1. Chapter 16: Darwin’s Theory of Evolution
Section 16-3: Darwin Presents His Case

2. Darwin’s Ideas
Darwin wanted to gather evidence before making his ideas public
Waited 20 years after returning
Published On the Origin of Species in 1858

3. The Struggle for Existence
More individuals are produced than survive
Members of a population compete for food, living space, other limited necessities
Malthus

4. Variation and Adaptation
Individuals have natural variation in heritable traits, some better suited for the environment
Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is an adaptation
Adaptations can be morphological, physiological, behavioral

5. Survival of the Fittest
An organism’s fitness as how well it can survive and reproduce in its environment
Adaptations increase fitness
Differences in fitness (survival rate and reproduction) called survival of the fittest
Survival = passing adaptations to next generation

6. Natural Selection
The process by which organisms with variations most suited to their local environment survive and leave more offspring
Mechanism of evolution
Environment influences fitness
Populations continue to change as they become better adapted and their environment changes
Acts only on inherited traits

7. Natural Selection

8. Natural Selection

9. Natural Selection

10. Natural Selection

11. Natural Selection Does not make organisms “better”
Does not move in a fixed direction
Extinction can occur if conditions change faster than species can adapt

12. Common Descent
Over many generations adaptation caused successful species to evolve into new species
Descent with modification from common ancestors
Life has been on Earth a long time
Evidence in fossil record
Explains diversity of life