Edexcel A Level (IAL) Biology -5.24 Isolation & Speciation- Study Notes- New Syllabus

Isolation and Speciation

🌿 Introduction

Speciation means the formation of a new species. It happens when populations stop exchanging genes and start evolving independently. Isolation blocks gene flow, so each group accumulates different mutations and adaptations.

🧬 Key Ideas

What is gene flow?

Movement of alleles between populations. It keeps populations genetically similar.

What is isolation?

Any barrier that reduces or completely stops gene flow between groups of the same species.

When isolation continues for many generations, allele frequencies change separately in each group. Eventually, they become so different that they cannot interbreed even if they meet again. That is speciation.

🌍 Types of Isolation

1️⃣ Allopatric Speciation (Geo = outside)

This occurs when populations are separated by a physical/geographical barrier.

Barriers include:

• Mountains
• Rivers or seas
• Deserts
• Islands forming
• Land breaking apart

What happens:

• No gene flow due to physical separation.
• Each population faces different selection pressures (climate, predators, food).
• Natural selection + mutation act separately.
• Over time, they become reproductively incompatible.
• New species form.

Simple example:

Birds split by a forming river → each side evolves different beak shapes based on available food.

2️⃣ Sympatric Speciation (Sym = same)

Speciation occurs without physical separation. Populations live in the same area but stop interbreeding due to reproductive barriers.

Possible barriers:

• Behavioural isolation – different mating calls, dances, pheromones.
• Temporal isolation – breed in different seasons/times.
• Mechanical isolation – incompatible reproductive structures.
• Gametic isolation – sperm cannot fertilise the egg.

Example:

Two insect groups living on the same tree but preferring different host plants → they rarely meet → become separate species.

🧪 Why Isolation Causes Speciation

• No gene flow means no mixing of alleles.
• Random mutations accumulate differently.
• Natural selection favours different traits in each population.
• Allele frequencies diverge → genetic differences grow.
• Eventually, members cannot interbreed successfully.

At this point, they are considered distinct species.

📊 Summary Table