1. REVIEW: Natural Selection Assumptions of Natural Selection
Process of change in populations over many generations
Individuals with certain traits survive local environmental conditions
Pass on favourable alleles to offspring
Environment exerts ‘selective pressure’
Assumptions of Natural Selection
Variation
Competition
Fitness
Adaptation

2. Mechanisms of Evolution

3. Allele Frequencies
When populations change in response to the environment or through predation, we see frequencies of alleles change, too.
Eg. More dark coloured pepper moths, fewer tall animals in colder climates, more redheads when they selectively breed and begin to take over the world…

4. FACTORS THAT CHANGE ALLELE FREQUENCIES:
Mutation
Gene Flow (Migration)
Non-Random Mating
Genetic Drift
Natural Selection

5. 1. Mutation Randomly introduces new alleles into pop’n.
Changes allele frequencies
Eg. Norway rats resistant to warfarin

6. 2. Gene Flow (Migration)
Occurs between 2 different interbreeding pop’ns with different allele frequencies
May change allele frequencies in either or both populations
Population A
Population B

7. 3. Non-random mating
Individuals in a pop’n. select mates, often on the basis of their phenotypes
Increases proportion of homozygous individuals in a pop’n., but does not change allele frequencies
Eg. Caribou females prefer more dominant males

8. Non-Random Mating: Inbreeding
When homozygous is more common - harmful recessive alleles more likely to be expressed
Pure bred = higher incidence of deformities and health problems
Eg. Shar-pei dogs’ wrinkles enhanced through inbreeding

9. 4. Genetic drift
Random change in genetic variation from generation to generation due to chance
More common in smaller populations
Changes frequency of alleles
Ex. Flip coin 10 times instead of 1000

10. Genetic Drift:

11. The Bottleneck Effect
Starvation, disease, natural disasters and severe weather can quickly reduce sizes of large pop’ns.
Survivors likely have a fraction of the alleles that were present before the reduction
Gene pool loses diversity
Eg typhoon devastated tiny island of Pingelap (Pacific Ocean)

12. The Founder Effect
Few individuals from a large population leave to establish a new population
Eg. Amish (descendants of about 30 people from Switzerland in 1720)

13. 5. Natural Selection
Result of the environment selecting for individuals with certain traits that make them better suited to survive and reproduce
Over many generations, frequency of alleles of many different genes may change, resulting in significant changes