Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 16
How Populations
Evolve
Lecture Outline
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Outline
• 16.1 Genes, Populations, and Evolution
• 16.2 Natural Selection
• 16.3 Maintenance of Diversity
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16.1 Genes, Populations, and
Evolution
• A population is all the members of a single
species occupying a particular area at the same
time.
• Diversity exists among members of a population.
• Population genetics is the study of this
diversity in terms of allele differences.
 Evaluates the diversity of a population by studying
genotype and phenotype frequencies over time
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Genes, Populations, and
Evolution
• In the 1930s, population geneticists began to
describe variations in a population in terms of
alleles
• Microevolution pertains to evolutionary
changes within a population.
 Various alleles at all the gene loci in all individuals
make up the gene pool of the population.
 The gene pool of a population can be described in
terms of:
• Genotype frequencies
• Allele frequencies
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The Genetic Basis of Body
Color in the Peppered Moth
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Phenotype:
Genotype:
DD
homozygous
Dd
heterozygous
dd
homozygous
Alleles:
D
D
d
D
d
d
chromosome
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Genes, Populations, and
Evolution
• Allele Frequencies
 The proportion of each allele within a population’s gene pool.
 Frequencies of the dominant and recessive allele must add up to
1.
• This relationship is described by the expression p + q = 1
• p is the frequency of one allele and q is the frequency of the other
 Microevolution involves a change in these allele frequencies
within populations over time.
• If the gene frequencies do not change over time, microevolution has
not occurred.
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How Hardy-Weinberg Equilibrium
Is Estimated
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Gene pool:
Allele frequencies:
Population = 25 moths, 50 alleles
Equilibrium
genotype frequencies:
d = 40
D = 10
DD
DD
Dd
Dd
Dd
D D D D D d d d
D
Dd
Dd
Dd
Dd
Dd
D D D D d d d d
dd
d
P2 + 2pq + q2 = 1
p+q=1
Dd
dd
Dd
dd
dd
dd
d d d d d d d d
p = frequency of D
p = frequency of D
q = frequency of d
q = frequency of d
Frequency of D
Frequency of DD
p = 10/50 alleles
= 0.20
p2 = freq D2
= (0.2)(0.2)
dd
dd
dd
dd
d d d d d d d d
Frequency of d
q = 40/50 alleles
Frequency of Dd
= 0.80
2pq = 2(freq D x freq d)
= 2(0.20 x 0.80)
dd
dd
dd
dd
d d d d d d d d
= freq d 2
= (0.08)(0.80)
dd
dd
dd
d d d d d d d d
= 0.32
Frequency of dd
q2
dd
= 0.04
1.00
= 0.64
1.00
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Genes, Populations, and
Evolution
• The Hardy-Weinberg Equalibrium states
that:
 Allele frequencies in a population will remain
constant assuming:
• No Mutations
• No Gene Flow
• Random Mating
• No Genetic Drift
• No Selection
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Hardy-Weinberg Equilibrium
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eggs
0.20 D
0.80 d
sperm
0.20
D
0.04 DD
0.16 Dd
0.16 Dd
Offspring
0.64 dd
0.80
d
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Genes, Populations, and
Evolution
• Hardy-Weinberg Equilibrium:
 Required conditions are rarely (if ever) met
 Deviations from a Hardy-Weinberg equilibrium
indicate that evolution has taken place
• Analysis of allele changes in populations over time
determines the extent to which evolution has occurred.
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Mechanisms of Microevolution
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F1 generation
Allele
frequencies:
F2 generation
Genotype
frequencies:
Allele
frequencies:
Genotype
frequencies:
DD
D
d
0.20 D
0.80 d
D
Dd
dd
Conclusion:
DD
If...
d
Random mating
No selection
No migration
No mutation
p2 + 2pq + q2 = 1
0.20 D
...then we expect
DD = 0.04
Dd
dd
p2 + 2pq + q2 = 1
No change in allele frequencies
No change in genotype frequencies
Evolution has not occurred
DD = 0.04
0.80 d
Dd = 0.32
Dd = 0.32
dd = 0.64
dd = 0.64
DD
If...
