Genes Within
Populations
Chapter 20
1
Genetic Variation and Evolution
• Darwin: Evolution is descent with
modification
• Evolution: changes through time
1. Species accumulate difference
2. Descendants differ from their
ancestors
3. New species arise from existing
ones
2
Natural selection: mechanism of
evolutionary change
Natural selection: proposed by Darwin as
the mechanism of evolution
• individuals have specific inherited
characteristics
• they produce more surviving offspring
• the population includes more individuals
with these specific characteristics
• the population evolves and is better adapted
3
to its present environment
Darwin’s
theory for
how long
necks
evolved in
giraffes
4
Natural selection: mechanism of
evolutionary change
Inheritance of acquired characteristics:
Proposed by Jean-Baptiste Lamarck
• Individuals passed on physical and
behavioral changes to their offspring
• Variation by experience…not genetic
• Darwin’s natural selection: variation a result
of preexisting genetic differences
5
Lamarck’s theory of how giraffes’ long necks
6
evolved
Gene Variation in Nature
• Measuring levels of genetic variation
– blood groups
– enzymes
• Enzyme polymorphism
– A locus with more variation than can be
explained by mutation is termed
polymorphic.
– Natural populations tend to have more
polymorphic loci than can be accounted
for by mutation.
• DNA sequence polymorphism
7
Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician
Wilhelm Weinberg: German physician
Concluded that:
The original proportions of the genotypes in a
population will remain constant from
generation to generation as long as five
assumptions are met
8
Hardy-Weinberg Principle
Five assumptions :
1. No mutation takes place
2. No genes are transferred to or from
other sources
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
9
Hardy-Weinberg Principle
•
•
•
•
Calculate genotype frequencies with a
binomial expansion
(p+q)2 = p2 + 2pq + q2
p = individuals homozygous for first allele
2pq = individuals heterozygous for both
alleles
q = individuals homozygous for second
allele
because there are only two alleles:
p plus q must always equal 1
10
Hardy-Weinberg Principle
11
Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
12
A population not in Hardy-Weinberg
equilibrium indicates that one or more of the
five evolutionary agents are operating in a
population
Five agents of evolutionary change
13
Agents of Evolutionary Change
• Mutation: A change in a cell’s DNA
– Mutation rates are generally so low they
have little effect on Hardy-Weinberg
proportions of common alleles.
– Ultimate source of genetic variation
• Gene flow: A movement of alleles from
one population to another
– Powerful agent of change
– Tends to homogenize allele frequencies
14
15
Agents of Evolutionary Change
• Nonrandom Mating: mating with specific
genotypes
– Shifts genotype frequencies
– Assortative Mating: does not change
frequency of individual alleles; increases
the proportion of homozygous individuals
– Disassortative Mating: phenotypically
different individuals mate; produce
excess of heterozygotes
16
Genetic Drift
• Genetic drift: Random fluctuation in
allele frequencies over time by chance
• important in small populations
–founder effect - few individuals
found new population (small allelic
pool)
–bottleneck effect - drastic
reduction in population, and gene
pool size
17
18
Genetic Drift: A bottleneck effect
19
Bottleneck effect: case study
20
Selection
• Artificial selection: a breeder selects for
desired characteristics
21
Selection
• Natural selection: environmental
conditions determine which individuals in a
population produce the most offspring
• 3 conditions for natural selection to occur
– Variation must exist among individuals in
a population
– Variation among individuals must result
in differences in the number of offspring
surviving
– Variation must be genetically inherited
22
Selection
23
Selection
Pocket mice from the Tularosa Basin
24
Selection to match climatic
conditions
• Enzyme allele frequencies vary with latitude
• Lactate dehydrogenase in Fundulus
heteroclitus (mummichog fish) varies with
latitude
• Enzymes formed function differently at
different temperatures
• North latitudes: Lactate dehydrogenase is a
better catalyst at low temperatures
25
Selection for pesticide resistance
26
Fitness and Its Measurement
• Fitness: A phenotype with greater
fitness usually increases in frequency
