Explain how geographic isolation of a small population from a larger parent population can lead to the formation of a new species through natural selection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Geographic isolation stops gene flow between the small isolated group and the larger parent population.
The two groups live in different environments and are therefore subject to different selection pressures, which favour different heritable traits or alleles.
Over many generations natural selection (and genetic drift) changes the allele frequencies in the isolated group so that they diverge from those in the parent population.
Eventually the genetic differences are so great that the two groups can no longer produce fertile offspring – reproductive isolation has arisen and the isolated group is considered a new species.
The two groups live in different environments and are therefore subject to different selection pressures, which favour different heritable traits or alleles.
Over many generations natural selection (and genetic drift) changes the allele frequencies in the isolated group so that they diverge from those in the parent population.
Eventually the genetic differences are so great that the two groups can no longer produce fertile offspring – reproductive isolation has arisen and the isolated group is considered a new species.
Examiner tips
- Use the word ‘geographic isolation’ first to show you understand the cause. Explain that different selection pressures act on each group. Show that allele frequencies diverge over many generations. Finish with reproductive isolation as the definition of a new species.
Common mistakes
- Mixing up gene flow with mutation. Forgetting to mention that the isolated group cannot interbreed. Using vague terms like ‘change’ instead of ‘diverge’ or ‘speciation’.
Mark scheme (4 marks)
- Geographic isolation prevents gene flow between the separated population and the parent population.
- Each population is subject to different selection pressures in its environment, favouring different heritable traits / alleles.
- Over many generations, allele frequencies in the isolated population diverge from those in the parent population through natural selection (and genetic drift).
- Eventually the two populations become so genetically different that they can no longer interbreed to produce fertile offspring, and are therefore classified as separate species (reproductive isolation / speciation).
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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