Explain how the accumulation of mutations over time can contribute to speciation.

IB DP Biology Standard Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks · View as Markdown

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

Mutations arise spontaneously in populations at a low but constant rate. Over many generations, different populations may accumulate distinct sets of mutations.

Model answer (4 marks)

Mutations introduce new alleles into a population, providing fresh genetic variation.
Natural selection then favours the alleles that improve fitness in each local environment, so the allele frequencies in each population change in different directions.
Over many generations the two populations become genetically distinct.
When the genetic differences are sufficient to prevent successful interbreeding or produce infertile hybrids, reproductive isolation occurs and the populations are recognised as separate species.

Examiner tips

Use the word ‘mutations’ first, then ‘natural selection’, then ‘genetic divergence’, finally ‘reproductive isolation’.
Show the causal chain: mutation → variation → selection → divergence → isolation.
Use UK spelling (e.g., ‘fertile’).

Common mistakes

Confusing mutation with natural selection as the cause of speciation.
Failing to mention reproductive isolation as the final criterion for species.

Mark scheme (4 marks)

Mutations introduce new alleles / new genetic variation into a population.
Natural selection acts on this variation, favouring alleles that increase fitness in the local environment, so different populations accumulate different allele frequencies.
Over time, the two populations diverge genetically / become genetically distinct from one another.
Eventually reproductive isolation arises / individuals from the two populations can no longer interbreed to produce fertile offspring, so they are considered separate species.

Key terms in this question

mutation · speciation