Explain how the concept of punctuated equilibrium differs from phyletic gradualism in describing the pattern of evolutionary change seen in the fossil record.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Phyletic gradualism proposes that evolutionary change occurs slowly and continuously over long periods of time.
Punctuated equilibrium proposes that species remain largely unchanged (stasis) for long periods, interspersed with rapid bursts of evolutionary change.
The fossil record tends to support punctuated equilibrium because transitional forms are rare and gaps between morphologically distinct forms are common.
Under punctuated equilibrium, rapid change is linked to environmental perturbation, isolation of small populations and allopatric speciation events, explaining why change appears sudden rather than gradual.
Punctuated equilibrium proposes that species remain largely unchanged (stasis) for long periods, interspersed with rapid bursts of evolutionary change.
The fossil record tends to support punctuated equilibrium because transitional forms are rare and gaps between morphologically distinct forms are common.
Under punctuated equilibrium, rapid change is linked to environmental perturbation, isolation of small populations and allopatric speciation events, explaining why change appears sudden rather than gradual.
Examiner tips
- Use the exact terminology (phyletic gradualism, punctuated equilibrium, stasis, rapid bursts).
- Show the contrast clearly – slow continuous vs. long stasis + rapid change.
- Mention fossil evidence (rare transitional forms, gaps).
- Explain the mechanism (environmental change, isolation, allopatric speciation).
Common mistakes
- Confusing gradualism with punctuated equilibrium or vice versa.
- Failing to mention the fossil record evidence.
- Using vague terms like ‘fast’ or ‘slow’ without specifying stasis and bursts.
Mark scheme (4 marks)
- Phyletic gradualism proposes that evolutionary change occurs slowly and continuously over long periods of time
- Punctuated equilibrium proposes that species remain largely unchanged (stasis) for long periods, interspersed with rapid bursts of evolutionary change
- The fossil record tends to support punctuated equilibrium because transitional forms are rare / gaps between morphologically distinct forms are common
- Under punctuated equilibrium, rapid change is linked to environmental perturbation / isolation of small populations / allopatric speciation events, explaining why change appears sudden rather than gradual
Key terms in this question
punctuated equilibrium · phyletic gradualism · fossil record
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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