REVIEW: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

Mechanisms of Evolution

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…

FACTORS THAT CHANGE ALLELE FREQUENCIES:

1. Mutation2. Gene Flow (Migration)3. Non-Random Mating4. Genetic Drift5. Natural Selection

1. Mutation• Randomly

introduces new alleles into pop’n.

• Changes allele frequencies

• Eg. Norway rats resistant to warfarin

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

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

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

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

Genetic Drift:

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. 1775 typhoon devastated tiny

island of Pingelap (Pacific Ocean)

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)

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