1. A Theory to Explain Change Over Time
Evolutionary Theory
A Theory to Explain Change Over Time

2. Modern Evolutionary Theory
A theory is: a broad range of data that has been scientifically tested and supported.
Modern evolutionary theory began when Charles Darwin presented evidence that evolution happens and offered an explanation of how it happens.

3. Evolutionary Theory Keeps Changing & Expanding
Like most scientific theories, evolutionary theory keeps developing & expanding.
Many scientists since Darwin have tested & added to his ideas.
Peter & Rosemary Grant-research Galapagos Finches

4. Darwin’s Ideas From Experience
In Darwin’s time, most people did not think that living things had changed over time.
In fact, many doubted that Earth itself had ever changed.

5. But Darwin saw evidence of gradual change.
Darwin’s experiences provided him with evidence of evolution at work.

6. The Voyage of the Beagle

7. Galapagos Islands
Darwin collected several different species of birds called finches.
Each of the finches are very similar, but have different bills.

8. Each bill is suited to a particular food available on the individual islands.

9. Galapagos Plants and Animals
Darwin noticed that many of the islands’ plants and animals were similar but not identical to species in South America.
He suggested they were descendants of S. American species with modifications.
Darwin’s original notes on descent with modification to explain the different species of birds, tortoises, etc. seen on the Galapagos.

10. Descent with modification = Evolution
Darwin suggested that all species have descended from ancient species with modifications to their environment.
Each of the Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists suggest that all 23 honeycreeper species apparently arose from a single species that migrated to Hawaii.

11. Years of Reflection
After returning from his voyage, Darwin spent many years studying his data.
He did not report his ideas for many years.
He took time to gather more data and to form a strong explanation for how evolution happens.

12. Breeding & Selection
Darwin bred exotic pigeons and studied the work of others who bred dogs, orchids, crops, and other livestock.
Breeders take advantage
of natural variation in traits within a species.
This is artificial selection.

14. Artificial Selection – The human practice of breeding animals or plants that have certain traits.

16. Natural Selection
Darwin suggested that nature “selects” which organisms survive and reproduce.
Survival of the Fittest

17. Darwin’s Ideas from Others
In Darwin’s time, most people—including scientists—believed that each species was created once and stayed the same forever.
But, this view could not explain fossils of organisms that no longer exist, such as dinosaurs.

18. Do Organisms Change over Time?
Some scientists tried to explain such observations by saying that species could die out but never change.
Others proposed various mechanisms to explain how species may change over time.

19. Lamarck
In 1809, the French scientist, Jean Baptiste Lamarck proposed an explanation for how organisms may change over generations.
He noticed that organisms are usually well adapted to their environment.

20. …Lamarck
Lamarck proposed (like Darwin) that organisms change over time as they adapt to changing environments.
The mechanism for that change, however, that was proposed by Lamarck was wrong.

21. Ex: building muscles would mean their baby would have bigger muscles.
Lamarck proposed that changes due to use or disuse of a characteristic would be passed on to the offspring. (Is this possible?)
Ex: building muscles would mean their baby would have bigger muscles.
Lamarck’s Idea of Acquired Characteristics was later shown to be incorrect!

22. Use and Disuse
Lamarck proposed a reason why giraffes have a long neck:
He said that giraffes stretched their necks to reach leaves high in the trees. Their offspring, therefore would have longer necks.
Is this possible?

23. The Theory of Evolution by Natural Selection (See page 381)

24. Natural Selection
The process by which individuals that are better adapted to their environment survive & reproduce more successfully than less well adapted individuals do.

25. 4 Steps to Natural Selection
1. Overproduction – Every population is capable of producing more offspring than can possibly survive. Therefore, there is competition for survival.

26. 2. Variation – Variations exist within every population
2. Variation – Variations exist within every population. Much of this variation is in the form of inherited traits. Some traits are better adapted than others.

27. 3. Selection – In a given environment, having a particular trait can make certain individuals more likely to survive & have successful offspring than others.

28. 4. Adaptation – Over time, those traits that improve survival & reproduction will become more common in a population.

29. Why do giraffes have long legs and necks?
What would the Theory of Evolution by Natural Selection state about why giraffes have longer legs and necks?

30. Natural Selection Animation

31. Definitions to Know!
Evolution: Change in a species over time; process by which modern organisms have descended from ancient organisms
Descent with modification: principle that each living species has descended, with changes, from other species over time
Theory: well-tested explanation, supported by substantial evidence, that unifies a broad range of observations

32. More Definitions to Know!
Lamarck’s Idea of Acquired Characteristics: organisms change over time because they acquire stronger characteristics for the traits they use often. There is no basis in heredity that accounts for these changes—they cannot be passed down to offspring
Natural Selection:process by which individuals that are better suited to their environment survive and reproduce most successfully; also called survival of the fittest