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A species is a group of organisms that can interbreed to produce fertile offspring, and speciation forms new ones. Free notes and 11 questions answered.
Formation of New Species is part of Variation and evolution in GCSE AQA Higher Biology.
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A species is a group of organisms that can interbreed to produce fertile offspring, and speciation forms new ones. Free notes and 11 questions answered.
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Use this lesson to stay inside Variation and evolution while connecting the detail back to the wider Inheritance, variation and evolution topic.
Read the core explanations from Formation of New Species before testing yourself from memory.
Picture two animals that look almost identical - a horse and a donkey. They can mate and produce offspring called mules. But here's the catch: mules are sterile. They can't have babies of their own.
This tells us something crucial about what makes a species. It's not just about looking similar or being able to mate - it's about producing fertile offspring that can continue the genetic line.
Trap: Many students think species are just animals that look alike. Actually, appearance can be misleading - some very different-looking animals are the same species, whilst some identical-looking ones are different species entirely.
Imagine a population of beetles living in a forest. A new river forms, splitting the forest in half. Now we have two separate populations that can't reach each other to mate.
Over many generations, these separated populations face different environmental pressures. The beetles on one side might encounter different predators, food sources, or climate conditions. Through natural selection, each population gradually develops different characteristics.
This separation can be the first step towards forming new species. But isolation alone isn't enough - the populations need time to change.
Here's where it gets fascinating. After thousands of generations apart, our beetle populations have changed so much that even if the river dried up, they couldn't successfully interbreed anymore.
Maybe their mating calls have changed, their breeding seasons have shifted, or their chromosomes have become incompatible. When they try to mate, either nothing happens, or they produce offspring that can't survive or reproduce.
This is the moment when one species has become two. The populations have crossed the biological point of no return - they are now separate species.
Darwin's finches in the Galápagos Islands show this process beautifully. Originally one species, different populations became isolated on different islands. Each island had different food sources - some had large, hard seeds, others had small, soft seeds, and some had nectar-rich flowers.
Over time, natural selection shaped each population differently. Birds with stronger, larger beaks survived better on islands with hard seeds. Birds with longer, thinner beaks thrived where nectar was the main food source.
Eventually, these populations became so different that they could no longer interbreed successfully. What started as one species of finch became multiple distinct species, each perfectly adapted to their island environment.
Speciation doesn't happen overnight. It typically takes thousands or even millions of years. The exact time depends on factors like generation time, population size, and the strength of natural selection pressures.
Organisms with short generation times (like bacteria or insects) can evolve new species relatively quickly. Organisms with long generation times (like elephants or humans) take much longer to accumulate enough genetic changes.
Scientists can observe this process happening in real time with rapidly reproducing organisms, but for most animals and plants, we see the end results of speciation that occurred long ago.
Exam questions on speciation often test your understanding of the key requirement: fertile offspring. Examiners love to include examples where animals can mate but produce sterile offspring - these are still the same species, not different ones.
Worked example: Worked Example: Applying Formation of New Species
Use this method to answer an exam-style question about what makes a species?.
Answer: A complete response links mechanism, evidence, and conclusion for formation of new species.
Command word focus: 'Explain' questions want you to give reasons why speciation occurs. Always link cause and effect - populations become isolated because of barriers, they evolve differently because of different selection pressures, they become separate species because they can no longer 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.
A species is a group of organisms that can interbreed to produce fertile offspring. Horses and donkeys can mate, but their offspring, mules, are sterile, which shows that producing fertile offspring, not just looking similar or being able to mate, defines a species.
Speciation is the evolutionary process by which populations evolve to become distinct species. It occurs when populations become so different, for example in mating calls, breeding seasons or chromosomes, that they can no longer interbreed to produce fertile offspring.
Mules, the offspring of a horse and a donkey, are sterile, which shows that horses and donkeys are separate species. A species is a group of organisms that can interbreed to produce fertile offspring.
Speciation typically takes thousands or even millions of years, depending on generation time, population size and the strength of natural selection pressures. Organisms with short generation times, like bacteria or insects, can evolve new species relatively quickly, while elephants or humans take much longer.
Populations become isolated by geographical barriers such as rivers or mountains, behavioural differences, or genetic changes. Once separated, they cannot interbreed, which can be the first step towards forming new species.
New species form when populations of one species become separated, for example by a river or mountains, and face different selection pressures. Natural selection shapes each population differently over many generations until they can no longer interbreed to produce fertile offspring, and they become separate species.
Darwin's finches started as one species, but populations became isolated on different Galápagos Islands with different food sources. Birds survived better with larger, stronger beaks on islands with hard seeds and longer, thinner beaks where nectar was the main food, until the populations could no longer interbreed successfully.
Animals that look alike can be different species, and some very different-looking animals are the same species. Appearance can be misleading; the key test is whether they can interbreed to produce fertile offspring.
After thousands of generations apart, separated populations may change so much that their mating calls have changed, breeding seasons have shifted or chromosomes have become incompatible. Mating then fails or produces offspring that cannot survive or reproduce.
Bacteria and insects can form new species relatively quickly because they have short generation times. Organisms with long generation times, like elephants or humans, take much longer to accumulate enough genetic changes.
A new river can split a beetle population into two groups that cannot reach each other to mate. Each group faces different predators, food sources or climate, so natural selection makes them develop differently until they can no longer produce fertile offspring together.