D
Nonrandom mating
Dd
...then we observe
dd
d
X
DD
X
Dd
X
p2 + 2pq + q2 = 1
DD = 0.10
0.80 d
or
Dd
0.20 D
No change in allele frequencies
Genotype frequencies change
Evolution has not occurred
Dd = 0.20
DD
dd = 0.70
dd
If...
Selection
...then we observe
d
Dd
D
0.80 D
0.20 d
DD
p2 + 2pq + q2 = 1
Allele frequencies change
Genotype frequencies change
Evolution has occurred
DD = 0.64
Dd = 0.32
dd = 0.04
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Genes, Populations, and
Evolution
• Causes of Microevolution
 Genetic mutations
• The raw material for evolutionary change
• Provide new alleles
• Some mutations might be more adaptive than
others
– Ex: Genetic mutations affecting pigment color in
peppered moths have provided the variation needed for
natural selection to occur
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Genes, Populations, and
Evolution
• Causes of Microevolution
 Gene Flow (gene migration)
• Movement of alleles between populations when:
– Gametes or seeds (in plants) are carried into another
population
– Breeding individuals migrate into or out of population
• Continual gene flow reduces genetic divergence
between populations
15
Gene Flow
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gene flow
Pisum arvense
Pisum sativum
Genes, Populations, and
Evolution
• Causes of Microevolution
 Genetic Drift
• Changes in the allele frequencies of a population due to
change rather than selection by the environment
• Does not necessarily lead to adaptation to the environment
• Occurs by disproportionate random sampling from population
– Can cause the gene pools of two isolated populations to
become dissimilar
– Some alleles are lost and others become fixed (unopposed)
• Likely to occur:
– After a bottleneck
– When severe inbreeding occurs, or
– When founders start a new population
• Stronger effect in small populations
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Genetic Drift
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10% of
population
natural disaster kills
five green frogs
20% of
population
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19
Genes, Populations, and
Evolution
• Bottleneck Effect
 A random event prevents a majority of
individuals from entering the next generation
 The next generation is composed of alleles
that just happened to make it
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Genes, Populations, and
Evolution
• Founder Effect
 When a new population is started from just a
few individuals
 The alleles carried by population founders are
dictated by chance
 Formerly rare alleles will either:
• Occur at a higher frequency in the new population,
or
• Be absent in new population
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Bottleneck and Founder Effects
Change Allele Frequencies
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a.
b.
c.
Original population
gene pool = 3,800 alleles*
d.
Remnant population
gene pool = 90 alleles*
11%
13%
8%
26%
44%
45%
53%
*1 marble = 10 alleles
Genes, Populations, and
Evolution
• Nonrandom Mating
 When individuals do not choose mates
randomly
• Assortative mating:
– Individuals select mates with the same phenotype with
respect to a certain characteristic
– Individuals reject mates with differing phenotype
– Increases the frequency of homozygotes for certain loci
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16.2 Natural Selection
• Adaptation of a population to the biotic and
abiotic environment
 Requires:
• Variation - The members of a population differ from
one another
• Inheritance - Many differences are heritable
genetic differences
• Differential Adaptiveness - Some differences affect
survivability
• Differential Reproduction – Some differences affect
the likelihood of successful reproduction
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Natural Selection
• Results in:
 A change in allele frequencies of the gene pool
 Improved fitness of the population
• Major cause of microevolution
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Natural Selection
• Most traits are polygenic - variations in the trait
result in a bell-shaped curve
• Three types of selection occur:
 (1) Directional Selection
• The curve shifts in one direction
– Bacteria become resistant to antibiotics
– Increasing body size in horse evolution
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Natural Selection
• Three types of selection occur (cont):
 (2) Stabilizing Selection
• The peak of the curve increases and tails decrease
• Example - human babies with low or high birth
weight are less likely to survive
 (3) Disruptive Selection
• The curve has two peaks
• Example – British land snails vary because a wide
geographic range causes selection to vary
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Three Types of Natural Selection
Number of Individuals
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Phenotype Range
Number of Individuals
stabilizing selection
Peak narrows.
a.
