Evolution changes a species. Speciation splits one species into two. The trick is knowing when the split has actually finished — and the answer is not “when they look different”. It is when they can no longer have fertile offspring together, even if you put them back in the same field.
📚 What you need to know
Speciation is the development of new species from pre-existing species over time.
It starts when gene flow between two populations stops, so genetic differences can build up.
Geographical isolation is a physical barrier splitting a population. It stops gene flow, but it can be temporary.
Differential selection means the two populations face different selection pressures, so they change in different directions.
Speciation is complete only when the populations are reproductively isolated — they can no longer interbreed to produce fertile offspring.
Speciation increases the number of species on Earth; extinction reduces it.
What speciation actually is
Definition
Speciation = the development of new species from pre-existing species over time
Remember what makes two organisms the same species: they can breed together and produce offspring that are themselves fertile. So the moment two groups permanently lose that ability, they stop being one species and become two.
Do this again and again for a few hundred million years and you get the millions of species alive today. In theory it all traces back to one original population that split, then split again, and again.
Gene flow: the thing that has to stop first
Gene flow is simply alleles moving between populations because individuals breed with each other. While gene flow continues, any new allele that appears on one side spreads to the other, and the two groups stay blended into a single species.
🤔 Why gene flow keeps a species together
Think of two ponds joined by a channel. Pour dye into one and it slowly spreads to both, so they stay the same colour. Block the channel and you can pour a different dye into each — now they drift apart and there is nothing pulling them back together. Gene flow is that channel. Mutations keep appearing in both populations either way, but only when the channel closes can those mutations build up into a real difference.
How a species splits, step by step
1
ONE SPECIESA single population, everyone breeding freely
2
BARRIERA river, sea or mountain range splits them
3
NO GENE FLOWAlleles can no longer move between the groups
4
DIVERGEDifferent pressures select different alleles
5
ISOLATEDThey can no longer make fertile offspring
The grey band in panels 2 and 3 is the physical barrier. Notice it has gone in panel 4 — that is deliberate. The split is only real if it survives the barrier being removed.
Geographical isolation vs reproductive isolation
This is the single most commonly confused pair in the whole topic, and examiners test it deliberately. They are not two names for the same thing.
GEOGRAPHICAL ISOLATION
A physical barrier keeps two populations apart: a river, a sea, a mountain range, or something we built like a motorway.
It stops gene flow, but it is reversible. Remove the barrier and they can breed again.
Speciation has not happened yet.
REPRODUCTIVE ISOLATION
Changes in alleles and phenotypes mean the two groups can no longer successfully breed with each other.
It is permanent. Even living in the same habitat, they cannot produce fertile offspring.
Speciation has happened.
Note the dot colours. In the bottom row the two groups have genuinely diverged, which is why bringing them together changes nothing.
What actually stops them breeding
Reproductive isolation happens because alleles and phenotypes drift apart until mating no longer works. A few of the ways this shows up:
Different timing. One population becomes sexually active, or flowers, at a different time of year, so the two never meet in breeding condition.
Different behaviour. Courtship displays, calls or mating rituals change until individuals no longer recognise or accept each other as mates.
Infertile offspring. Sometimes they can still mate, but the offspring cannot reproduce. That still counts as separate species, because the alleles go no further.
Careful with the definition: “same species” means producing fertile offspring. Two populations that produce healthy but sterile young are already two species.
Case study: bonobos and chimpanzees
Chimpanzees live north of the Congo river and bonobos live south of it. That is a real example of every step on this page.
🧩 The story in order
One ancestral population of apes lived across the region.
The river formed a barrier. Neither group crosses it easily, so the populations became geographically isolated and gene flow stopped.
Different selection pressures acted on each side — different food availability, different levels of competition. This is differential selection.
They diverged. Chimpanzees became more aggressive with male-dominated social groups, possibly because competition for resources was harsher. Bonobo groups are led by females and are far less aggressive.
Reproductive isolation. Eventually the two groups could no longer interbreed successfully, so speciation was complete.
You do not need the Latin names in an exam. “Bonobos” and “chimpanzees” is completely fine, so do not lose time worrying about spelling Pan paniscus.
Extinction: the other direction
Speciation adds species. Extinction removes them. A species is extinct when no individuals remain anywhere, and the alleles it carried are gone for good — the passenger pigeon and the woolly mammoth are the usual examples.
The number of species alive at any moment is the balance between these two processes. Speciation has clearly won over the long run, which is why Earth has millions of species rather than one.
Worked examples
WORKED EXAMPLE
A new river cuts through a forest, splitting a population of frogs. Explain how this could lead to speciation. [5]
Step 1: name the isolationThe river is a geographical barrier, so the two frog populations become geographically isolated.Step 2: gene flowGene flow between the two populations stops, so alleles can no longer be exchanged.Step 3: differences build upRandom mutations occur separately in each population, and genetic differences accumulate.Step 4: differential selectionThe two sides of the river have different selection pressures, so natural selection favours different alleles in each population.Step 5: the finish lineOver many generations they become reproductively isolated and can no longer produce fertile offspring — speciationFive marks, five clear stages. Write them as separate sentences.
WORKED EXAMPLE
Two lizard populations were separated by a lake for 3,000 years. The lake dried up and the lizards now breed together and produce fertile young. Has speciation occurred? Explain. [3]
Step 1: answer the question directlyNo — speciation has not occurredStep 2: say what did happenThey were geographically isolated, which stopped gene flow, but that isolation was temporary.Step 3: apply the testThey can still interbreed to produce fertile offspring, so they are not reproductively isolated and remain one species.The 3,000 years is a distractor. Time alone does not create a species.
WORKED EXAMPLE
Two plant populations in the same meadow now flower two months apart. Explain how this can lead to speciation without any physical barrier. [3]
Step 1: identify the isolating factorA change in alleles has shifted flowering time, so the two groups are never in flower at the same moment.Step 2: link it to gene flowPollen cannot be exchanged between them, so gene flow stops even though they share a habitat.Step 3: finish the chainGenetic differences accumulate until they are reproductively isolated, and speciation occursThis shows geographical isolation is one route to speciation, not the only one.
💡 Exam tip
Always finish a speciation answer with “reproductively isolated, so they can no longer produce fertile offspring”. That line is usually the final mark.
Use the chain in order: barrier → no gene flow → mutations accumulate → different selection pressures → reproductive isolation.
Say fertile offspring, not just offspring. One word, one mark.
Name the barrier the question gives you. “The river prevents gene flow” beats “they were separated”.
Use the term differential selection when the two populations face different environments.
If a question mentions extinction, remember it is the counterweight: speciation increases species number, extinction decreases it.
⚠ Common mix-up
Treating geographical isolation as speciation. It only starts the process. It can be undone if the barrier disappears.
Forgetting the word “fertile”. Horses and donkeys interbreed, but mules are sterile, so they stay separate species.
Saying “they became so different they were new species”. Looking different is not the test. Being unable to interbreed successfully is.
Assuming speciation always needs a physical barrier. Different flowering times or courtship behaviour can do it in one habitat.
Confusing gene flow with mutation. Mutation makes new alleles; gene flow moves existing ones between populations.
Writing that the barrier caused the mutations. The mutations happen anyway. The barrier just stops them being shared.
Up next: Classification & Cladistics — once you have millions of species, how do you sort them into groups that reflect real ancestry?
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