# Explain how a punctuated equilibrium model of evolution differs from a gradualist model in describing the pattern of evolutionary change over geological time.

> IB DP Biology Standard Level (2023 syllabus) — D4.2 Stability and change · Explain · 4 marks

## Mark scheme (4 marks)

1. In gradualism, evolutionary change occurs slowly and continuously over long periods of time
2. In punctuated equilibrium, long periods of little or no change (stasis) are interrupted by short, rapid bursts of evolutionary change
3. The fossil record shows few transitional forms, which supports punctuated equilibrium since rapid change in small, isolated populations leaves little trace
4. 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

- [punctuated equilibrium](https://www.gradenine.co.uk/glossary/punctuated-equilibrium)

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