1. Evolution Notes
Part II

2. Natural Selection Stabilizing selection Directional selection
Natural selection has three modes of action:
Stabilizing selection
Directional selection
3. Disruptive selection

3. 1. Stabilizing Selection
Favors the average, acts against the extremes.
Number of
Individuals
Size of individuals
Small Large

4. 2. Directional Selection
Favors one of the two extremes.
Number of
Individuals
Size of individuals
Small Large

5. 3. Disruptive Selection
Favors both extremes, acts against the average.
Number of
Individuals
Size of individuals
Small Large

6. How do new species evolve?
Speciation-the formation of new species In order for new species to form, there must first be reproductive isolation among parts of a population.

7. Isolating Mechanisms Geographic Isolation Behavioral Isolation
Temporal Isolation

8. Geographic Isolation
Populations are separated physically by geographic barriers such as rivers, mountains, or stretches of water.

9. Behavioral Isolation
Populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding.

10. Temporal Isolation
Two populations reproduce at different times.

11. Adaptive Radiation
Emergence of numerous species from a common ancestor introduced to new and diverse environments.

12. Coevolution
Evolutionary change resulting from the interaction between two different species. Example: Pollinators and Flowering plants

13. Biodiversity
The sum total of different species in a given area.

14. Extinction

15. Interpretations of Speciation
Two theories:
1. Gradualist Model (Neo-Darwinian):
Slow changes in species over time.
2. Punctuated Equilibrium:
Evolution occurs in spurts of relatively rapid change.