The Process of Speciation Revision Notes | GCSE AQA Higher Biology

1. What Is Speciation?

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.

2. The Steps of Speciation

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?

1. One population — All individuals can potentially interbreed and share genetic material
2. Geographical barrier separates groups — Mountain range, river, or distance prevents interbreeding
3. Each group faces different environmental challenges — Different climates, food sources, or predators in each location
4. Natural selection favours different traits in each group — The populations become increasingly different over time
5. Groups can no longer interbreed successfully — Even if brought together, they cannot produce fertile offspring

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.

3. Types of Isolation

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.

4. Natural Selection Drives Divergence

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.

5. Real-World Examples

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.

6. Exam Technique and Common Mistakes

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'.