The big idea: A species is a group of organisms that can interbreed and produce fertile offspring.
Speciation is the splitting of one species into two separate species.
It happens when two populations stop interbreeding — they become reproductively isolated — and then change in different ways until they can no longer breed together.
- Species
- A group of organisms that can interbreed and produce fertile offspring.
- Population
- A group of the same species living in the same area at the same time.
- Speciation
- The formation of a new species from an existing one.
- Reproductive isolation
- When two populations can no longer interbreed to produce fertile offspring.
The test for one species: Two organisms belong to the same species only if they can interbreed AND their offspring are fertile.
A horse and a donkey can mate, but their offspring (a mule) is sterile — so they are different species.
How to test whether two forms are the same species: Suppose you find two similar-looking forms and you are not sure whether they are one species or two. Looking alike is not evidence — you need a breeding (crossing) test.
Try to interbreed them, then check whether their offspring are fertile — that is, whether the offspring can themselves go on to reproduce.
- Fertile offspring are produced ⇒ the two forms are the same species. - No offspring at all, or only sterile offspring (like a mule) ⇒ the two forms are different species.
Definition — fertile offspring: offspring that are themselves able to reproduce. Sterile offspring cannot reproduce.
| Breeding test result | What it tells you |
|---|---|
| They interbreed and the offspring are fertile | Same species |
| They cannot produce any offspring | Different species |
| They produce offspring, but the offspring are sterile (e.g. a mule) | Different species |
Tested as 'suggest further evidence': If a question shows you two similar forms (for example two salamander or two beetle forms) and asks you to suggest how to gather further evidence that they are the same or different species, the expected answer is the breeding test: attempt to interbreed them and check whether the offspring are fertile.
Say what each result would mean: fertile offspring ⇒ same species; no offspring or sterile offspring ⇒ different species.
Allopatric speciation: a new river splits one population into two (still one species, no gene flow across the barrier). Over many generations the isolated groups diverge until they can no longer interbreed — two species.
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Speciation needs two steps.
First, the populations must be separated so that members of one population stop breeding with members of the other. This is reproductive isolation. A very common cause is a geographic barrier — a river, a mountain range, or the sea — that physically splits one population into two.
Second, the two separated populations diverge. Once gene flow between them has stopped, each population faces its own environment. Mutation creates different new alleles in each group, and natural selection favours different alleles in each place. Over many generations the two gene pools drift apart.
- Gene flow
- The movement of alleles between populations through interbreeding.
- Geographic isolation
- Separation of populations by a physical barrier such as a river, mountain or ocean.
- Divergence
- The build-up of genetic differences between two separated populations over time.
- Gene pool
- All the alleles present in a population.
When are they finally two species?: The two populations have become two separate species only once they have diverged so much that they can no longer interbreed to produce fertile offspring, even if they are brought back together.
Until that point they are still one species — just two isolated populations.
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How this is tested: Paper 1A (multiple choice) often tests the species concept (interbreed + fertile offspring) or asks for the consequence of a barrier splitting a population (reproductive isolation).
Paper 2 asks you to Explain how a new species forms after a population is split — you must put the steps in order: barrier → isolation → mutation + natural selection → divergence → no longer interbreed.
IB-style question — lizards split by a new river
A single population of lizards lives across a valley. A new river forms and divides the valley, separating the lizards into two groups that can no longer reach each other. Explain how this could lead to the formation of two separate species. [4]
How to score all four marks
- Isolation. The river is a geographic barrier that stops the two groups interbreeding, so there is no gene flow between them — they are reproductively isolated.
- Different variation and conditions. Mutation produces different new alleles in each group, and the two sides of the river have different environments / selection pressures.
- Natural selection drives divergence. In each group, natural selection favours different alleles, so over many generations the two gene pools become more and more different.
- Answer the command term (Explain): eventually the two populations have diverged so much that they can no longer interbreed to produce fertile offspring — so they are now two separate species.
Final answer
The river reproductively isolates the two groups (no gene flow); mutation gives different alleles and the two sides have different selection pressures; natural selection makes the gene pools diverge; eventually they can no longer interbreed to produce fertile offspring, so two species have formed.
Mark the order, not just the words: Examiners award the marks for the sequence: barrier → isolation (no gene flow) → mutation + different selection → divergence → can no longer interbreed.
An answer that lists 'natural selection' and 'isolation' with no order usually drops marks.