1. Types of Natural Selection
SBI3U

2. Sickle-Cell Anemia
Mutation + genetic variation + natural selection = evolution of a population
Sickle- cell allele resulted from a single base mutation in the DNA coding for hemoglobin
Heterozygous individuals, Ss, are resistant to malaria  better chance of survival
Sub-Sahara Africa- if you carry the allele, S, than you will survive to reproduce, and pass on the allele to the next generation
Therefore, an increase in the frequency of the allele

4. Directional Selection
Occurs when selection favours an increase or decrease in the value of a trait from the current population average.
Favours individuals with a more extreme variation of a trait
Results in a shift away the average condition
Read Hummingbird example on the bottom of pg. 326

5. Directional Selection Example
-The soil in the original environment was a medium brown
- After a volcano the soil becomes darker
Mouse population evolved to blend in with the darker surroundings
Therefore, environment favoured individuals with more extreme variations of the trait

6. Stabilizing Selection
Selection against individuals exhibiting traits that deviate from the current population
The average phenotype within a population is favoured by the environment
Read the human birth weight example on the bottom of page 327.

7. Stabilizing Selection Example
The soil remains medium brown
Medium brown mice survive from predation
Medium brown mice pass of their allele for medium brown fur to their offspring
Light/ dark brown mice are eaten more -> those alleles are not passed on

8. Disruptive Selection
Selection that favours two or more variations of a trait that differ from the current population average.
Favours opposite extremes of a trait over individuals with intermediate variations

9. Disruptive Selection Example
Mouse move to a new environment with light brown rock and dark brown volcanic rock
Light brown and dark brown mice blend in with their environment, but medium brown mice do not
Medium brown mice are eaten by predators
Light/ dark brown mice pass on their genes to future generations

10. Hardy- Weinberg Equilibrium
If allele frequencies change over time, the population is NOT in Hardy-Weinberg Equilibrium
Therefore, a ‘disturbing factor’ is acting on the population
Natural Selection
Small population size
Mutation
Immigration/ Emigration
Sexual selection