Explain how natural selection can lead to the formation of a new species (speciation).

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.4 Evolution and speciation · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A population of birds lives on a large continent. Over thousands of years, a mountain range rises up, splitting the population into two groups that can no longer interbreed. Eventually, the two groups become so different that they can no longer produce fertile offspring, even if they meet again.

Model answer (5 marks)

Genetic variation exists within the original population due to mutations and sexual reproduction.
The mountain range acts as a physical barrier, isolating the two groups so they cannot interbreed and preventing gene flow.
Each isolated population experiences different selection pressures or environmental conditions.
Individuals with alleles better suited to their local environment survive, reproduce and pass on those alleles (survival of the fittest).
After many generations the allele frequencies in each population change so much that the two groups can no longer interbreed to produce fertile offspring – they have become separate species (speciation).

Examiner tips

Use the exact terms: genetic variation, isolation, selection pressures, survival of the fittest, allele frequencies, speciation.
Show the causal chain: variation → isolation → different selection → change in allele frequencies → reproductive isolation.
Keep each point short and to the point – 1 mark per bullet.

Common mistakes

Confusing ‘isolation’ with ‘migration’; forgetting that isolation stops gene flow.
Using vague terms like ‘differences’ instead of ‘different selection pressures’.
Not linking the change in allele frequencies to the final reproductive isolation step.

Mark scheme (5 marks)

Genetic variation exists within the original population due to mutations (and sexual reproduction)
The mountain range acts as a physical barrier, isolating the two groups so they cannot interbreed / preventing gene flow between the two populations
Each isolated population experiences different selection pressures / different environmental conditions
Individuals with alleles better suited to their local environment survive, reproduce and pass on their alleles (natural selection / survival of the fittest)
Over many generations the allele frequencies in each population change so much that the two groups can no longer interbreed to produce fertile offspring — they have become separate species (speciation)

Key terms in this question

natural selection · speciation