1. 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

2. 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

3. 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

4. Persistent Selection Changes Allele Frequencies
(i.e. Evolution by Natural Selection)
Strength of selection is given by the
magnitude of the selection differential

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

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

7. 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 (Africa)
High selection/transmisson (Africa)

8. What is the frequency of A1 in the next generation?
p2w11 + pqw12
pt + 1 =
p2w11 + 2pqw12 + q2w22
What is the change in frequency of A2 per generation?
After some algebra
Dp = pt pt =
p / w (pw11 + qw12 - w )
With this equation we can substitute values for relative
fitness and analyze various cases of selection.