Explain how the founder effect can lead to speciation in a small population that becomes geographically isolated from its parent population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The founder effect occurs when a very small number of individuals establish a new population, carrying only a subset of the genetic variation present in the parent population.
Because the new population is so small, allele frequencies differ from the parent population by chance (genetic drift). This initial genetic difference makes the two populations genetically distinct.
Geographic isolation prevents any gene flow between the founder and parent populations, so mutations and natural selection act independently on each group.
Over time, the accumulated genetic differences can lead to reproductive isolation, meaning members of the two populations can no longer produce fertile offspring. This completes speciation.
Because the new population is so small, allele frequencies differ from the parent population by chance (genetic drift). This initial genetic difference makes the two populations genetically distinct.
Geographic isolation prevents any gene flow between the founder and parent populations, so mutations and natural selection act independently on each group.
Over time, the accumulated genetic differences can lead to reproductive isolation, meaning members of the two populations can no longer produce fertile offspring. This completes speciation.
Examiner tips
- Use the term "founder effect" and explain it as a small founding group. Show that allele frequencies differ by chance. Mention geographic isolation stops gene flow. End with reproductive isolation as the final step.
Common mistakes
- Confusing founder effect with natural selection. Failing to mention that isolation stops gene flow. Not linking genetic drift to the initial allele frequency change.
Mark scheme (4 marks)
- The founder effect occurs when a small number of individuals establish a new population, carrying only a subset of the genetic variation present in the original population.
- Allele frequencies in the founder population differ from those in the parent population due to chance sampling (genetic drift), so the two populations begin to diverge genetically.
- Geographic isolation prevents gene flow between the founder and parent populations, so mutations and selection pressures act independently on each population.
- Over time, sufficient genetic divergence accumulates so that members of the two populations can no longer interbreed successfully to produce fertile offspring, completing speciation (reproductive isolation is established).
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
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