Explain how genetic drift can lead to rapid evolutionary change in small, isolated populations.

IB DP Biology Higher Level (2023 syllabus) — D4.2 Stability and change · Explain · 4 marks · View as Markdown

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

A volcanic island is colonised by a small founding group of birds that became separated from a large mainland population. Over several generations, the island population shows markedly different allele frequencies from the mainland population, despite no apparent selection pressure.

Model answer (4 marks)

Genetic drift is the random change in allele frequencies caused by chance events rather than natural selection.

In a small population, sampling error has a larger proportional effect, so allele frequencies fluctuate more dramatically from one generation to the next.

Because of this randomness, alleles can become fixed (reach 100 % frequency) or be lost entirely, which reduces genetic variation.

The founder effect is a special case of drift: when a small number of individuals colonise a new area, they carry only a subset of the original gene pool, so the new population diverges genetically from the source population.

Examiner tips

Define genetic drift and emphasise it is random, not selection.
Explain why small populations are more affected – larger proportional sampling error.
Mention fixation/loss of alleles and loss of variation.
Give the founder effect as a clear example of drift.

Common mistakes

Confusing drift with natural selection or mutation.
Failing to note that drift is random and not driven by environment.
Not recognising the founder effect as a specific form of drift.

Mark scheme (4 marks)

Genetic drift refers to random changes in allele frequencies due to chance events rather than natural selection
In small populations, chance sampling errors have a proportionally larger effect, so allele frequencies fluctuate more dramatically between generations
Alleles can become fixed (reach 100% frequency) or lost entirely by chance, reducing genetic variation
The founder effect is a specific example of genetic drift where the founding population carries only a subset of the original gene pool, so the island population diverges genetically from the mainland population

Key terms in this question

genetic drift