Phenotype Range
Phenotype Range
directional selection
disruptive selection
Peak shifts.
b.
Two peaks result.
c.
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Human Birth Weight
(stabilizing selection)
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100
70
50
15
30
20
10
10
7
5
5
Percent Infant Mortality
Percent of Births in Population
20
3
2
.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5
Birth Weight (in kilograms)
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Directional Selection
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After More Time
Number of
Individuals
After Time
Number of
Individuals
Number of
Individuals
Initial Distribution
Body Size
Body Size
Body Size
a.
Hyracotherium
Merychippus
Equus
b.
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Disruptive Selection
Initial
Distribution
Number of
Individuals
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After
Time
Number of
Individuals
Banding Pattern
After
More Time
Number of
Individuals
Banding Pattern
Banding Pattern
a.
b.
b: © Bob Evans/Peter Arnold/Photolibrary
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Natural Selection
• Sexual selection - adaptive changes in
males and females lead to an increased
ability to secure a mate.
 Males - increased ability to compete with other
males for a mate
 Females choose to select a male with the best
fitness (ability to produce surviving offspring).
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Natural Selection
• Female Choice
 Choice of a mate is a serious consideration because
females produce few eggs
• Good genes hypothesis: Females choose mates on the basis
of traits that improve the chance of survival.
• Runaway hypothesis: Females choose mates on the basis of
traits that improve male appearance.
• Male Competition
 Males can father many offspring because they
continuously produce sperm in great quantity.
 Compete to inseminate as many females as possible.
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Dimorphism
• The drab
females tend to
choose
flamboyant
males as
mates.
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Sexual Selection:
Competition
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© Barbara Gerlach/Visuals Unlimited
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Sexual Selection:
Competition
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a.
b.
a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images
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Natural Selection
• A study of sexual selection among humans
shows that female choice and male competition
apply to humans too
 Women must invest more in having a child than men.
 Men need only contribute sperm
• Generally more available for mating than are women.
 More available men results in competition
 Men also have a choice
• Prefer women who are most likely to present them with
children.
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16.3 Maintenance of Diversity
• Genetic Variability
 Populations with limited variation may not be able to
adapt to new conditions
 Maintenance of variability is advantageous to the
population
• Only exposed alleles are subject to natural
selection
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Maintenance of Diversity
• Natural selection causes imperfect adaptations
 Depends on evolutionary history
 Imperfections are common because of necessary
compromises
• The environment plays a role in maintaining
diversity
 Disruptive selection due to environmental differences
promotes polymorphisms in a population
 If a population occupies a wide range, it may have
several subpopulations designated as subspecies
 The environment includes selecting agents that help
maintain diversity
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Subspecies Help Maintain Diversity
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Pantheropsis obsoleta obsoleta
Pantheropsis obsoleta quadrivittata
Pantheropsis obsoleta lindheimeri
Pantheropsis obsoleta rossalleni
Pantheropsis obsoleta spiloides
(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.;
(E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited
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Maintenance of Diversity
• Recessive alleles:
 Heterozygotes shelter recessive alleles from selection
 Heterozygotes allow even lethal alleles to remain in the
population at low frequencies virtually forever
 Sometimes recessive alleles confer an advantage to
heterozygotes
• The sickle-cell anemia allele is detrimental in homozygote
• However, heterozygotes are more likely to survive malaria
• The sickle-cell allele occurs at a higher frequency in malaria
prone areas
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Maintenance of Diversity
• Heterozygote Advantage
 Assists the maintenance of genetic, and
therefore phenotypic, variations in future
generations.
 In sickle cell disease heterozygous individuals
don’t die from sickle-cell disease, and they
don’t die from malaria.
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Sickle Cell Disease
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malaria
sickle-cell
overlap of both
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