Speciation
When Australia and Antarctica separated approximately 45 million years ago during the Gondwana breakup, the marsupial lineages isolated on the drifting Australian continent had no further gene flow with placental mammals elsewhere. Over 45 million years of isolation, 155 marsupial species radiated to fill every available ecological niche — including the thylacine, which evolved wolf-like anatomy, hunting behaviour, and body form from a completely different ancestor than the placental wolf. Two animals, the same ecological niche, zero shared ancestry since the Cretaceous: speciation through geographic isolation is the most powerful explanation for this pattern.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- The definition of speciation and the biological species concept.
- The stages of allopatric speciation.
- The difference between pre-zygotic and post-zygotic isolation.
Understand
- Why geographic isolation can lead to divergence over time.
- Why speciation is not goal-directed or intentional.
- How sympatric speciation differs from allopatric speciation.
Can Do
- Explain an allopatric-speciation sequence clearly.
- Classify reproductive barriers as pre- or post-zygotic.
- Use Australian marsupials or Galapagos finches as evidence-based examples.
Core Content
What has to happen before one lineage becomes two
When Australia separated from Antarctica approximately 45 million years ago during the Gondwana breakup, the marsupial ancestors isolated on the drifting continent could no longer interbreed with the placental mammals evolving on other landmasses. Over millions of generations with no gene flow, mutations accumulated independently, selection pressures differed, and the two lineages diverged so completely that they can no longer produce fertile offspring together — they became separate species. This is speciation: one ancestral species diverges into 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. The biological species concept: organisms that can interbreed and produce fertile offspring belong to the same species. Speciation = divergence into two or more reproductively isolated lineages that can no longer exchange genes through fertile offspring. That makes the horse and donkey separate species even though mating and fertilisation can occur.
Interbreed
Members of the same species can mate naturally.
Fertile Offspring
The offspring must be able to reproduce too.
Gene Flow
Speciation requires gene flow to stop between diverging populations.
Pause — copy the highlighted biological species concept definition into your book before the check below.
Which statement best defines speciation?
Population split → barrier → divergence → reproductive isolation
We just saw that speciation requires gene flow to stop between populations. That raises a question: what is the most common way for gene flow to stop? This card answers it → a geographic barrier physically separates one population into two isolated groups, beginning allopatric speciation.
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. Allopatric speciation sequence: geographic barrier → gene flow reduced → independent divergence under different pressures → reproductive isolation builds up → speciation. Even removing the barrier later cannot restore successful interbreeding once sufficient divergence has occurred. At that point, speciation has occurred.
The barrier starts the separation, but speciation depends on enough divergence building up that interbreeding no longer restores gene flow.
| 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 |
Pause — copy the highlighted allopatric speciation sequence into your book before the check below.
What is the first key step in allopatric speciation?
How barriers block gene flow before or after fertilisation, and when new species can arise without geographic separation
We just saw that allopatric speciation requires a physical barrier followed by divergence. That raises a question: can reproductive isolation happen in more specific ways, and can it ever arise without a geographic barrier? This card answers it → isolation can act before or after fertilisation (pre- vs post-zygotic), and sympatric speciation requires neither geography nor a barrier.
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 × donkey is sterile |
Pre-Zygotic
Stops the zygote from forming.
Post-Zygotic
The zygote forms, but successful long-term gene flow still fails.
Key Outcome
Both types block gene flow and keep lineages separate.
Most of this lesson focuses on allopatric speciation, but speciation can also happen without geographic isolation. Pre-zygotic barriers prevent mating or fertilisation (e.g. different mating seasons, mating calls, mechanical incompatibility). Post-zygotic barriers allow fertilisation but hybrids are inviable or sterile (e.g. the mule). Sympatric speciation occurs within the same area without a geographic barrier — most commonly in plants through polyploidy. Bread wheat is a classic example: a hexaploid lineage formed through hybridisation involving three ancestral species.
Pause — copy the highlighted pre/post-zygotic and sympatric speciation summary into your book before the check below.
