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Chapter 17
Processes of
Evolution
Sections 1-6
Albia Dugger • Miami Dade College
17.1 Rise of the Super Rats
• When warfarin was used to control rats, natural selection
favored individuals with a mutation in the VKORC1 gene
which resulted in warfarin resistance
• When warfarin resistance increased in rat populations, people
stopped using warfarin to kill rats
• The frequency of the warfarin-resistance allele in rat
populations declined, probably because rats that carry the
allele are not as healthy as ones that do not
Rats Infesting Rice Fields in the Philippines
Natural Selection for Resistant Rats
17.2 Individuals Don’t Evolve,
Populations Do
• Mutations in individuals are the source of new alleles in a
population’s gene pool
• A change in an allele’s frequency in a population is called
microevolution
Variation In Populations
• All individuals of a species share certain morphological,
physiological, and behavioral traits
• A population is a group of interbreeding individuals of the
same species in a specified area
• Individuals of a population with different alleles of shared
genes vary in the details of their shared traits
Morphs
• Many traits have two or more distinct forms (morphs)
• A trait with only two forms is dimorphic
• Traits with more than two distinct forms are polymorphic
• Traits that vary continuously among individuals of a
population may be influenced by alleles of several genes
Phenotypic Variations
An Evolutionary View of Mutations
• Mutations are the source of new alleles that give rise to
differences in details of shared traits
• Lethal mutations result in death
• Neutral mutations have no effect on survival or reproduction
• Beneficial mutations convey an advantage
Table 17-1 p272
The Gene Pool
• Gene pool
• All genes found in one population
• Alleles
• Different forms of the same gene
• Determine genotype and phenotype
• Dimorphism and polymorphism
Allele Frequencies and Microevolution
• Allele frequency refers to the relative abundance of a
particular allele of a given gene in a population
• Changes in allele frequency of a population (or a species) is
called microevolution
• Microevolutionary processes include mutation, natural
selection, genetic drift, and gene flow
Take-Home Message:
What mechanisms drive evolution?
• Individuals of a natural population share morphological,
physiological, and behavioral traits characteristic of the
species
• Different alleles are the basis of differences in the details of a
population’s shared traits
• All alleles of all individuals in a population make up the
population’s gene pool
• Changes in allele frequency (microevolution) are always
occurring in natural populations
17.3 Genetic Equilibrium
• Natural populations are always evolving
• Researchers trace evolution within a population by tracking
deviations from a baseline of genetic equilibrium – a
theoretical state in which a population is not evolving
• Natural populations are never in genetic equilibrium
Genetic Equilibrium
• Under certain ideal conditions, the frequency of an allele in a
sexually reproducing population’s gene pool should remain
stable from one generation to the next
• Five conditions required for a genetic equilibrium:
• Mutations do not occur
• Population is infinitely large
• Population is isolated (no gene flow)
• Mating is random
• All individuals survive and reproduce equally
The Hardy-Weinberg Formula
• The Hardy-Weinberg formula can be used to determine if a
population is in genetic equilibrium
p2(BB) + 2pq (Bb) + q2(bb) = 1.0
• The frequency of the dominant allele (B) plus the recessive
allele (b) equals 1.0
p + q = 1.0
A Two Allele System
one type of gametes
two types of gametes
one type of gametes
Predicted Proportions in a Population
Applying the Rule
• A population consists of 1,000 plants: 490 homozygous (BB),
420 heterozygous (Bb), and 90 homozygous (bb)
• Each plant makes two gametes:
• All gametes made by BB individuals have the B allele
• All gametes made by bb individuals have the b allele
• Bb individuals have half B gametes half b gametes
Applying the Rule
• Frequency of the B allele:
p (B) = (980 + 420) ÷ 2,000 = 1,400 ÷ 2,000 = 0.7
• Frequency of the b allele:
q (b) = (180 + 420) ÷ 2,000 = 600 ÷ 2,000 = 0.3
Applying the Rule
• Predicted proportion of individuals in the next generation:
BB (p2) = (0.7)2 = 0.49
Bb (2pq) = 2 (0.7 × 0.3) = 0.42
bb (q2) = (0.3)2 = 0.09
Real World Situations
• Researchers used genetic equilibrium to determine the carrier
frequency of an allele that causes hereditary
hemochromatosis (HH)
• The allele’s frequency (q) was found to be 0.14
• Frequency of the normal allele (p) = 1.0 – 0.14 = 0.86
• Carrier frequency (2pq) is calculated at 0.24
Take-Home Message: How do we know
when a population is evolving?
