1. Population Genetics:
Populations change in genetic characteristics over time
Ways to measure change:
Allele frequency change (B and b)
Genotype frequency change (BB, Bb, & bb)

2. Hardy-Weinberg Equation
p + q = 1
p = the frequency of dominant allele (B)
q = the frequency of recessive allele (b)
Total frequency = 1
Assumes a 2 allele simple traits
© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

3. How to Calculate Frequencies
Imagine there are 1,000 plants in a population
Only two flower colors are possible:
red (R = dominant) and white (r = recessive).
Let there be
160 RR individuals (red homozygotes)
480 Rr individuals (red heterozygotes)
360 rr individuals (white homozygotes)

4. How to Calculate Frequencies
Total population = 1,000
The genotype frequency is:
RR = 160/1000 = .16 or 16%
Rr = 480/1000 = .48 or 48%
rr = 360/1000 = .36 or 36%

5. How to Calculate Frequencies
Each plant has two alleles for flower color
Since there are 1,000 individuals, there are 2,000 alleles
160 RR Rr rr = 1000 plants
2x160 RR + 2x480 Rr + 2x360 rr = 2000 alleles
Allele frequency:
R = (2 x ) = 800/2000 = .40 (40%)
r = ( x 360) = 1200/2000 = .60 (60%)

6. How to Calculate Frequencies
In 1908, Hardy & Weinberg independently discovered another way to calculate gene frequencies in an equilibrium population
Equilibrium populations do not change, and assume:
No mutation
No gene flow (no movement of alleles in or out)
No selection
Mating is random
The population is large

7. Given a Hardy-Weinberg equilibrium:
Allele frequencies and genotype frequencies do not change in a population
p + q = 1
p2 + 2pq + q2 = 1
160 RR Rr rr = 1000 plants
p2= freq of genotype RR (p is freq of allele R)
2pq = freq of Rr
q2 = freq of rr =.36
q = √¯ q2 = .6 (p = 1 – q)

8. Genetic Variation: The Raw Material of Change
Genetic variation - differences in DNA sequences in a population
Population changes occur because of these differences
Two major sources of variation:
Mutations
Recombination

9. Four Mechanisms of Change
Changes in allele frequencies in a population occur in four ways:
Mutations
Gene flow
Genetic drift
Natural selection
Animation on mechanisms of change

10. Mutations Mutations are random events
Ultimately, all new genetic variation comes from mutations
Mutations can be harmful, beneficial, or neutral

11. Gene Flow
Gene flow -movement of alleles between two different populations
May result in new alleles being introduced or rare alleles being lost by migration

12. Genetic Drift The result of chance events
Leads to random changes in allele frequencies
The smaller the population, the greater the genetic drift
By chance, the white allele at right decreased

13. Genetic Drift May reduce genetic variation
May fix alleles that are neutral, harmful, or beneficial in a population
Occurs in a genetic bottleneck

14. Genetic Bottlenecks Occur when there is a drop in population size
Environmental disaster
Founder effect
New population is less genetically diverse than original population

15. Natural Selection
When individuals with favorable traits have a higher reproductive rate
Artificial selection is similar, but people pick the favorable trait not nature.
Three types of Natural Selection:
Directional selection
Stabilizing selection
Disruptive selection

16. Directional Selection
Individuals of one extreme phenotype are favored
Peppered moths

17. Stabilizing Selection
Individuals with an intermediate phenotype are favored
The extreme phenotypes are selected against
Human birth weights

18. Disruptive Selection Where both extreme phenotypes are favored
The intermediate phenotypes are selected against
African seed cracker birds

19. Three Types of Natural Selection

20. Sexual Selection A form of natural selection
Females place selective pressure on males
Peacock’s feathers
Lion’s mane

21. How Populations Change
Mutations and recombination occur randomly
These events create genetic variation
Gene flow, genetic drift, and natural selection change allele frequencies
Gene flow tends to equalize allele frequencies between 2 pops
Genetic drift an natural selection tend to diverge allele frequencies between 2 pops

22. Animations on the causes of differences in the alpine skypilot population
Differences in the population
Variation in the population
The variation is inherited
There is variable survival
There is variation in offspring production
Conclusion
Answer B is correct.
If a squirrel has more than 3 pups in a littler, she has too many mouths to feed and their survival is in jeopardy.
Having less than three pups in a litter may be an advantage to the single pup, but the mother will not be passing on as many of her genes compared to other mothers in the population.

23. Concept Quiz The average litter size for a squirrel is three.
Above and below three is not favored. What
type of selection is occurring?
Disruptive selection
Stabilizing selection
Directional selection
Answer B is correct.
If a squirrel has more than 3 pups in a littler, she has too many mouths to feed and their survival is in jeopardy.
Having less than three pups in a litter may be an advantage to the single pup, but the mother will not be passing on as many of her genes compared to other mothers in the population.

24. Concept Quiz A population of slugs has 230 individuals that are
AA, 570 that are Aa, and 200 that are aa. What is
the frequency of the A allele?
.50 or 50%
.65 or 65%
.52 or 52%
Answer C is correct.
There are 1,000 total individuals in the population.
Each individual has 2 alleles; there are 2,000 alleles in the population.
2 x 230 (AA) (Aa) + 0 (aa) = 1,030/2,000 = .52

25. Concept Quiz A few birds manage to start their own
population on a new island. What best
describes this situation?
Genetic drift
Gene flow
Natural selection
Answer A is correct.
When a few individuals start their own population, this is known as the founder effect.
The founder effect is a type of genetic drift.
The birds that made it to the island are not necessarily better flyers; this is a random event. Therefore, natural selection is incorrect.
Gene flow occurs when there is exchange of genes between two populations.

26. Process Animation 17.1 The Hardy-Weinberg Equation
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