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Speciation is the formation of new species when isolated populations evolve until they can no longer interbreed. Free notes and 11 questions answered.
The Process of Speciation is part of The development of understanding of genetics and evolution in GCSE AQA Higher Biology.
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Speciation is the formation of new species when isolated populations evolve until they can no longer interbreed. Free notes and 11 questions answered.
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Use this lesson to stay inside The development of understanding of genetics and evolution while connecting the detail back to the wider Inheritance, variation and evolution topic.
Read the core explanations from The Process of Speciation before testing yourself from memory.
Imagine two groups of the same animal living on opposite sides of a mountain range. Over thousands of years, they become so different they can no longer breed together. This is speciation - one of evolution's most dramatic tricks.
The key word here is 'fertile'. A horse and donkey can mate to produce a mule, but mules are sterile - they cannot reproduce. This proves horses and donkeys are separate species, despite being similar.
Trap: Many students think species are just animals that look similar. Actually, appearance can be misleading - some very different-looking animals are the same species, while some identical-looking ones are different species.
Speciation doesn't happen overnight. It's a step-by-step process that can take thousands or millions of years. Here's how populations split into separate species:
Worked example: The Process of Speciation
How does one population become two separate species?
Answer: Two distinct species have formed from one original population
The crucial point is step 2 - isolation. Without some barrier preventing interbreeding, gene flow continues and the population remains as one species. This barrier is usually geographical, but it can also be behavioural or temporal.
Not all isolation is the same. The type of barrier that separates populations affects how quickly and dramatically speciation occurs.
Geographical isolation is the most common cause of speciation. When continents drift apart, mountains form, or sea levels change, populations get cut off from each other. The Galápagos finches are a classic example - different islands led to different species.
But isolation isn't always about physical barriers. Populations can become reproductively isolated in other ways:
Behavioural isolation: Different mating rituals or preferences prevent interbreeding, even when populations live in the same area.
Temporal isolation: Populations breed at different times of year, so they never encounter potential mates from other groups.
Once populations are isolated, natural selection becomes the engine of speciation. Each group faces different environmental pressures, so different traits become advantageous.
Picture two groups of the same bird species separated by a mountain range. On one side, the climate is dry with hard seeds. Birds with stronger, larger beaks can crack these seeds more easily, so they survive and reproduce more successfully. Their offspring inherit these stronger beaks.
On the other side of the mountain, the climate is wet with soft fruits. Here, birds with smaller, more delicate beaks are better at extracting fruit pulp. These birds have more offspring, passing on their beak characteristics.
Over many generations, the two populations become increasingly different. Eventually, their beaks become so specialised that even if the populations meet again, they cannot successfully interbreed - they have become separate species.
Speciation isn't just theory - we can see it happening in nature. Here are some compelling examples that demonstrate the process in action.
Darwin's Finches: These birds on the Galápagos Islands evolved from a single ancestor into 14 different species. Each island's unique environment selected for different beak shapes - large beaks for cracking seeds, curved beaks for extracting nectar, sharp beaks for catching insects.
Cichlid Fish in African Lakes: Lake Victoria contains over 500 species of cichlid fish, all evolved from a few ancestral species in just 15,000 years. Different feeding strategies led to different mouth shapes and behaviours.
Apple Maggot Flies: These flies originally only lived on hawthorn trees. When apple trees were introduced to North America, some flies switched to apples. The two populations now breed at different times (apples ripen earlier than hawthorns) and are becoming separate species.
Speciation questions often trip students up because they require you to link several concepts together. Here's how to tackle them successfully.
Command word focus: 'Describe' questions want you to state what happens in sequence. 'Explain' questions need you to say why each step occurs, using scientific principles.
Common mistake: Students often forget that speciation takes a very long time. Don't write about changes happening in a few years - use phrases like 'over many generations' or 'over thousands of years'.
Mark-winning phrases: 'Populations become reproductively isolated', 'gene flow is prevented', 'different selection pressures', 'cannot produce fertile offspring'.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Darwin's finches evolved from a single ancestor into 14 species with different beak shapes; Lake Victoria has over 500 cichlid fish species that evolved in just 15,000 years; and apple maggot flies that switched from hawthorn to apple trees now breed at different times and are becoming separate species.
Horses and donkeys are separate species. They can mate to produce a mule, but mules are sterile and cannot reproduce, and members of the same species must be able to interbreed to produce fertile offspring.
Speciation is the formation of new species: a population becomes isolated, usually by a geographical barrier, each group faces different selection pressures, natural selection makes them evolve differently, and eventually they can no longer interbreed to produce fertile offspring.
Geographical isolation is the physical separation of populations by geographical barriers such as mountains, rivers or oceans that prevent interbreeding. It is the most common cause of speciation, because without a barrier gene flow continues and the population remains one species.
Populations can become reproductively isolated by geographical isolation, where barriers like mountains or rivers separate them; behavioural isolation, where different mating rituals or preferences prevent interbreeding in the same area; and temporal isolation, where populations breed at different times of year.
Behavioural isolation is when different mating rituals or preferences prevent interbreeding, even when populations live in the same area. It is one way populations can become reproductively isolated without a physical barrier.
Temporal isolation is when populations breed at different times of year, so they never encounter potential mates from other groups. Apple maggot flies are an example: flies on apples breed earlier than those on hawthorns because apples ripen earlier.
Lake Victoria contains over 500 species of cichlid fish, all evolved from a few ancestral species in just 15,000 years. Different feeding strategies led to different mouth shapes and behaviours.
Once populations are isolated, each faces different selection pressures, so different traits become advantageous. For example, birds on a dry side of a mountain with hard seeds survive better with stronger beaks, while birds on a wet side with soft fruits do better with delicate beaks, until the populations cannot interbreed.
Darwin's finches on the Galápagos Islands evolved from a single ancestor into 14 different species. Each island's environment selected for different beak shapes: large beaks for seeds, curved beaks for nectar and sharp beaks for insects.
Isolation is needed for speciation because without a barrier preventing interbreeding, gene flow continues and the population remains one species. Once isolated, each group can evolve separately under different selection pressures.