanical incompatibility, geographic separation |
| Post-zygotic | Fertilisation occurs, but hybrids die young or are infertile | Mule from horse x donkey is sterile |
Most of this lesson focuses on allopatric speciation, but speciation can also happen without geographic isolation. Sympatric speciation occurs within the same area, most commonly in plants through polyploidy, where chromosome number changes suddenly create reproductive barriers. Bread wheat is a classic example: a hexaploid lineage formed through hybridisation involving three ancestral species.
Activities
A river changes course and splits one frog population in two. Over many generations, the two groups evolve different mating calls. Explain how this could lead to speciation using the terms geographic isolation, divergence and reproductive isolation.
Write the sequence in order rather than listing disconnected facts.
Sketch the sequence in your book first, then write the explanation here.
Classify each barrier as pre-zygotic or post-zygotic and justify one example: different flowering times in plants, incompatible reproductive structures in insects, and sterile mule offspring.
A strong answer should connect the category to whether fertilisation happens.
Make a two-column table in your book first, then write the final answer here.
The cleanest way to explain speciation is as a breakdown of gene flow. Barriers create separation, divergence builds up over generations, and reproductive isolation locks the split in place.
If your first answer focused only on organisms looking different, the key correction is this: species status depends on reproductive isolation and fertile offspring, not just visible change.
Assessment
Answer first, then read the explanation
1. Which statement best defines speciation?
2. What is the first key step in allopatric speciation?
What is NOT the first key step in allopatric speciation?
3. Which example is post-zygotic isolation?
4. Why can isolated populations diverge over time?
5. Which statement about sympatric speciation is most accurate?
1. Explain the sequence of events in allopatric speciation. (4 marks)
1 mark: barrier splits population | 1 mark: independent divergence | 1 mark: reproductive isolation | 1 mark: clear sequence
2. Distinguish between pre-zygotic and post-zygotic isolation using one example of each. (3 marks)
1 mark each for correct definition/example pairing, plus 1 mark for clear distinction
3. Explain why Australian marsupials are a useful example when discussing allopatric speciation. (3 marks)
1 mark: isolation of Australia | 1 mark: divergence over time | 1 mark: link to diversification/speciation
Answers
SA1: Allopatric speciation begins when a geographic barrier such as a river, mountain range or ocean divides one ancestral population into isolated groups. Because gene flow is reduced, the two populations accumulate different mutations, experience different selection pressures and evolve different adaptations over time. As allele frequencies diverge further, reproductive barriers build up. Eventually, even if the barrier is removed, the populations can no longer interbreed successfully to produce fertile offspring, so separate species exist.
SA2: Pre-zygotic isolation prevents mating or fertilisation from happening in the first place. An example is different flowering times in plants or different mating calls in animals. Post-zygotic isolation occurs after fertilisation, but the hybrid offspring fail to survive or are infertile. A mule from a horse and a donkey is a classic example because the hybrid is sterile. The difference is whether the barrier acts before or after the zygote forms.
SA3: Australian marsupials are a useful allopatric-speciation example because Australia became geographically isolated after separating from Gondwana. That long isolation reduced gene flow with mammal populations elsewhere and allowed marsupial lineages to accumulate different adaptations over time. The result was diversification into many distinct marsupial species, showing how geographic isolation can support speciation.
Say each answer aloud before moving to the next prompt