Which example is post-zygotic isolation?
Activities
Barrier to Species
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.
Pre or Post?
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.
True or false: sympatric speciation can only occur when a physical geographic barrier separates two populations.
Speciation Definition
- Speciation is the formation of new species from an ancestral species.
- It requires reproductive isolation so gene flow stops.
Allopatric Speciation
- Barrier divides the population.
- Different selection pressures and mutations cause divergence.
- Enough divergence leads to reproductive isolation.
Reproductive Isolation
- Pre-zygotic barriers stop fertilisation.
- Post-zygotic barriers allow fertilisation but hybrids fail or are sterile.
Sympatric Speciation
- Occurs in the same geographic area.
- Often linked to polyploidy in plants.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3–4(4 marks) 1. Explain the sequence of events in allopatric speciation.
AnalyseBand 3–4(3 marks) 2. Distinguish between pre-zygotic and post-zygotic isolation using one example of each.
EvaluateBand 4–5(3 marks) 3. Explain why Australian marsupials are a useful example when discussing allopatric speciation.
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Barrier to Species
The river change provides geographic isolation: the river acts as a barrier that prevents the two frog populations from interbreeding. Over many generations, each population accumulates different mutations and faces different selection pressures, causing divergence. As divergence builds up, mating calls become different enough that even if the barrier is removed, the frogs no longer recognise each other's calls — reproductive isolation (pre-zygotic) has been established. Speciation has occurred.
Activity 2 — Pre or Post?
Different flowering times are pre-zygotic: they prevent fertilisation from occurring because the two species do not flower simultaneously. Incompatible reproductive structures in insects are pre-zygotic: mating physically cannot succeed, so fertilisation never occurs. Mule sterility is post-zygotic: fertilisation does occur between horse and donkey, but the resulting hybrid (the mule) is sterile, so gene flow does not continue.
Short Answer Model Responses
SA1 (4 marks): Allopatric speciation begins when a geographic barrier such as a river, mountain range or ocean divides one ancestral population into isolated groups [1]. Because gene flow is reduced, the two populations accumulate different mutations, experience different selection pressures and evolve different adaptations over time [1]. As allele frequencies diverge further, reproductive barriers build up [1]. Eventually, even if the barrier is removed, the populations can no longer interbreed successfully to produce fertile offspring, so separate species exist [1].
SA2 (3 marks): Pre-zygotic isolation prevents mating or fertilisation from happening in the first place [1]. 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 [1]. 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 [1].
SA3 (3 marks): Australian marsupials are a useful allopatric-speciation example because Australia became geographically isolated after separating from Gondwana [1]. That long isolation reduced gene flow with mammal populations elsewhere and allowed marsupial lineages to accumulate different adaptations over time [1]. The result was diversification into many distinct marsupial species, showing how geographic isolation can support speciation [1].
Biological species concept
Interbreed + fertile offspring. The mule matters because fertilisation occurs but the hybrid is sterile — horse and donkey are separate species.
Allopatric sequence
Barrier → isolated populations → diverge under different pressures → reproductive isolation → speciation.
Pre vs post-zygotic
Pre: stops fertilisation. Post: fertilisation happens but hybrids fail or are sterile. Both block gene flow.
Sympatric route
No geographic barrier needed. Polyploidy in plants creates instant reproductive isolation in the same area.
Rapid-fire questions on speciation types, reproductive barriers and Australian examples. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
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.
The Australia-Antarctica separation approximately 45 million years ago during the Gondwana breakup is the most dramatic real-world example: 155 marsupial species evolved in isolation, including the thylacine — which developed wolf-like anatomy, body form, and hunting behaviour from a completely different ancestral lineage to the placental wolf. The two populations (marsupial and placental lineages) are now so diverged that they cannot produce fertile offspring together. Geographic isolation was the initial barrier; over 45 million years of independent evolution, that isolation became permanent biological incompatibility. That is speciation by allopatric divergence.