# Explain how the concept of gradualism differs from punctuated equilibrium as models of the rate of evolutionary change, and outline one line of evidence from the fossil record that supports punctuated equilibrium.

> IB DP Biology Higher Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks

## Mark scheme (4 marks)

1. Gradualism proposes that evolutionary change occurs slowly and continuously over long periods of time / that species accumulate small, incremental changes generation by generation.
2. Punctuated equilibrium proposes that long periods of little or no change (stasis) are interrupted by relatively rapid bursts of evolutionary change / speciation events.
3. The fossil record often shows species remaining morphologically unchanged for millions of years (stasis) before being abruptly replaced by a new form, with few intermediate fossils between them — consistent with punctuated equilibrium rather than gradual transition.
4. A specific named example: e.g. the Cambrian explosion, in which most major animal phyla appear in the fossil record within a geologically short interval (~20 million years) after long preceding periods of simpler life forms, supports rapid bursts of diversification.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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