• Researchers measure genetic change by comparing it with a
theoretical baseline of genetic equilibrium
• Allele frequencies are always changing in natural populations
because ideal conditions can never be met
ANIMATED FIGURE: How to find out if a
population is evolving
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17.4 Patterns of Natural Selection
• Natural selection occurs in different patterns depending on
the organisms involved and their environment
Natural Selection
• Natural selection results from the differential survival and
reproduction among individuals of a population that vary in
details of their shared traits
• Natural selection occurs in three recognizable patterns
depending on the organisms and their environment:
• Directional selection
• Stabilizing selection
• Disruptive selection
Three Modes of Natural Selection
population
before selection
directional
selection
stabilizing
selection
disruptive
selection
Take-Home Message: Does evolution
occur in recognizable patterns?
• Natural selection, the most influential process of evolution,
occurs in patterns that depend on the organisms and their
environment
17.5 Directional Selection
• Changing environmental conditions can result in a directional
shift in allele frequencies
• Directional selection
• Changing environmental conditions can shift allele
frequencies in a consistent direction
• Forms of traits at one end of a range of phenotypic
variation become more common
Number of individuals
in population
Time 1
Range of values for the trait
Time 2
Time 3
Stepped Art
Figure 17-5 p276
ANIMATED FIGURE: Directional selection
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Directional Selection in Peppered Moths
• In preindustrial England, most moths were light-colored, and a
dominant allele that resulted in darker coloration was rare
• In post-industrial England, pollution from coal-burning
factories changed the colors of the forests
• Predatory birds ate more light-colored moths in soot-darkened
forests, and more dark-colored moths in clean forests
• Light color is adaptive in areas of low pollution; dark color is
adaptive in areas of high pollution
Directional Selection in Peppered Moths
ANIMATED FIGURE: Change in moth
population
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Directional Selection in Rock-Pocket Mice
• In rock-pocket mice, two alleles of a single gene control coat
color
• Night-flying owls are the selective pressure that directionally
shifts the allele frequency
• Most of the mice in populations that inhabit dark rock have
dark gray coats
• Most of the mice in populations that inhabit light brown rock
have light brown coats
Directional Selection in Rock-Pocket Mice
Directional Selection in Rock-Pocket Mice
Directional Selection
in Antibiotic Resistant Bacteria
• A typical two-week course of antibiotics can exert selection
pressure on over a thousand generations of bacteria
• Antibiotics are used preventively in humans, cattle, pigs,
chickens, fish, and other animals raised on factory farms
• Bacteria with alleles that allow them to survive antibiotic
treatment (antibiotic resistant strains) are now common in
hospitals and schools
Take-Home Message:
What is the effect
of directional selection?
• Directional selection causes allele frequencies underlying a
range of variation to shift in a consistent direction
17.6 Stabilizing and Disruptive Selection
• Stabilizing selection
• Natural selection that favors an intermediate phenotype
and eliminates extreme forms
• Disruptive selection
• Natural selection that favors extreme forms of a trait and
eliminates the intermediate forms
Number of individuals
in population
Time 1
Range of values for the trait
Time 2
Time 3
Stepped Art
Figure 17-8 p278
ANIMATED FIGURE: Stabilizing selection
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Stabilizing Selection: Sociable Weavers
• Body weight in sociable weavers is a trade-off between risks
of starvation and predation
• Food supplies are limited – leaner birds do not store enough
fat to avoid starvation
• Fatter birds may be more attractive to predators, and not as
agile when escaping
• Birds of intermediate weight have the selective advantage
Sociable Weavers
Body Weight of Sociable Weavers
Number of individuals
in population
Time 1
Range of values for the trait
Time 2
Time 3
Stepped Art
Figure 17-10 p279
ANIMATED FIGURE: Disruptive selection
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Disruptive Selection in African Finches
• The bill of a blackbellied seedcracker is either 12 millimeters
wide, or wider than 15 millimeters
• These finches feed on seeds of two types of sedge – one
with hard seeds, one with soft seeds
• Small-billed birds are better at opening soft seeds; large-billed
birds are better at opening hard seeds
Bill Size in African Finches
ANIMATED FIGURE: Disruptive selection
among African finches
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Take-Home Message: Natural selection can
favor intermediate or extreme forms of traits
• With stabilizing selection, an intermediate phenotype is
favored, and extreme forms are selected against
• With disruptive selection, an intermediate form of a trait is
selected against, and extreme phenotypes are favored