A species of ground-dwelling bird lives on a large island. The birds feed on seeds found on the ground. Over thousands of years, a wide sea channel forms, permanently separating the island into two smaller islands. Each island has a different climate and different types of seeds available. Explain how this separation could eventually lead to the formation of two new species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A species of ground-dwelling bird lives on a large island. The birds feed on seeds found on the ground. Over thousands of years, a wide sea channel forms, permanently separating the island into two smaller islands. Each island has a different climate and different types of seeds available.
Model answer (5 marks)
The sea channel becomes a geographical barrier, isolating the two bird populations so they cannot interbreed.
Both populations contain genetic variation from random mutations, giving rise to different alleles.
Because each island has a distinct climate and seed types, different alleles are advantageous in each environment.
Individuals carrying the advantageous alleles are more likely to survive and reproduce, passing those alleles on to their offspring (natural selection).
After many generations, the accumulated differences are so great that the two populations can no longer produce fertile offspring, resulting in two new species (speciation).
Both populations contain genetic variation from random mutations, giving rise to different alleles.
Because each island has a distinct climate and seed types, different alleles are advantageous in each environment.
Individuals carrying the advantageous alleles are more likely to survive and reproduce, passing those alleles on to their offspring (natural selection).
After many generations, the accumulated differences are so great that the two populations can no longer produce fertile offspring, resulting in two new species (speciation).
Examiner tips
- Use the word "geographical barrier" to show understanding of isolation. Include "genetic variation" and "alleles" to cover variation. Explain how different environments favour different alleles. Show natural selection and the eventual reproductive isolation that defines speciation.
- common_mistakes
- :
- Failing to mention the sea channel as a barrier. Omitting the role of genetic variation or natural selection. Not stating that reproductive isolation leads to speciation.
Mark scheme (5 marks)
- The sea channel acts as a geographical barrier / the two populations are now isolated from each other and cannot interbreed
- Each population has genetic variation due to random mutations producing different alleles
- Different environmental conditions (different climate / seed types) mean different alleles are advantageous in each population
- Individuals with advantageous alleles are more likely to survive, reproduce and pass on those alleles to offspring (natural selection)
- Over many generations the two populations become so different that they can no longer interbreed to produce fertile offspring, forming two new species (speciation)
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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