1. Mechanisms of Evolution
and their Effect on Populations

2. How did you determine that the evolutionary process took place in your rabbit population?

3. What are the factors that change allele frequencies in populations???

4. Mutation
A mutation is a change in the DNA of an individual. If heritable, the mutation can affect the entire gene pool over time by changing the gene frequency.

5. Gene Flow (migration)
Two different interbreeding populations with different allele frequencies meet as individuals migrate from one population to another
During the move, alleles are removed from one population and added to another
May change allele frequencies in either population, but, over time, it tends to reduce genetic differences between populations

6. Gene Flow (migration)

7. Non-random Mating
Mating among individuals based on selecting for specific phenotypes
Preferred mates pass on alleles in greater proportions
Increases proportion of homozygous individuals, which means harmful recessive alleles are more likely to be expressed
Artifical Selection in domesticated animals

8. Genetic Drift
Random change in genetic variation based on CHANCE!

9. Genetic Drift : Small Populations
Chance fluctuations can change allele frequencies in small populations
Small populations will experience greater fluctuations, less likely that the parent gene pool will be reflected in the next generation

10. Genetic Drift : Founder effect
Imagine a newly mutated gene which causes an organism to grow 10 cm taller than average (a gene that causes the growth spurt at puberty to last for a bit longer).
Just by chance, this individual happened to be washed out to sea on a raft with his partner and be washed up on a desert island and found a new population.
All members of the new population, being his descendants, have a good chance of being taller than any representative of the old population.
This is not natural selection, ie. being taller wasn’t selected for because of its survival value.
This is evolution by chance!

11. Genetic Drift : Founder effect
When a few individuals start a new, isolated population, resulting in a change in the gene pool
Founder effect occurs frequently on islands, since they are isolated habitats.

12. Genetic Drift : Bottleneck effect
Change in gene pool caused by a rapid decrease in population size (e.x. natural disaster or human intervention)
The survivors will carry a fraction of the alleles that were present in the initial population
The gene pool has lost genetic variation

13. Bottleneck Effect: Elephant Seals
Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s.
Hunting for their oil reduced their population size to as few as 20 individuals at the end of the 19th century.
Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
Why do you think that Southern Elephant Seals are significantly larger than Northern Elephant Seals?

14. Bottleneck Effect in Human populations
The Afrikaner population are descendants of a small group of Dutch settlers in South Africa in the 17th century.
They exhibit high frequencies of Huntington’s disease
Scientists explain this through the Bottleneck Effect and inbreeding
The original Dutch Settlers happened to carry the gene for Huntington’s Disease, which was inherited in future generations as the individuals continued to inbreed.

15. Natural Selection
Environment responsible for selecting individuals better suited to survive
Over many generations this can lead to significant changes in populations, even formation of new species

17. Homework
Read pages
Pg. 352 #1-6
Pg. 356 # 7-12

18. Patterns of Selection
Natural Selection causes changes in the allele frequencies of a population, which can lead to evolutionary change.
We are going to analyze how natural selection can affect allele frequencies in populations

19. Patterns of Selection

20. Stabilizing Selection
Most common phenotypes (the intermediate ones) are most favoured by the environment
Extreme phenotypes are acted against
Environment is stable

21. Directional Selection
Gradually changing environment favours individuals at one extreme phenotype over the other

22. Directional Selection
Peccaries eat cacti. If they discover an untouched population of cacti, they quickly exploit the new food source.
They naturally consume those cactus plants with the fewest spines. Even with their tough mouths, they prefer to eat the cacti with 70 spines first, before going on to tackle the plants with 80 or more spines.
Thus the cacti with more spines are better adapted, more fit, and get eaten less often.
And at flowering time there are more cacti with higher spine numbers;
Thus, there are more of their alleles going into pollen, eggs, and seeds for the next generation.
Over time, in the population of cacti will gradually shift in the direction of the more heavily spined cactus varieties. The later (newer) distribution curves will show a virtual absence of low-spine-numbered plants and a much greater proportion of high-spine-numbered plants.

23. Disruptive Selection Polymorphism exists Fluctuating environment
Extremes are favoured over the intermediate phenotypes

25. Sexual Selection
Favours selection of any trait that influences mating success of individuals
female mate choice based on physical traits
male-versus-male competition
e.g. sexual dimorphism
e.g. behavioural differences between the sexes (courtship displays)

26. Sexual Dimorphism
Difference in physical characteristics of the males and females of the same species

27. Female Mate Choice

28. Male Competition

29. Homework
Read pgs
Activity 9.1
Pg. 350 # 6-11