1. Mechanisms of Evolution

2. Is this population evolving?
What do you think?
Is this population evolving?

3. This population is NOT evolving
Genetic Equilibrium - Population in which allelic frequency does NOT change over generations, i.e. population is NOT evolving

4. Connecting Darwin & Mendel
Darwin’s Handicap ~ He didn’t know how heredity worked
Mendel’s work (completed during Darwin’s lifetime) was not understood for decades
Two gaps in Darwin’s thinking:
Didn’t know how traits were inherited
Variation in heritable traits was central to his theory, but he had no idea how that variation appeared

5. Two Main Sources of Genetic Variation
1) Mutations 2) Gene Shuffling

6. Mutations can be beneficial, harmful, or harmless
Any change in a sequence of DNA.
Caused by mistakes during replication or environmental factors such as radiation or chemicals.
Source of new phenotypes
Mutations can be beneficial, harmful, or harmless

7. 2) Gene Shuffling
~ Mixing of genes during the production of gametes in meiosis
Recombination as homologous chromosomes move independently during meiosis
Crossing over: the exchange of genetic material during prophase I
EX: Shuffling a deck of cards; produces many different combinations, but does not change the frequency of each type of allele in the population

8. Gene Pool & Allelic Frequency
Gene Pool: Entire collection of all the alleles in a population
Allelic Frequency: The number of times an allele appears in the gene pool

9. Gene Pool & Allelic Frequency
Gene Pool: 12 green alleles and 8 brown alleles
Allelic Frequency of green: 12/20
Allelic Frequency of brown: 8/20

10. Evolution is the change in allele frequency
Occurs in one of three ways:
Gene Flow
Genetic Drift
Natural Selection
Remember…evolution occurs
Over time
In populations…NOT individuals
On phenotypes

11. 1) Gene Flow ~ The movement of alleles into or out of a population
Population’s genetic variation increases when alleles are added to the population (migration)
Decreases when alleles leave (emigration)

12. 2. Genetic Drift
Random change in allelic frequencies due to chance.

13. Types of Genetic Drift
Founder Effect ~ A small group of individuals colonize a new habitat
Allele frequencies change due to the random change in the population

14. Genetic drift ~ Founder Effect
Sample of
Original Population
Founding Population A
Founding Population B
Descendants

15. Types of Genetic Drift
Bottleneck ~ a sharp reduction in the size of a population due to environmental events (flooding, earthquake, etc.)
Allele frequencies change due to the random chance of surviving the bottleneck event

17. 3) Natural Selection ~ Most common source of evolutionary change
Can lead to an increase or decrease in allele frequency in single gene traits
Effects are more complex in polygenic traits
3 types

18. Single gene traits vs. polygenic traits

19. 1. Directional Selection
One of the extreme phenotypes is selected against
Ex: Rainfall and Bird Beaks Gizmo

20. 2. Stabilizing Selection
BOTH of the extreme phenotypes are selected against
Ex: Baby’s birth weight, gall fly size

21. Stabilizing Selection

22. 3. Disruptive Selection The middle phenotype is selected against
Ex. Large and Small Bird Beaks

23. Disruptive Selection

24. Evolution leads to new species
When the gene pool of two populations become separated, they become two separate species.
Natural selection is the key mechanism of evolution that leads to speciation

25. Evolution vs. Genetic Equilibrium
Hardy-Weinberg Principle ~ Five conditions are required to maintain genetic equilibrium
Random Mating: All members of the population must have an equal opportunity to produce offspring
Large Population: To prevent genetic drift
No Migration
No Mutations
No Natural Selection

26. Hardy-Weinberg Principle
p= dominant allele q= recessive allele

27. Hardy-Weinberg Principle
p2 + 2pq + q2=1 ; p+ q = 1
p2 = frequency of homozygous dominant individuals
p = frequency of the dominant allele
q2 = frequency of homozygous recessive individuals
q = frequency of the recessive allele
2pq = frequency of heterozygous individuals 

28. PROBLEM #1
You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Calculate the following:
The frequency of the "aa" genotype.
The frequency of the "a" allele.
The frequency of the "A" allele.
The frequencies of the genotypes "AA" and "Aa."

29. PROBLEM #1 (aa) is 36%. The frequency of the "aa" genotype.
The frequency of the "a" allele.
The frequency of the "A" allele.
The frequencies of the genotypes "AA" and "Aa."

31. PROBLEM #2
A very large population of randomly-mating laboratory mice contains 64% white mice. White coloring is caused by the double recessive genotype, "aa". Calculate allelic and genotypic frequencies for this population.

32. Problem #2 “aa” = 64%

33. PROBLEM #3
PROBLEM #4.Within a population of butterflies, the color brown (B) is dominant over the color white (b). 40% of all butterflies are white. Calculate the following:
The percentage of butterflies in the population that are heterozygous.
The frequency of homozygous dominant individuals.

35. PROBLEM #3 “bb” = 40%
The percentage of butterflies in the population that are heterozygous.
The frequency of homozygous dominant individuals.