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Population:10 individuals
Phenotype frequency:30% blue eyes (or 0.3)70% brown eyes (or 0.7)
Population10 individuals
Phenotype frequency:30% blue eyes (or 0.3)70% brown eyes (or 0.7)
= bb
= BB
= Bb
Genotype frequency:f(bb) = 30% (or 0.3)f(Bb) = 50% (or 0.5)F(BB) = 20% (or 0.2)
Population10 individuals
= bb
= BB
= Bb
Genotype frequency:f(bb) = 30% (or 0.3)f(Bb) = 50% (or 0.5)f(BB) = 20% (or 0.2)
b b b bb b
B b
B b
B b
B b
B b
B B B B
Number of alleles:b = 11B = 9
Population10 individuals
Genotype frequency:f(bb) = 30% (or 0.3)f(Bb) = 50% (or 0.5)f(BB) = 20% (or 0.2)
b b b bb b
B b
B b
B b
B b
B b
B B B BNumber of alleles:b = 11B = 9
bb
bb
b
b
Bb
Bb
Bb
Bb
Bb
BB
BB
Allele frequency:f(b) = 11/20 = 0.55f(B) = 9/20 = 0.45
bb
bb
b
b
Bb
Bb
Bb
Bb
Bb
BB
BB
=
Gene pool of a population
= bb
= BB
= Bb
b b b bb b
B b
B b
B b
B b
B b
B B B B
Possible gametes of each individual
bb
bb
b
b
Bb
Bb
Bb
Bb
Bb
BB
BB
=
Possible gametes in the population
Population10 individuals
Genotype frequency:f(bb) = 30% (or 0.3)f(Bb) = 50% (or 0.5)f(BB) = 20% (or 0.2)
b b b bb b
B b
B b
B b
B b
B b
B B B BNumber of alleles:b = 11B = 9
bb
bb
b
b
Bb
Bb
Bb
Bb
Bb
BB
BB
Allele frequency:f(b) = 11/20 = 0.55f(B) = 9/20 = 0.45
Genetic Drift
(a) Founder effect
(b) Genetic bottleneck
High mortalitystrikes
individualsat random
Time 1 Time 2
Bottleneckedpopulation is likelyto have differentallele frequenciesthan originalpopulation,by chance
New populationis likely to havedifferent allelefrequenciesthan the sourcepopulation,by chance
Immigrantsestablish new
population
Homozygous for allele A1
Homozygous for allele A2
Heterozygous
Time 1
Population 1
At time 1,populationsdiffer in allelefrequencies
Gene flowcauses allelefrequenciesin the twopopulationsto be more alike
Time 2
Population 2
Population 1 Population 2
Homozygous for allele A1Homozygous for allele A2
Geneflow
Geneflow
Gene Flow
(a) Directional selection changes the average valueof a trait.
Before selection
During selection
Normal distribution
Nu
mb
er
of
ind
ivid
ua
ls
Lowfitness
Highfitness
Change inaveragevalueAfter selection
Value of a trait
Figure 25-3 Slide 6
Original population(N = 2880)
Body size class
For example, directional selection caused averagebody size to increase in a cliff swallow population.
Per
cen
tag
e o
f b
ird
s
Change inaveragevalue
Survivors(N = 1027)
(a) Stabilizing selection reduces the amount ofvariation in a trait.
Normal distribution
Before selection
During selection
After selection
Nu
mb
er
of
ind
ivid
ua
ls
Value of a trait
Lowfitness
Lowfitness
Reductionin variation
High fitness
(b) For example, very small and very large babies are themost likely to die, leaving a narrower distribution of birthweights.
Pe
rce
nta
ge
of
ne
wb
orn
po
pu
lati
on
Mortality
Birth weight (pounds)
Pe
rce
nta
ge
of
mo
rta
lity
Heavymortalityon extremes
(a) Disruptive selection increases the amount of variationin a trait.
Normal distribution
Before selection
During selection
After selection
Nu
mb
er
of
ind
ivid
ua
ls
Value of a trait
HighHighfitnessfitness
Highfitness
Increase invariation
Low fitness
(b) For example, only juvenile black-bellied seedcrackers thathad very long or very short beaks survived long enough tobreed.
Only theextremessurvived
Only theextremessurvived
Nu
mb
er
of
ind
ivid
ua
ls
Beak length (mm)