SESSION 3

Engage

Explore

Explain

Part A: 

Fossil Evidence

Part B: 

Biogeographical Evidence

Part C: 

Anatomical Evidence

Elaborate

Evaluate

Resources

Facilitator Notes

SESSION 3: What Is the Evidence for Evolution?

Explain Part B: Biogeographical Evidence

Darwin's travels during the voyage of the Beagle allowed him to see the similarities and differences in organisms in very diverse geographic areas. He noticed some important patterns in the diversity and distribution of species. As you read the following passage from Darwin's autobiography, jot down the three key observations he made about the distribution of related species:

From September 1854 onwards I devoted all my time to arranging my huge pile of notes, to observing, and experimenting, in relation to the transmutation of species. During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the Continent; and thirdly, by the South American character of most of the productions of the Galapagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of these islands appearing to be very ancient in a geological sense. It was evident that such facts as these, as well as many others, could be explained on the supposition that species gradually become modified; and the subject haunted me.

(From The Autobiography of Charles Darwin [New York: W.W. Norton and Company, Inc., 1996] p.118)

Compare your observations to this summary of Darwin's observations.

Investigate the Web sites listed below to gather information on biogeographical evidence.

	•	Read Chapters 11 and 12 of The Origin of Species, at the Online Literature Library Web site.
	•	Read about the distribution of placental and marsupial mammals at the Scientific American Web site.
	•	For some examples of how biogeography provides evidence for evolution, see "The Distribution of Species" section of the National Academy of Sciences Web site.

Look at the image "Biogeography." The north and south polar regions are inhabited by very different organisms. Why are there no polar bears in the Antarctic, and no penguins in the Arctic?

The continental drift theory, first proposed by Alfred Wegener in 1912, has been supported by oceanographic evidence. Continental drift has also influenced the distribution of species. For more information on continental drift and species distribution, see the NASA Web site page on Evidence Supporting Continental Drift and the United States Geological Survey Web site page on Polar Dinosaurs.

Now, answer these questions.

	•	What are some key examples that illustrate how biogeographical evidence supports the theory of evolution?
	•	What questions remain unanswered using just biogeographical evidence?

Next: Explain Part C: Anatomical Evidence