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Chapter 20
Lecture and
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Genes Within
Populations
Chapter 20
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Genetic Variation and Evolution
• Genetic variation
– Differences in alleles of genes found within
individuals in a population
– Raw material for natural selection
• Evolution
– How an entity changes through time
– Development of modern concept traced to
Darwin
• “Descent with modification”
5
• “Through time, species accumulate
differences; as a result, descendants
differ from their ancestors. In this way,
new species arise from existing ones.”
– Charles Darwin
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• Darwin was not the first to propose a
theory of evolution
• Unlike his predecessors, however,
Darwin proposed natural selection as
the mechanism of evolution
• Rival theory of Jean-Baptiste Lamarck
was evolution by inheritance of acquired
characteristics
7
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Stretching
Proposed ancestor of
giraffes has characteristics
of modern-day okapi.
Stretching
The giraffe ancestor lengthened its
neck by stretching to reach tree
leaves, then passed the change to
offspring.
Reproduction
a. Lamarck’s theory: acquired variation is passed on to descendants.
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Some individuals born happen to have longer necks due to
genetic differences.
Reproduction
Individuals pass on their traits to next generation.
Natural
selection
Reproduction
Over many generations, longer-necked individuals are more
successful, perhaps because they can feed on taller trees, and
pass the long-neck trait on to their offspring.
b. Darwin’s theory: natural selection or genetically-based
variation leads to evolutionary change.
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• Population genetics
– Study of properties of genes in a
population
– Evolution results in a change in the genetic
composition of a population
– Genetic variation is the raw material
for selection
– In nature, genetic variation is the rule
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• Polymorphic variation
– More than one allele at
frequencies greater
than mutation alone
• SNPs
– Used to assess
patterns in human and
natural populations.
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Hardy–Weinberg principle
• Changes in allele frequency
• Hardy–Weinberg equilibrium
– Proportions of genotypes do not change in
a population if
1. No mutation takes place
2. No genes are transferred to or from other
sources (no immigration or emigration)
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
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• Principle can be written as an equation
• Used to calculate allele frequencies
• For 2 alleles, p and q
– p = B for black coat color
• Black cat is BB or Bb
– q = b for white coat color
• White cats are bb
p2 + 2pq + q2 = 1
BB + Bb + bb = 1
13
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Generation One
Phenotypes
84%
16%
Genotypes
BB
Bb
bb
Frequency of
genotype in population
0.36
0.48
0.16
Frequency of gametes
0.36 + 0.24 = 0.60B
0.24 + 0.16 = 0.40b
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Generation Two
B
p = 0.60
b
q = 0.40
B
p = 0.60
BB
p2 = 0.36
Bb
pq = 0.24
b
q = 0.40
Bb
pq = 0.24
bb
q2 = 0.16
Eggs
Sperm
p2 + 2 pq + q2 = 1
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• If all 5 assumptions for Hardy-Weinberg
equilibrium are true, allele and genotype
frequencies do not change from one
generation to the next
• In reality, most populations will not meet all 5
assumptions
• To determine this, look for changes in
frequency
• Suggest hypotheses about what process or
processes are at work to cause changes to
the frequencies
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5 agents of evolutionary change
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• Mutation
– Rates generally low
– Other evolutionary
processes usually
more important in
changing allele
frequency
– Ultimate source of
genetic variation
– Makes evolution
possible
Mutation
Mutagen
DNA
C
T
G
G
C
G
A
G
a. The ultimate source of
variation. Individual
mutations occur so
rarely that mutation
alone usually does
not change allele
frequency much.
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• Gene flow
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– Movement of alleles
from one population
to another
– Animal physically
moves into new
population
– Drifting of gametes
or immature stages
into an area
– Mating of individuals
from adjacent
populations
Gene Flow
b. A very potent agent of
change. Individuals or
gametes move from one
population to another.
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• Nonrandom mating
– Assortative mating
• Phenotypically similar
individuals mate
• Increases proportion
of homozygous
individuals
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Nonrandom Mating
Self-fertilization
– Disassortative
mating
• Phenotypically
different individuals
mate
• Produces excess of
heterozygotes
c. Inbreeding is the most
common form. It does
not alter allele
frequency but reduces
the proportion of
heterozygotes.
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• Genetic drift
– In small populations,
allele frequency may
change by chance
alone
– Magnitude of genetic
drift is negatively
related to population
size
– Founder effect
– Bottleneck effect
Genetic Drift
d. Statistical accidents.
The random fluctuation
in allele frequencies
increases as population
size decreases.
