Explain how a punctuated equilibrium model of evolution differs from a gradualist model in describing the pattern of evolutionary change over geological time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In gradualism, evolutionary change is slow and continuous over long periods of time.
In punctuated equilibrium, long periods of stasis are interrupted by short, rapid bursts of change.
The fossil record shows few transitional forms, supporting punctuated equilibrium because rapid change in small, isolated populations leaves little trace.
Rapid change in punctuated equilibrium is often linked to environmental disruption or geographic isolation of small founder populations, allowing faster divergence by natural selection or genetic drift.
In punctuated equilibrium, long periods of stasis are interrupted by short, rapid bursts of change.
The fossil record shows few transitional forms, supporting punctuated equilibrium because rapid change in small, isolated populations leaves little trace.
Rapid change in punctuated equilibrium is often linked to environmental disruption or geographic isolation of small founder populations, allowing faster divergence by natural selection or genetic drift.
Examiner tips
- Use the word ‘gradualism’ and ‘punctuated equilibrium’ explicitly. Mention stasis, rapid bursts, fossil evidence and small isolated populations. Keep each point concise to match the 4 marks.
Common mistakes
- Confusing gradualism with punctuated equilibrium. Omitting the role of small isolated populations. Failing to mention the fossil record evidence.
Mark scheme (4 marks)
- In gradualism, evolutionary change occurs slowly and continuously over long periods of time
- In punctuated equilibrium, long periods of little or no change (stasis) are interrupted by short, rapid bursts of evolutionary change
- The fossil record shows few transitional forms, which supports punctuated equilibrium since rapid change in small, isolated populations leaves little trace
- Rapid change in punctuated equilibrium is often linked to environmental disruption or geographic isolation acting on small founder populations, enabling faster divergence by natural selection or genetic drift
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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