1. What evolutionary forces alter
Population Genetics
Evolution depends upon mutation
to create new alleles.
Evolution occurs as a result of allele frequency
changes within/among populations.
What evolutionary forces alter
allele frequencies?

2. How do allele frequencies change
in a population from generation
to generation?

3. Allele frequencies
in the gene pool:
A: 12 / 20 = 0.6
a: 8 / 20 = 0.4
Alleles Combine to Yield Genotypic Frequencies

4. Our mice grow-up and generate gametes for next generations gene pool

5. Allele frequency across generations:
A General Single Locus, 2 Allele Model
Freq A1 = p
Freq A2 = q
Genotypic frequencies are given
by probability theory

6. One locus, 2 Allele Model Genotype A1A1 A1A2 A2A2 Given:
In a diploid organism, there are two alleles for each locus.
Therefore there are three possible genotypes:
Genotype A1A1 A1A2 A2A2
Given:
Frequency of allele A1 = p
Frequency of allele A2 = 1 - p = q
Then:
Genotype A1A1 A1A2 A2A2
Frequency p2 2pq q2
A population that maintains such frequencies
is said to be at Hardy-Weinberg Equilibrium

7. Hardy-Weinberg Principle
When none of the evolutionary forces (selection, mutation, drift,
migration, non-random mating) are operative:
Allele frequencies in a population will not change, generation after generation.
If allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2

8. Hardy-Weinberg Principle Depends Upon the Following Assumptions
There is no selection
There is no mutation
There is no migration
There are no chance events
5. Individuals choose their mates at random

9. The Outcome of Natural Selection Depends Upon:
Relationship between phenotype and fitness.
(2) Relationship between phenotype and genotype.
These determine the relationship between
fitness and genotype.
Outcome determines if there is evolution

10. % survival to reproduction: A = 0.05 B = 0.10
12.2 Growth of 2 genotypes in an asexually reproducing population w/ nonoverlapping generations
% survival to reproduction:
A = 0.05
B = 0.10
Fecundity (eggs produced):
A = 60
B = 40
C:\Figures\Chapter12\high-res\Evolution-Fig jpg
Fitness A = 0.05 x 60 = 3
Fitness B = 0.01 x 40 = 4

11. R = Per Capita Growth Rate = Represents Absolute Fitness
The rate of genetic change in a populations depends upon
relative fitness:
Relative Fitness of A = Absolute fitness A
Highest Absolute Fitness
WA = /4 = 0.75
Often by convention, fitness is expressed relative to the genotype with highest absolute fitness.
Thus,
WB = 4/4 = 1.0

12. The fitness of a genotype is the average lifetime
contribution of individuals of that genotype to the
population after one or more generations, measured
at the same stage in the life history.

13. 12.3 Components of natural selection that may affect the fitness of a sexually reproducing organism
C:\Figures\Chapter12\high-res\Evolution-Fig jpg

14. 12.1(2) Modes of selection on a polymorphism consisting of two alleles at one locus
C:\Figures\Chapter12\high-res\Evolution-Fig jpg

15. 12.1(1) Modes of selection on a heritable quantitative character
C:\Figures\Chapter12\high-res\Evolution-Fig jpg

16. Individuals may differ in fitness because of their underlying genotype
Incorporating Selection
Individuals may differ in fitness because
of their underlying genotype
Genotype A1A1 A1A2 A2A2
Frequency p2 2pq q2
Fitness w11 w12 w22
Average fitness of the whole population:
w =
p2w11 + 2pqw12 + q2w22

17. Given variable fitness, frequencies after selection:
Genotype A1A1 A1A2 A2A2
Freq p2 w11 2pq w12 q2 w22 w w w
New allele frequencies after mating:
p2 w11
pq w12
pq w12
q2w22 + + w w
New Frequency of A1
New Frequency of A2

18. Fitness: Probability that one’s genes will be represented
in future generations.
Hard to measure. Often, fitness is indirectly measured:
(e.g. survival probability given a particular genotype)
WAA WAa Waa s
Fitness is often stated
in relative terms
Selection coefficient
gives the selection differential

19. Persistent Selection Changes Allele Frequencies
Strength of selection is given by the
magnitude of the selection differential

20. Selection Experiments Show Changes in Allele FrequenciesHW
Cavener and
Clegg (1981)
Food spiked
with ethanol

21. Selection can drive genotype frequencies
away from Hardy Weinberg Expectations

22. Predicted change in allele frequencies at CCR5
High frequency (Europe)
High selection/transmisson (Africa)
Predicted
change
in allele
frequencies
at CCR5
High frequency (Europe)
Low selection/transmisson (Europe)
Low frequency (Europe)
High selection/transmisson (Africa)

23. What is the frequency of A1 in the next generation?
p2w11 + pqw12
pt + 1 =
p2w11 + 2pqw12 + q2w22
What is the change in frequency of A1 per generation?
Dp = pt pt =
p / w (pw11 + qw12 - w )
With this equation we can substitute values for relative
fitness and analyze various cases of selection.

24. Gene Action Fitness Relationship A1A1 A1A2 A2A2 1+s 1 + s 1 Dominance
Recessivity
A1A1 A1A2 A2A2
1 + s t
Overdominance
A1A1 A1A2 A2A2
1 + s t
Underdominance

25. Dominance
Genotype A1A1 A1A2 A2A2
Fitness 1 + s s
S = 0.01 A1

26. Recessive
Genotype A1A1 A1A2 A2A2
Fitness 1 + s
S = 0.01 A1

27. Evolution in lab
populations of
flour beetles
support theoretical
predictions.
Dawson (1970)

28. Overdominance/Heterozygote Superiority
Genotype A1A1 A1A2 A2A2
Fitness 1 + s t
S = t =
Stable equilibrium
is reached A1
Genetic diversity
is maintained

29. Viable allele did not fix in the population
Mukai and Burdick 1958

30. Underdominance
Genotype A1A1 A1A2 A2A2
Fitness 1 + s t
S = t = 0.02
Unstable equilibrium A1
A1 maybe fixed or
lost from the
population

31. Frequency-Dependent Selection
Allele frequencies in a population remain near an equilibrium
because selection favors the rarer allele.
As a result, both alleles are maintained in the population.

32. Frequency-Dependent Selection
Perissodus

33. Incorporating Mutation
Mutation alone is a weak
evolutionary force

34. However, mutation and selection acting in concert
are a powerful evolutionary force