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Genetic drift
• Alters allele frequencies in small
populations
– Population must be large
– Small number of individuals drift from
population
– Can lead to the loss of alleles in isolated
populations
21
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Parent
population
Bottleneck
(drastic reduction
in population)
Surviving
individuals
Next
generation
• Genetic drift can lead to the loss of alleles in isolated
populations
• Alleles that initially are uncommon are particularly
vulnerable
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Bottleneck effect
• If organisms do not move from place to
place their populations may be drastically
reduced
• Survivors may constitute a random
genetic sample of the original population
• Results in loss of genetic variability
23
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UNITED
STATES
population in 1890,
reduced to inhabiting
Guadalupe only
current population
Guadalupe
MEXICO
• Northern Elephant Seal
– Bottleneck case study
– Nearly hunted to extinction in 19th century
– As a result, species has lost almost all of its
genetic variation
– Population now numbers in tens of thousands
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• Selection
– Some individuals leave behind more
progeny than others, and the rate at
which they do so is affected by
phenotype and behavior
– Artificial selection
– Natural selection
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• 3 conditions for natural selection to
occur and to result in evolutionary
change
1. Variation must exist among individuals in
a population
2. Variation among individuals must result in
differences in the number of offspring
surviving in the next generation
3. Variation must be genetically inherited
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• Natural selection and evolution are not
the same
– Natural selection is a process
• Only one of several processes that can result in
evolution
– Evolution is the historical record, or
outcome, of change through time
• Result of evolution driven by natural
selection is that populations become
better adapted to their environment
28
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Light coat color pocket mouse
is vulnerable on lava rock
Light coat color favored by
natural selection because
it matches sand color
Dark coat color favored by
natural selection because
it matches black lava color
• Pocket mice come in different colors
– Population living on rocks favor dark color
– Populations living on sand favor light color
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• Housefly has
pesticide
resistance
alleles at
– pen gene
decreases
insecticide
uptake
– kdr and dld-r
genes
decrease
target sites for
insecticide
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Pesticide molecule
Resistant target site
Target site
Insect cell membrane
a. Insect cells with resistance allele at pen gene:
decreased uptake of the pesticide.
Target site
b. Insect cells with resistance allele at kdr gene:
decreased number of target sites for the pesticide.
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Fitness and its measurement
• Fitness
– Individuals with one phenotype leave more
surviving offspring in the next generation
than individuals with an alternative
phenotype
– Relative concept; the most fit phenotype is
simply the one that produces, on average,
the greatest number of offspring
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• Fitness has many components
– Survival
– Sexual selection – some individuals more
successful at attracting mates
– Number of offspring per mating
– Traits favored for one component may be a
disadvantage for others
• Selection favors phenotypes with the greatest
fitness
– Phenotype with greater fitness usually increases
in frequency
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4
2
0
12
13
14
15
16
Length of Adult Female Water Strider (mm)
200
Number of Eggs Laid
During Lifetime
6
50
Life Span of Adult
Female (days)
Number of Eggs
Laid per Day
8
40
30
20
10
0
12
13
14
15
16
Length of Adult Female Water Strider (mm)
150
100
50
0
12
13
14
15
16
Length of Adult Female Water Strider (mm)
• Larger female water striders lay more eggs per day
• Large females survive for a shorter period of time
• As a result, intermediate-sized females produce the
most offspring over the course of their entire lives
and thus have the highest fitness
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Interactions
• Mutations and genetic drift may counter
selection
– In nature, mutation rates are rarely high
enough to counter selection
– Selection is nonrandom but genetic drift is
random
• Drift may decrease an allele favored by
selection
• Selection usually overwhelms drift except in
small populations
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• Gene flow can be
– Constructive
• Spread beneficial mutation to other
populations
– Constraining
• Can impede adaptation by continual flow
of inferior alleles from other populations
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Maintenance of variation
• Frequency-dependent selection
– Fitness of a phenotype depends on its
frequency within the population
– Negative frequency-dependent selection
• Rare phenotypes favored by selection
• Rare forms may not be in “search image”
– Positive frequency-dependent selection
• Favors common form
• Tends to eliminate variation
• “Oddballs” stand out
36
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Percent of Color Type
Taken by Fish Predators
• Negative frequencydependent selection
• In water boatman,
fish eat the most
common color type
more than they
would by chance
alone
100
80
60
40
Color type of
water boatman
dark brown
medium brown
light brown
20
20
40
60
100
80
Color Type Frequency in Population
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Positive frequency-dependent selection 38
• Oscillating selection
– Selection favors one phenotype at one
time and another phenotype at another
time
– Effect will be to maintain genetic variation
in the population
– Medium ground finch of Galápagos Islands
• Birds with big bills favored during drought
• Birds with smaller bills favored in wet conditions
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• Heterozygote advantage
– Heterozygotes are favored over
homozygotes
– Works to maintain both alleles in the
population
– Sickle cell anemia
• Hereditary disease affecting hemoglobin
• Causes severe anemia
• Homozygotes for sickle cell allele usually die
before reproducing (without medical treatment)
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• Why is the sickle
cell allele not
eliminated?
