1. 9.1-Mechanisms of Evolution & Their Effect on Populations
SBI3U1

2. Evolution & Population
Individuals organisms do not evolve POPULATIONS DO!
Therefore when we study evolutionary processes we study the changes in population

3. What is a gene pool? A A A A a a A a A a A A a
A population that consists of all the alleles of all genes of each individual in that population= GENE POOL.
This provides the population’s genetic variation. A A A a A a A a A a A A a

4. MICROEVOLUTION
Allele Frequency: the number of copies of a particular allele compared to the total number of alleles in a population
Microevolution = change in % or frequency of alleles within a population
Therefore, small changes to allele frequency can lead to evolution

5. 5 Factors that Affect Allele Frequencies
Mutation
Gene Flow ( migration)
Non-Random Mating
Genetic Drift
*Natural Selection
*Note: natural selection is
the most significant factor

6. 1) Mutations Recall: Mutation is a change in DNA of an individual
A heritable mutations may affect an entire gene pool
More genetic variation in a population = greater diversity = greater chance of selective advantage (in changing environment)
(ie. Norway rats resistant to poison- see next slide)

7. Example: Norway Rat Population
It is likely that an allele already existed in the Norway rat population that gave some individuals resistance to the poison warfarin. When warfarin was applied, those individuals survived, reproduced, and passed on the allele. This selective advantage changed the allele frequency of this characteristic. 

8. 2) Gene Flow
The net movement of alleles from one population to another due to the migration of individuals
Through gene flow, modelled here, genetic information is exchanged between individuals of different populations.

9. Example: Grey Wolves
-Grey wolves will travel great distances to find a neighbouring pack & mate -This brings new alleles into the distant pack -This will change the allele frequencies & increase genetic diversity

10. 3)Non-Random Mating
Non-random mating is mating among individuals on the basis of a particular phenotype or due to inbreeding
In animals, individuals may choose mates based on their physical or behavioural characteristics.
This will affect the alleles in the gene pool of the next generation

11. Non-Random Mating Examples
Peacocks choose mates based on the tail/dance
Male Caribou spar for a mate
Humans choose mates based on certain characteristics/values

12. 3)Non-Random Mating Cont’d…
Inbreeding: When closely related individuals breed
This increases the frequency of homozygous genotypes.
Therefore, as homozygous genotypes become more common, harmful recessive alleles are more likely to be expressed.
Ex: Self fertilization in plants Purebred dogs

13. 4)Genetic Drift
Genetic Drift= the frequency of certain alleles can be changed due to chance ( in small popl’n)
The smaller the popl’n the less likely the parent gene pool will be reflected in certain populations.

14. In each generation, only some of the plants in this population reproduce. When the light pink (aa) and heterozygous roses (Aa) in the second generation did not reproduce, the allele for light pink petals was lost from the gene pool.

15. 4)Genetic Drift Cont’d…
Large populations are less susceptible to the effects of genetic drift, However….
2 situations that can lead to significant genetic drift in large populations are called:
Founder Effect
Bottleneck Effect

16. 4)Genetic Drift: The Founder Effect
Founder Effect: a new population is started by a few members of the original population
Reduces genetic variation/
Gene pool is reduced
Common in islands
New “founders” carry some
but not all of the alleles from t
he original population’s
gene pool.
Higher health concerns->
Polydactylism has a high frequency in the Amish founder population in Philadelphia

17. 4)Genetic Drift: The Bottleneck Effect
When a popl’n is quickly reduced by starvation, disease, or a natural catastrophe, the surviving population likely has only a fraction of the alleles that were present before the population declined. The gene pool will have lost its diversity, and allele frequencies will have changed

18. 5)Natural Selection
Recall: Selective forces such as competition and predation affect populations. Some individuals are more likely to survive and reproduce than others. Alleles that are selected for by environmental conditions will increase in frequency over time.
3 Types of natural selection :
Stabilizing selection
Directional selection
Disruptive selection

19. 5) Natural Selection: Stabilizing Selection
Stabilizing Selection: favours intermediate phenotypes and acts against extreme variants of the phenotype
Reduces variation
E.g. Birth weight in humans

20. 5) Natural Selection: Directional Selection
Directional Selection: favours phenotypes at one extreme over the other.
Common during environmental change
E.g. Peppered moths

21. 5) Natural Selection Disruptive Selection
Disruptive Selection: favours the extremes of a range of phenotypes rather than intermediate are favoured
Intermediate phenotypes can be eliminated
E.g. Male coho salmon (ranges from g)

22. 5)Natural Selection Cont’d…
Sex Selection: competition for mates between males through combat or through visusal displays.
Combat (caribou)
Visual display (Peacocks)
Sexual dimorphism:
difference in appearance
Btw. males and females
Ie. Green head in
male mallard ducks

23. Evolution Simulations
hill.com/sites/ /student_view0/chapter20/ani mation_-_mechanisms_of_evolution.html
Genetic Drift Simulation:

24. Homework: Read and make notes 9.1 Genetic Drift Dry Lab- TBA
Complete pg. 352 Q#1-4
Pg.356 Q #7-12