Biology (Holt)

The Tempo of Evolution

For decades, most biologists have understood evolution as a gradu-

al process that occurs continuously. The model of evolution in

which gradual change over a long period of time leads to species

formation is called .But American biologists Stephen

Jay Gould and Niles Eldredge have suggested that successful

species may stay unchanged for long periods of time. Gould and

Eldredge have hypothesized that major environmental changes in

the past have caused evolution to occur in spurts. This model of

evolution, in which periods of rapid change in species are separated

by periods of little or no change, is called punctuated equilibrium.

gradualism

282 CHAPTER 13The Theory of Evolution

Punctuated Equilibrium

How could major environmental changes lead to

spurts in evolution? The fossil record shows that

drastic environmental changes have occurred very

infrequently, separated by periods of time that

often last tens of millions of years. Events such as

volcanic eruptions, asteroid impacts, and ice ages

have been linked to sudden and drastic changes

in climate. Such changes have also been linked to

the extinction of many groups of organisms. As a

result, environments that were once inhabited

became empty. This provided opportunities for

colonization by species that could quickly adapt

to the new conditions through natural selection.

What Fossils Reveal

Although there are large gaps in the fossil record as

a result of erosion and other destructive geologic

processes, the fossil record seems to provide

evidence of both gradualism and punctuated

equilibrium. Many groups of organisms appear

suddenly in the fossil record. Some of these groups

remain virtually unchanged for millions of years,

while other groups disappear as suddenly as they

appear. Still other groups of organisms appear to

change slowly through time, as predicted by the

gradualism model of evolution. More study of the

fossil record may reveal additional examples of one

or both types of evolution.

Exploring FurtherFurther

Gradualism Punctuated equilibrium

Listtwo observations made by Charles Darwin

during his 5-year voyage that led him to conclude

that living species evolved from extinct species.

Describehow natural selection occurs. 7B

Summarizethe modern theory of evolution by

natural selection. 3A

Comparethe punctuated equilibrium model of

evolution with the gradualism model. 3A 3E

Speciation can result when two

populations have become

Aextinct. Cinterbred.

Breproductively isolated. Done population.

TAKS Test PrepTAKS Test Prep

Section 1 Review

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Topic: Fossil Record

Keyword: HX4088

3C 3F

7B

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Topic: Evolution

Keyword: HX4074