1. Patterns of Natural Selection & Genetic Drift
Ch 17.2

2. State Standard
8a. Students know how natural selection determines the differential survival of groups of organisms.
8c. Students know the effects of genetic drift on the diversity of organisms in a population.

3. Objectives:
What are the possible patterns that result from natural selection?
What are genetic drifts and how can they change populations?

4. Natural Selection is Limited
The environment does the selecting.
This limits the process
Natural Selection acts only to change the relative frequency of alleles in a population.
Does not create new ones, or delete those that are not adaptive.

5. Natural Selection is Limited
Natural selection does not act directly on genes.
Only allows individuals with favorable traits to pass them on.
It acts by indirectly removing unsuccessful phenotypes from a population.
Only characteristics that are expressed can be targets of natural selection.

6. Patterns of Natural Selection
Three major patterns are possible in the way that natural selection affects the distribution of polygenic traits over time.
Directional Selection
Stabilizing Selection
Disruptive Selection

7. Directional Selection
The peak of normal distribution moves in one direction along its range, shifting toward one end (or the other).
Eliminates one of the extremes.
Key
Low mortality, high fitness
Food becomes scarce.
High mortality, low fitness

8. Stabilizing Selection
Bell curve becomes narrower, tightening up the “normal” range.
Eliminates individuals at both extremes.
Stabilizing Selection
Key
Low mortality, high fitness
Selection against both extremes keep curve narrow and in same place.
High mortality, low fitness
Percentage of Population
Birth Weight

9. Disruptive Selection
Bell curve becomes disrupted, pushing it apart into two separate peaks.
Eliminates the average or middle phenotype values, and favors extreme forms of a trait.
Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality, high fitness
Population splits into two subgroups specializing in different seeds.
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size

10. Genetic Drift The random change in allele frequency in a population.
Can be significant in small populations.
Nature is unpredictable

11. Causes: (random changes in allele frequency)
Few organisms in population
Parents having only a few offspring, so they don’t give off all alleles
Only a fraction of offspring live to maturity and have offspring
Events of Nature
Fire, Flood, physical separation
Mutations

12. Bottleneck Effect & Founder’s Effect
Bottleneck - A few organisms survive an event to pass on traits
Founders Effect – A few organisms leave and begin a new population, pass on traits
Human influence
often leads these

13. Examples: