1. The Pace of Evolution

2. Common Ancestry
Most scientists believe that all forms of life can trace their origins back to a single common ancestor. Different species that are more closely related have a much more recent common ancestor.

3. Common Ancestry
Scientists use evidence from fossils and the study of homologies to draw conclusions about the evolutionary relationships between organisms and their ancestors.

4. The Fossil Record
Fossils are preserved remains of an organism, and they can tell us about organisms that are now extinct as well as about the history of an ecosystem.

5. The Fossil Record
Fossils can be used to study the ancestors of a species. By studying the sequence of fossils found in layers of rock, scientists can make observations about how species have changed over time.
Change = Evolution

6. Punctuated Equilibrium
The Fossil Record
The fossil record is not complete, and there are sometimes gaps between species. Because of this, there are two major theories about the rate of evolution based on the fossil record.
Punctuated Equilibrium
Gradualism

7. The Pace of Evolution
Gradualism describes evolution as a process of slow and continuous change. Populations are gradually changing over time into new species.
Gradualism

8. Punctuated Equilibrium
The Pace of Evolution
Punctuated equilibrium describes evolution as having long periods of stability that are interrupted by geologically brief periods of significant change.
Punctuated Equilibrium

9. Punctuated Equilibrium
The Pace of Evolution
When the populations are stable, they are in equilibrium. These periods are punctuated by periods of great change.
Punctuated Equilibrium
Rapid Change
Period of Stability

10. Gradualism vs. Punctuated Equilibrium
Both of these theories can be applied to the fossil record to try and explain the sequence of fossils in each layer of rock.
Diagram from

11. Gradualism vs. Punctuated Equilibrium
Gradualism states that evolution is slow and continuous. This means that gaps in the fossil record would be because there was a large gap between the formation of the two layers of rock.
Diagram from

12. Gradualism vs. Punctuated Equilibrium
If there was no rock layer formed to capture the intermediate species resulting from the gradual process of evolution, there would be no record of their existence.
Diagram from

13. Gradualism vs. Punctuated Equilibrium
Punctuated Equilibrium states that evolution occurs in rapid bursts followed by long stable periods. Because the change is quick in terms of rock formation, the intermediates do not appear.
Diagram from

14. Gradualism vs. Punctuated Equilibrium
If the intermediate species do not exist long enough to be captured in the fossil record, then we will only have fossils of the species that exist in equilibrium for long periods of time.
Diagram from

15. Gradualism vs. Punctuated Equilibrium
There is evidence to support both of these theories, and scientists believe that there may have been times when evolution occurred according to both of these theories.

16. Gradualism vs. Punctuated Equilibrium Punctuated Equilibrium
We depict these processes differently to suggest the different rates. Gradualism shows the slow, gradual changes with winding lines while punctuated equilibrium is depicted with rapid change followed by straight periods of stability.
Punctuated Equilibrium
Gradualism