IB Biology HLEvolution & SpeciationPaper 1 & 2~11 min read
Speciation
Evolution changes a species. Speciation splits one into two. The difference between those two sentences is the single most examined idea in this topic, and it comes down to one thing: whether the two groups can still breed together.
📚 What you need to know
Speciation is the development of new species from pre-existing species over time.
Speciation happens when gene flow between two populations is prevented.
With no gene flow, genetic differences accumulate, and this happens faster if different selection pressures act on each population.
A speciation split has occurred when the two populations can no longer interbreed to produce fertile offspring — they are reproductively isolated.
Gradual evolutionary change on its own is not enough; there must be reproductive isolation.
Geographical isolation by a natural or man-made barrier is a common cause, and leads to differential selection.
Extinction works in the opposite direction: it reduces the number of species on Earth.
What speciation means
Definition
Speciation is the development of new species from pre-existing species over time
Run that process backwards far enough and everything collapses to one point. In theory, at the origin of life there was a single species. It split into separate new species. Those species split again, each forming new species. Over millions of years, countless speciation events produced the millions of species alive today.
Gene flow is the thing that has to stop
Gene flow is simply the exchange of genes between populations, which happens whenever members of those populations breed with each other. While gene flow continues, the two populations keep getting mixed back together and stay one species.
Speciation can occur when gene flow is prevented, for example because the two populations end up separated on different islands.
Once gene flow stops, genetic differences accumulate between the two populations.
This happens faster if different selection pressures are acting on each of them, because natural selection then pushes them in different directions.
Think of gene flow as stirring. Keep stirring and the mixture stays uniform, no matter how many new alleles appear. Stop stirring and the two halves are free to become different — slowly by chance, quickly if something is actively pushing them apart.
Reproductive isolation: the finish line
Organisms of the same species share characteristics and can breed together to produce fertile offspring. Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals prevent them from breeding successfully with individuals that do not carry those changes.
A speciation split has occurred at the moment the two populations can no longer interbreed to produce fertile offspring. Before that point you have two populations of one species. After it, you have two species.
The line examiners look for. Gradual evolutionary change on its own is not enough. For speciation to have occurred, there must be reproductive isolation. Two populations can look very different and still be one species.
Changes that can cause it
Type of change
What happens
Seasonal changes
Some individuals develop different mating or flowering seasons, becoming sexually active at different times of the year, so they never meet a partner from the other group
Behavioural changes
Some individuals develop changes in their courtship behaviour, so they can no longer attract individuals of the opposite sex from the other group
Changes like these often follow on from geographical isolation of the two populations, which is where most speciation stories begin.
Geographical isolation
Reproductive isolation can begin when populations of a species become separated by geographical barriers. The separated populations are then said to be geographically isolated.
Barriers can be natural — a body of water, or a mountain range.
They can also be man-made, such as a motorway.
Geographical isolation creates two populations of the same species between which no gene exchange can occur.
The two populations may then be affected by different selection pressures, so natural selection acts differently on each one. This is called differential selection.
Over time the two populations may become so different that they are reproductively isolated, and speciation has occurred.
The barrier does not create the new alleles. It only stops them being shared, which lets the two sides drift apart.
Bonobos and chimpanzees
This is the example to learn, because it shows every step of the process in one real case.
Chimpanzees are found to the north of the Congo river, and bonobos to the south.
At some point in their evolutionary past, the river caused two populations of their ancestor species to become geographically isolated.
Different selection pressures acted on the two sides, so differential selection occurred and differences built up. Chimpanzees tend to be more behaviourally aggressive than bonobos, which may have arisen from more intense competition for resources. Chimpanzees also have male-dominated social structures, while bonobos have dominant females.
Eventually the two groups became reproductively isolated, and were two separate species.
Put the two groups together today and they still would not interbreed, which is exactly why they count as separate species.
Do not confuse the two kinds of isolation. Geographical isolation prevents gene flow, but it may be temporary — if the populations came back together, successful breeding could still happen. Reproductive isolation means speciation has occurred: they can no longer breed together successfully, even in the same habitat.
Extinction: the other direction
Speciation increases the number of species on Earth, but it is only half the story. Not all the species that have evolved over evolutionary time still exist today. Many have gone extinct, meaning they no longer exist — the passenger pigeon and the woolly mammoth are two well-known examples.
So the number of species alive at any moment is the balance between two opposing processes: speciation increases diversity, and extinction reduces it.
Worked examples
WE 1
From one population to two species
A river changes course and divides a population of lizards in two. Explain how this could lead to speciation. (4 marks)
Point 1: isolation
The river is a geographical barrier, so the two populations are geographically isolated and there is no gene flow between them.
Point 2: differences accumulate
Genetic differences build up, because mutations in one population cannot spread to the other.
Point 3: differential selection
Different selection pressures on each side mean natural selection acts differently, so different alleles become more frequent in each population.
Point 4: the finish line
Eventually the two populations are so different that they cannot interbreed to produce fertile offspring: they are reproductively isolated and speciation has occurred.
No gene flow → differences accumulate → reproductive isolationthe last step is the one students leave out, and it is almost always worth a mark
WE 2
Has speciation happened?
Two populations of a beetle separated by a mountain range now differ noticeably in colour and size. State, with a reason, whether they are separate species. (2 marks)
Point 1: the judgement
You cannot say yet. Looking different is not the test.
Point 2: the reason
They are only separate species if they are reproductively isolated, meaning they can no longer interbreed to produce fertile offspring.
Different appearance is not speciation; failure to interbreed isa question that looks too easy is usually testing exactly this distinction
WE 3
The bonobo and chimpanzee split
Explain how bonobos and chimpanzees became separate species. (3 marks)
Point 1: the barrier
The Congo river geographically isolated two populations of their ancestor species, chimpanzees to the north and bonobos to the south.
Point 2: differential selection
Different selection pressures acted on each population, producing differences such as aggression levels and which sex is dominant.
Point 3: the outcome
Eventually the two groups became reproductively isolated and were two separate species.
River → differential selection → reproductive isolationyou do not need the Latin names in an exam; “bonobos” and “chimpanzees” are fine
💡 Exam tips
Learn the definition of speciation as a phrase: new species from pre-existing species over time.
Use the term gene flow, and say clearly that it has been prevented.
Always finish with reproductive isolation — no answer about speciation is complete without it.
Use differential selection when different pressures act on two separated populations.
Know the bonobo and chimpanzee example, including one behavioural difference.
Remember that extinction reduces the number of species, in case a question asks about diversity overall.
⚠ Common mistakes
Treating “looks different” as “different species”. The test is fertile offspring, not appearance.
Confusing geographical with reproductive isolation. The first can be reversed; the second cannot.
Forgetting the word “fertile”. A horse and a donkey can breed — the offspring just is not fertile.
Saying the barrier causes the mutations. It only stops alleles being shared.
Stopping the answer at “they became different”. Say they can no longer interbreed.
Up next: Types of Speciation — allopatric and sympatric, and why speciation does not always need a barrier at all.
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