10.5.1 Time Frame for Evolution: Gradualism vs. Punctuated Equilibrium

Question 1

gradualism vs. punctuated equilibrium

Answer 1

• Two theories present differing views as to how species arise:
1. Gradualism is the theory that new species gradually diverge as a result of accumulations of small changes.
2. Punctuated equilibrium is the theory that new species develop in periods of rapid change followed by long periods of no change.
• Evaluating the theories involves estimating the life span of a species and examining the fossil record for evidence of morphological change

Question 2

gradualism

Answer 2

• According to the theory of gradualism, evolution is a gradual, constant, and slow process. A new species gradually diverges as a result of the accumulation of small changes.
• Because of the lack of transition fossils, some scientists dispute this theory. Supporters argue that gradual changes may not be recorded in the fossil record. A transition fossil is evidence of extinct life-forms that shows the link between a new species and its parent species.

Question 3

punctuated equilibrium

Answer 3

• According to the theory of punctuated equilibrium, new species develop in periods of rapid change followed by long periods of no change.
• The average life span of a species is about 4–5 million years. If the period of rapid change of a species takes place in the first 1% of its existence, that still allows for 50,000 years of chang

Question 4

note

Answer 4

• The illustration on the left shows why many transition fossils may not have been found. If punctuated equilibrium was the most common mechanism of speciation, the period of rapid change would take place over the course of 50,000 years. In geological time measured in the earth’s layers, 50,000 years is an extremely thin layer, reducing the likelihood of finding
  transition fossils. The transition fossils found to date,
  however, appear to support the punctuated equilibrium model of evolution.

Question 5

According to one view of evolution, species exist for long periods of time without changing. A sudden environmental change causes a sudden change in the gene pool. This theory of evolution is known as

Answer 5

• punctuated equilibrium

Question 6

The processes shown in the illustration left to right are ____________.

Answer 6

• gradualism and punctuated equilibrium, respectively.

Question 7

What is punctuated equilibrium?

Answer 7

• A process that describes periods of rapid evolution followed by periods of stability

Question 8

Why is it difficult to trace speciation through time?

Answer 8

• Transition fossils are hard to find

Question 9

What is the average lifespan of a species?

Answer 9

• 4 to 5 million years

Question 10

Why will the debate over gradualism versus punctuated equilibrium not be resolved in the near future?

Answer 10

• Not enough fossil evidence to support either view.