Transcript Document

Population:
10 individuals
Phenotype frequency:
30% blue eyes (or 0.3)
70% brown eyes (or 0.7)
= bb
= Bb
= BB
Population
10 individuals
Phenotype frequency:
30% blue eyes (or 0.3)
70% brown eyes (or 0.7)
Genotype frequency:
f(bb) = 30% (or 0.3)
f(Bb) = 50% (or 0.5)
F(BB) = 20% (or 0.2)
= bb
b b
b b
b b
B b
B b
B b
B b
B b
BB
BB
= Bb
= BB
Population
10 individuals
Genotype frequency:
f(bb) = 30% (or 0.3)
f(Bb) = 50% (or 0.5)
f(BB) = 20% (or 0.2)
Number of alleles:
b = 11
B=9
Population
10 individuals
b b
b
b b
b
b
b b
b
b
b
Genotype frequency:
f(bb) = 30% (or 0.3)
f(Bb) = 50% (or 0.5)
f(BB) = 20% (or 0.2)
B b bB b bB b b
B
B
B
Number of alleles:
b = 11
B=9
B B BB B
B
B
Allele frequency:
f(b) = 11/20 = 0.55
f(B) = 9/20 = 0.45
B b bB b b
B
B
B
b
b
b
b
=
B
B
b
b
b
b
B
B
b
B
b
B
B
B
b
B
Gene pool of a population
= bb
b b
b b
b b
B b
B b
B b
B b
B b
BB
BB
= Bb
= BB
Possible gametes of each individual
b
b
b
b
=
B
B
b
b
b
b
B
B
b
B
b
B
B
B
b
B
Possible gametes in the population
Population
10 individuals
b b
b
b b
b
b
b b
b
b
b
Genotype frequency:
f(bb) = 30% (or 0.3)
f(Bb) = 50% (or 0.5)
f(BB) = 20% (or 0.2)
B b bB b bB b b
B
B
B
Number of alleles:
b = 11
B=9
B B BB B
B
B
Allele frequency:
f(b) = 11/20 = 0.55
f(B) = 9/20 = 0.45
B b bB b b
B
B
B
Genetic Drift
Homozygous for allele A1
Homozygous for allele A2
Heterozygous
(a) Founder effect
Immigrants
establish new
population
New population
is likely to have
different allele
frequencies
than the source
population,
by chance
(b) Genetic bottleneck
High mortality
strikes
individuals
at random
Time 1
Time 2
Bottlenecked
population is likely
to have different
allele frequencies
than original
population,
by chance
Gene Flow
Homozygous for allele A1
Homozygous for allele A2
Time 1
At time 1,
populations
differ in allele
frequencies
Population 2
Population 1
Time 2
Gene
flow
Gene flow
causes allele
frequencies
in the two
populations
to be more alike
Gene
flow
Population 1
Population 2
(a) Directional selection changes the average value
of a trait.
Normal distribution
During selection
After selection
Number of individuals
Before selection
Low
fitness
Change in
average
value
Value of a trait
High
fitness
Figure 25-3
Slide 6
For example, directional selection caused average
body size to increase in a cliff swallow population.
Percentage of birds
Original population
(N = 2880)
Change in
average
value
Body size class
Survivors
(N = 1027)
(a) Stabilizing selection reduces the amount of
variation in a trait.
Normal distribution
During selection
After selection
Number of individuals
Before selection
Low High fitness Low
fitness
fitness
Reduction
in variation
Value of a trait
Mortality
Heavy
mortality
on extremes
Birth weight (pounds)
Percentage of mortality
Percentage of newborn population
(b) For example, very small and very large babies are the
most likely to die, leaving a narrower distribution of birth
weights.
(a) Disruptive selection increases the amount of variation
in a trait.
Normal distribution
During selection
Number of individuals
Before selection
High Low fitness
fitness
High
fitness
Increase in
variation
After selection
Value of a trait
Number of individuals
(b) For example, only juvenile black-bellied seedcrackers that
had very long or very short beaks survived long enough to
breed.
Only the
extremes
survived
Only the
extremes
survived
Beak length (mm)