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”
2
• “Through time, species accumulate
differences; as a result, descendants
differ from their ancestors. In this way,
new species arise from existing ones.”
– Charles Darwin
3
• 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
4
5
6
• 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
7
Hardy–Weinberg principle
• 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 takes
place)
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
8
• 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
9
10
11
• If all 5 assumptions for 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
• Look for changes in frequency
• Suggest hypotheses about what process or
processes at work
12
5 agents of evolutionary change
• Mutation
– Rates generally low
– Other evolutionary
processes usually
more important in
changing allele
frequency
– Ultimate source of
genetic variation
– Makes evolution
possible
13
• Gene flow
– 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
14
• Nonrandom mating
– Assortative mating
• Phenotypically similar
individuals mate
• Increases proportion
of homozygous
individuals
– Disassortative
mating
• Phenotypically
different individuals
mate
• Produces excess of
heterozygotes
15
• 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
16
• Genetic drift can lead to the loss of alleles in isolated
populations
• Alleles that initially are uncommon are particularly
vulnerable
17
• 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
18
• 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
19
• 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
20
• 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
21
• Common sulphur
butterfly
– Caterpillar usually
pale green
– Excellent
camouflage
– Bright blue color
morph rare and kept
at low frequency by
predation
22
• Pocket mice come in different colors
– Population living on rocks favor dark color
– Populations living on sand favor light color
23
• Housefly has
pesticide
resistance
alleles at
– pen gene
decreases
insecticide
uptake
– kdr and dld-r
genes
decrease
target sites for
insecticide
24
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
25
• 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
26
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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
27
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
28
• Gene flow can be
– Constructive
• Spread beneficial mutation to other
populations
– Constraining
• Can impede adaptation by continual flow
of inferior alleles from other populations
29
• 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
30
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
31
• Negative frequencydependent selection
• In water boatman,
fish eat the most
common color type
more than they
would by chance
alone
32
Positive frequency-dependent selection 33
• 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 Galapagos Islands
• Birds with big bills favored during drought
• Birds with smaller bills favored in wet conditions
34
• 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)
35
• 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
36
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
37
• Disruptive selection
• Acts to eliminate
intermediate types
• Different beak sizes
of African blackbellied seedcracker
finch
– Available seeds fall
into 2 categories
– Favors bill sizes for
one or the other
38
• 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
39
• 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
– Now fewer have that
behavior
40
Directional selection for negative
phototropism in Drosophila
41
• 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
42
Stabilizing selection for birth weight in
humans
43
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
44
• 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
45
• Pike cichlid
(predator) rare above
waterfall
– Killifish rarely eats
guppies
– Guppy males larger
and gaudier
• Predator common
below waterfall
– Individuals more drab
and reproduce earlier
46
• 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
47
48
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
49
Selection for increased speed in racehorses
is no longer effective
50
Differences in the number of ommatidia in
fly eyes does not have a genetic basis
51