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5.24: Macroevolution

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Fast or slow?

Which is better, the direct route or the scenic route? Each has its advantages, depending on the situation. And that describes evolution. It can be fast or slow, depending on the situation.

Timing of Macroevolution

Is evolution slow and steady? Or does it occur in fits and starts? It may depend on what else is going on, such as changes in climate and geologic conditions.

  • When geologic and climatic conditions are stable, evolution may occur gradually. This is how Darwin thought evolution occurred. This model of the timing of evolution is called gradualism .
  • When geologic and climatic conditions are changing, evolution may occur more quickly. Thus, long periods of little change may be interrupted by bursts of rapid change. This model of the timing of evolution is called punctuated equilibrium . It is better supported by the fossil record than is gradualism.

Two theories of evolutionary change - gradualism vs. punctuated equilibrium - are still debated. The former proposes continuous change, while the latter suggests that species remain constant for long periods of time and that change, when it occurs, is rapid.

Summary

  • Darwin thought that evolution occurs gradually. This model of evolution is called gradualism.
  • The fossil record better supports the model of punctuated equilibrium. In this model, long periods of little change are interrupted by bursts of rapid change.

Practice

Use this resource to answer the questions that follow.

  1. What is meant by "gradual changes"?
  2. Define punctuated equilibrium.

Review

1. What is gradualism? When is it most likely to apply?

2. Describe the timing of evolutionary change according to the punctuated equilibrium model.

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Feb 24, 2012

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Sep 15, 2014
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