• Leading cause of
death in central
Africa is malaria
• Heterozygotes for
sickle cell allele
do not suffer
anemia and are
much less
susceptible to
malaria
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Normal red
blood cells
Sickled red
blood cells
Sickle cell
allele in Africa
1–5%
5–10%
10–20%
Geographic
distribution of
P. falciparum
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Selection
• Many traits affected by more than one gene
• Selection operates on all the genes for the
trait
• Changes the population depending on which
genotypes are favored
• Types of selection
– Disruptive
– Directional
– Stabilizing
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– Available seeds fall
into 2 categories
– Favors bill sizes for
one or the other
0
25
50
75
100
125
Body Size (g)
Selection for small and large individuals
Number of Individuals
• Disruptive selection
• Acts to eliminate
intermediate types
• Different beak sizes
of African blackbellied seedcracker
finch
Number of Individuals
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Two peaks
form
0
25
50
75
100
125
Body Size (g)
a. Disruptive selection
43
• Birds with intermediate-sized beaks are
at a disadvantage with both seed types
– they are unable to open large seeds
and too clumsy to efficiently process
small seeds
44
– Now fewer have that
behavior
Number of Individuals
0
25
50
75
100
125
Body Size (g)
Selection for larger individuals
Number of Individuals
• Directional selection
• Acts to eliminate
one extreme
• Often occurs in
nature when the
environment
changes
• In Drosophila,
artificially selected
flies that moved
toward the light
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Peak shifts
0
25
50
75
100
125
Body Size (g)
b. Directional selection
45
Light
Dark
Average Tendency to Fly Toward Light
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11
10
9
8
7
6
5
4
3
2
1
0
2
4
6
8 10 12 14 16
Number of Generations
18
Directional selection for negative
phototropism in Drosophila
20
46
0
25
50
75
100
125
Body Size (g)
Selection for mid-size individuals
Number of Individuals
• Stabilizing selection
• Acts to eliminate
both extremes
• Makes intermediate
more common by
eliminating extremes
• In humans, infants
with intermediate
weight at birth have
the highest survival
rate
Number of Individuals
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Distribution
gets narrower
0
25
50
75
100
125
Body Size (g)
c. Stabilizing selection
47
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births in population
infant mortality
100
70
15
50
30
20
10
10
7
5
Percent Infant Mortality
Percent of Births in Population
20
5
3
2
2
3
4
5
6
7
8
9 10
Birth Weight in Pounds
Stabilizing selection for birth weight in
humans
48
Experimental studies
• To study evolution, biologists have
traditionally investigated what has
happened in the past
– Fossils or DNA evidence
• Laboratory studies on fruit flies common
for more than 50 years
• Only recently started with lab and field
experiments
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50
• Guppy coloration
– Found in small streams in northeastern
South America and Trinidad
– Some are capable of colonizing portions of
streams above waterfalls
• Different dispersal methods
• Other species not able to make it upstream
– Dispersal barriers create 2 different
environments
• Predators rare above waterfall
51
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• Pike cichlid
(predator) rare above
waterfall
Killifish
(Rivulus hartii)
Guppy
(Poecilia reticulata)
– Killifish rarely eats
guppies
– Guppy males larger
and gaudier
• Predator common
below waterfall
– Individuals more drab
and reproduce earlier
Pikecichlid
(Crenicichla alta)
Guppy
(Poecilia reticulata)
52
• Guppy lab study
– Other explanations are possible for field
results
– 10 large pools
– Added pike cichlids to 4, killifish to 4, and 2
left as controls
– 14 months and 10 guppy generations later
– Guppies in killifish and control pool – large
and colorful
– Guppies in pike cichlid pools – smaller and
drab
53
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no predation
low predation
high predation
14
Spots per Fish
13
12
11
10
9
8
0
4
8
12
Duration of Experiment (months)
54
Constraints on evolutionary change
• Mutation and genetic drift
– May counter or promote selection
• Natural in abandoned mine sites in
Great Britain
– Copper tolerance in plants
55
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Index of Copper Tolerance
Pollen
Prevailing wind
Mine Site
Nonmine
60
Nonmine
Bent grass
(Agrostis tenuis)
40
20
0
20
0
Distance Upwind
from Mine (m)
0
20
40
60
80
100
120
140
160
Distance Away
from Mine (m)
• Slender bent grass at copper mines
– Resistance allele occurs at intermediate levels in many
areas
– Individuals with resistance gene grow slower on
uncontaminated sites
– Gene flow between sites high enough to counteract selection
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Limits of selection
• Multiple phenotypic effects of alleles
– Larger clutch size leads to thinner shelled eggs
• Lack of genetic variation
– Gene pool of thoroughbreds limited and
performance times have not improved for more
than 50 years
– Phenotypic variation may not have genetic basis
• Interactions between genes – epistasis
– Selective advantage of an allele at one gene may
vary from one genotype to another
57
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Kentucky Derby Winning Time
(seconds)
130
125
120
115
110
1900
1920
1940
1960
1980
2000
Year
Selection for increased speed in racehorses
is no longer effective
58
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Right eye
of insect
Left eye
of insect
Ommatidia
Differences in the number of ommatidia in
fly eyes does not have a genetic basis
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