Explain how natural selection acting on variation within a population can lead to the formation of a new species over time.

IB DP Biology Standard Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Variation exists within a population because of mutation and sexual reproduction.
Natural selection favours individuals with advantageous traits, so they survive and reproduce more successfully – this is differential reproductive success.
Over many generations allele frequencies change, accumulating genetic differences.
When populations become reproductively isolated (for example by a geographical barrier), independent natural selection and genetic divergence mean the groups can no longer interbreed to produce fertile offspring, thereby forming a new species.

Examiner tips

Use the exact terms: variation, mutation, sexual reproduction, natural selection, differential reproductive success, allele frequencies, reproductive isolation, fertile offspring.
Show the causal chain: variation → selection → allele frequency change → isolation → speciation.
Keep the answer concise – 4 points, one sentence each.

Common mistakes

Confusing mutation with natural selection as the cause of variation.
Losing the step of reproductive isolation or saying speciation happens instantly.
Using vague terms like ‘change’ without specifying allele frequencies or reproductive isolation.

Mark scheme (4 marks)

Variation exists within a population due to mutation and/or sexual reproduction
Natural selection favours individuals with advantageous traits, which survive and reproduce more successfully (differential reproductive success)
Allele frequencies in the population change over many generations, accumulating genetic differences
When populations become reproductively isolated (e.g. by a geographical barrier), independent natural selection / genetic divergence leads to populations that can no longer interbreed to produce fertile offspring, forming a new species

Key terms in this question

natural selection · variation