A population of freshwater fish lives in a large lake. After a period of volcanic activity, a lava flow divides the lake into two smaller, completely separate lakes. Explain how the two isolated fish populations could eventually become two separate species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A population of freshwater fish lives in a large lake. After a period of volcanic activity, a lava flow divides the lake into two smaller, completely separate lakes. The two smaller lakes have different water temperatures, food sources and levels of dissolved oxygen.
Model answer (5 marks)
The lava flow separates the fish into two isolated populations, preventing interbreeding and gene flow.
In each isolated group random mutations create new alleles, giving genetic variation.
Because the lakes differ in temperature, food and oxygen, natural selection favours different traits in each population.
Fish with advantageous traits survive and reproduce more, passing those alleles to their offspring.
After many generations the accumulated genetic differences mean the two groups can no longer produce fertile offspring, so they are distinct species.
In each isolated group random mutations create new alleles, giving genetic variation.
Because the lakes differ in temperature, food and oxygen, natural selection favours different traits in each population.
Fish with advantageous traits survive and reproduce more, passing those alleles to their offspring.
After many generations the accumulated genetic differences mean the two groups can no longer produce fertile offspring, so they are distinct species.
Examiner tips
- Use the word "isolated" to show lack of gene flow. Mention "random mutations" and "alleles". Explain how natural selection acts differently. Show that advantageous traits are passed on. Conclude with reproductive isolation after many generations.
Common mistakes
- Failing to mention gene flow is blocked. Using vague terms like "differences" instead of "alleles". Not linking selection to survival and reproduction. Omitting the final step of reproductive isolation.
Mark scheme (5 marks)
- The two populations are separated and can no longer interbreed / exchange genetic material (gene flow is prevented)
- Random mutations occur in each population, producing genetic variation / different alleles in each population
- Natural selection acts differently in each lake because the environments are different
- Individuals with advantageous characteristics are more likely to survive, reproduce and pass on their alleles / genes to offspring
- Over many generations the two populations become so genetically different that they can no longer interbreed to produce fertile offspring, so they are now two separate species
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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