–Most fit is given a value of 1
• Fitness is a combination of:
–Survival: how long does an
organism live
–Mating success: how often it mates
–Number of offspring per mating that
survive
27
Fitness and its Measurement
Body size and egg-laying in water striders
28
Interactions Among Evolutionary
Forces
• Mutation and genetic drift may counter
selection
• The magnitude of drift is inversely related to
population size
29
Interactions Among Evolutionary
Forces
• Gene flow may promote or constrain
evolutionary change
– Spread a beneficial mutation
– Impede adaptation by continual flow of
inferior alleles from other populations
• Extent to which gene flow can hinder the
effects of natural selection depends on the
relative strengths of gene flow
– High in birds & wind-pollinated plants
– Low in sedentary species
30
Interactions Among Evolutionary
Forces
Degree of copper tolerance
31
Maintenance of Variation
• Frequency-dependent selection:
depends on how frequently or infrequently
a phenotype occurs in a population
– Negative frequency-dependent selection:
rare phenotypes are favored by selection
– Positive frequency-dependent selection:
common phenotypes are favored;
variation is eliminated from the
population
• Strength of selection changes through time
32
Maintenance of Variation
Negative frequency - dependent selection
33
Maintenance of Variation
Positive frequency-dependent selection 34
Maintenance of Variation
• Oscillating selection: selection favors
one phenotype at one time, and a
different phenotype at another time
• Galápagos Islands ground finches
– Wet conditions favor big bills
(abundant seeds)
– Dry conditions favor small bills
35
Maintenance of Variation
• Fitness of a phenotype does not depend
on its frequency
• Environmental changes lead to
oscillation in selection
36
Maintenance of Variation
• Heterozygotes may exhibit greater fitness
than homozygotes
• Heterozygote advantage: keep
deleterious alleles in a population
• Example: Sickle cell anemia
• Homozygous recessive phenotype: exhibit
severe anemia
37
Maintenance of Variation
• Homozygous dominant phenotype:
no anemia; susceptible to malaria
• Heterozygous phenotype: no anemia;
less susceptible to malaria
38
Maintenance of Variation
Frequency of sickle cell allele
39
Maintenance of Variation
Disruptive selection acts to eliminate
intermediate types
40
Maintenance of Variation
Disruptive selection for large and small
beaks in black-bellied seedcracker finch of
west Africa
41
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
42
Maintenance of Variation
Directional selection for negative
phototropism in Drosophila
43
Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
44
Maintenance of Variation
Stabilizing selection for birth weight in
humans
45
Experimental Studies of Natural
Selection
• In some cases, evolutionary change can
occur rapidly
• Evolutionary studies can be devised to test
evolutionary hypotheses
• Guppy studies (Poecilia reticulata) in the lab
and field
– Populations above the waterfalls: low
predation
– Populations below the waterfalls: high
predation
46
Experimental Studies
• High predation environment - Males
exhibit drab coloration and tend to be
relatively small and reproduce at a younger
age.
• Low predation environment - Males
display bright coloration, a larger number of
spots, and tend to be more successful at
defending territories.
47
Experimental Studies
The evolution of protective coloration in
48
guppies
Experimental Studies
The laboratory experiment
–10 large pools
–2000 guppies
–4 pools with pike cichlids (predator)
–4 pools with killifish (nonpredator)
–2 pools as control (no other fish
added)
–10 generations
49
Experimental Studies
The field experiment
– Removed guppies from below the
waterfalls (high predation)
– Placed guppies in pools above the
falls
– 10 generations later, transplanted
populations evolved the traits
characteristic of low-predation
guppies
50
Experimental Studies
Evolutionary change in spot number
51
The Limits of Selection
• Genes have multiple effects
– Pleiotropy: sets limits on how much
a phenotype can be altered
• Evolution requires genetic variation
– Thoroughbred horse speed
– Compound eyes of insects: same
genes affect both eyes
– Control of ommatidia number in left
and right eye
52
Experimental Studies
Selection for increased speed in racehorses
is no longer effective
53
Experimental Studies
Phenotypic variation in insect ommatidia
54