1. The Evolution of Populations & Speciation
Chapter 16

2. Genetic Equilibrium
Population Genetics: the study of evolution from a genetic viewpoint
Most popltns have variation amongst their traits
i.e. different heights, skin color, etc.
If graphed, looks like a bell

3. Effects of Natural Selection
Natural Selection can cause a shift in traits within a population
Depends on which traits are favorable in that environment
Those traits will increase over time
4 Different Effects:
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection

4. Stabilizing Selection
Occurs when the average form of a trait is considered the highest fitness level (best for that environment)
i.e. height
Causes decrease in variation

5. Directional Selection
Occurs when one extreme is considered highest fitness level
i.e.: an anteater’s tongue length
Causes a shift in variation

6. Disruptive Selection
Occurs when both extremes are considered highest fitness level
i.e.: black/white limpets on barnacles
Causes splitting of phenotypes

7. Sexual Selection
Occurs when female organisms select a mate based on phenotype
May require extreme male traits
i.e. bright colors, unique mating calls, etc.

8. Formation of Species Speciation: process of forming new species
Results in many related popltns
May remain similar or become quite different
Manchester
Terrier
Dandie
Terrier

9. Isolating Mechanisms
Speciation occurs when members of a species stop interbreeding and split
Become isolated from each other
2 Types Isolating Mechanisms:
1) Geographic
2) Reproductive

10. Geographic Isolation
Geographic Isolation: the physical separation of members of a popltn
i.e.: by canyons, mountains, rivers, etc.
Once physically separated, will continue to diverge until they can no longer mate

11. Reproductive Isolation
Reproductive Isolation: members of a popltn no longer able to mate successfully
i.e. due to disruptive selection, changes in mating habits, etc.
Once separated, will continue to diverge

12. Rates of Speciation
Gradualism: some species evolve gradually & at stable rate
Although speciation may take millions of years, it is not always gradual
Punctuated Equilibrium: when species evolve in rapid spurts of great change
Involves long periods of no change between “growth spurts”
Usually due to natural disasters