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Learn7402Paper 2 · Paper 2: Topics 5–8Genetics, populations, evolution and ecosystemsSpeciation and evolution
Genetics, populations, evolution and ecosystems · Subtopic 3 of 4

Speciation and Evolution

A species (biological concept) is a group of organisms that can interbreed to produce fertile offspring. Speciation requires reproductive isolation, which stops gene flow. Allopatric speciation: a geographical barrier (river, mountain, sea) separates populations; different mutations, selection pressures and genetic drift make their allele frequencies diverge until they can't interbreed. Sympatric speciation happens in the same area through ecological, behavioural or temporal isolation, or polyploidy in plants. Isolating mechanisms may be prezygotic (different breeding seasons, courtship, incompatible genitalia) or postzygotic (hybrid inviability or sterility, e.g. mules). Molecular evidence (DNA base sequences, mitochondrial DNA, amino acid sequences of proteins such as cytochrome c) shows relationships: fewer differences mean more recent common ancestry.

Cambridge 9700 Paper 4Edexcel YBI11 WBI14AQA 7402 Paper 2≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Define species.
  • Explain allopatric and sympatric speciation.
  • Describe isolating mechanisms.
  • Use molecular evidence to infer relationships.
Pause and think

Horses and donkeys can mate and produce mules. Why are they still separate species?

Section 2 of 14

Core idea

Start hereOne species becomes two

Squirrels on either side of the Grand Canyon have been separated for thousands of years and now look different.

The big idea

When gene flow stops, populations evolve separately until they can no longer produce fertile offspring together.

Allopatric speciation

Isolation, then divergence.

Worked exampleMolecular comparison

Proteins differ at 12 of 104 positions. % difference?

Goal
%.
Strategy
Differences ÷ total.
  1. 12104=0.115\frac{12}{104}=0.115
    Fraction.
  2. 11.5%11.5\%
    ×100.
Answer11.5%.
✓ CheckSmaller % = closer relatives ✓.

Summary

  • No gene flow.
  • Allopatric vs sympatric.
  • Molecular evidence.
Key words
Gene flow
Transfer of alleles between populations.
Reproductive isolation
Barrier preventing interbreeding.
HistoryDarwin's finches. The Galápagos finches, separated on different islands, evolved beaks suited to different foods: a classic example of allopatric speciation.
Fun factRing species. Greenish warblers form a ring around the Tibetan plateau; neighbouring populations interbreed, but the two ends of the ring don't.
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