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)