GCSE · Biology · AQA · Spec 8461
Speciation (biology only)
One population, one species. Then a river splits it in two. Nothing about the organisms has changed yet. So how can you end up with two species?
From one species to two
Open each step and ask the same question every time: could these groups breed together and have fertile offspring?
Zooming in on step 4
Reason it through
Why do two isolated groups end up with different characteristics?
First link · your turn
Start with what's inside each group. Are all its members identical?
Apply the species test
One species or two?
For each pair of groups, ask the one question that decides it: could they breed together and have fertile offspring?
Still to sort
Still one species (0)
They can breed together and their offspring are fertile.
Where the line is: Fertile offspring is the line. Offspring that can never reproduce don't pass the test.
Separate species (0)
They can't interbreed to give fertile offspring.
Where the line is: Being kept apart doesn't put groups here. What matters is whether they could have fertile offspring together.
The bigger picture
Natural selection is the explanation scientists give today for how evolution works. Commit to an answer before you open it.
Why do most scientists accept natural selection as the explanation for evolution?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
How one population can end up as two species, one step at a time.
What you need to know
- A species is a group of similar organisms that can breed with each other and have offspring that are themselves fertile.
- A physical barrier, such as a river or a mountain range, can split a population so that the groups can't reach each other to breed. They are reproductively isolated.
- Each isolated group contains variation. Conditions differ between the habitats, so natural selection favours different characteristics in each group, and those characteristics become more common over many generations.
- Eventually the groups can't interbreed to give fertile offspring, even if brought back together. They are then separate species.
- Darwin and Wallace's theory proposes that every species alive today descended, through evolution, from earlier species. Natural selection is now the accepted explanation for how evolution works.
The big picture
Speciation is how one species becomes two. A physical barrier splits a population into groups that can't reach each other to breed. Each group contains variation, and because conditions differ between their habitats, natural selection favours different characteristics in each one over many generations. In time the groups change so much that, even if brought back together, they could not interbreed to give fertile offspring. They are now separate species.
Key points
Worked example
Problem
A population of one species of insect lives across a wide area. The area becomes divided by a river that the insects cannot cross. Describe how the two groups could become separate species.
⚠ Watch out
Stopping at 'they can't breed any more'. Separate species means the groups can't interbreed to give FERTILE offspring. Leave out 'fertile' and the description is incomplete, because groups that breed but only produce infertile offspring are still separate species.
Memory hook
Four S's: Split, Stuck apart, Selected differently, Separate species. And the judge at the end is always the same question: fertile offspring, yes or no?
Check yourself
Cover the page. Name the three things that happen, in order, between a barrier splitting a population and there being two species. Then say what test tells you it has happened.
Flashcards
(13)What is a species?
Give two examples of physical barriers that can keep groups of a species apart.
What does 'reproductively isolated' mean?
Does the barrier itself change the organisms?
Why does natural selection favour different characteristics in two isolated groups?
What does natural selection act on in each isolated group?
Over what timescale do the favoured characteristics become more common?
When have two isolated groups become separate species?
Two groups can breed, but all their offspring are infertile. One species or two?
Two groups are kept apart by a river, but when they meet they have fertile offspring. One species or two?
What does Darwin and Wallace's theory propose about the species alive today?
What do scientists now give as the explanation for how evolution works?
Why is natural selection accepted by most scientists?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
Learning with Lightbulb is opening soon
You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.
Keep me postedMore AQA GCSE Biology topics
- Active transport
- Adaptations
- Advantages and disadvantages of sexual and asexual reproduction (biology only)
- Animal and plant cells
- Antibiotic-resistant bacteria
- Antibiotics and painkillers
- Biodiversity
- Cell differentiation
- Cell specialisation
- Classification of living organisms
- Control of blood glucose and diabetes
- Control of body temperature/thermoregulation (biology only)
How this lesson was checked. This AQA GCSE Biology (specification 8461)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.