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Processes of Evolution
Chapter 18 Part 1
Impacts, Issues
Rise of the Super Rats
 When warfarin was used to control rats, natural
selection favored individuals with a mutation for
warfarin resistance – now warfarin is rarely used
18.1 Individuals Don’t Evolve,
Populations Do
 Evolution starts with mutations in individuals;
mutation is the source of new alleles
 Sexual reproduction can quickly spread a
mutation through a population
 Population
• Individuals of the same species in the same area
Variation In Populations
 All individuals of a species share certain traits
 Individuals of a population vary in the details of
their shared traits
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
Key Events in Inheritance
Mutation Revisited
 Mutations are the source of new alleles that give
rise to differences in details of shared traits
• Lethal mutations usually result in death
• Neutral mutations have no effect on survival or
reproduction
• Beneficial mutations convey an advantage
Stability and Change
in Allele Frequencies
 Allele frequencies
• Relative abundance of alleles of a given gene in a
population
 Natural populations are never in genetic
equilibrium
• A theoretical state which occurs when a population
is not evolving
Microevolution
 Four processes of microevolution (small-scale
changes in a population's allele frequencies)
prevent genetic equilibrium
•
•
•
•
Mutation
Natural selection
Genetic drift
Gene flow
18.2 A Closer Look
at Genetic Equilibrium
 Researchers know whether or not a population
is evolving by tracking deviations from a
baseline of genetic equilibrium
Genetic Equilibrium
 Five conditions required for a stable gene pool:
•
•
•
•
•
Mutations do not occur
Population is infinitely large
No gene flow
Random mating
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(AA) + 2pq (Aa) +q2(aa) = 1.0
 The frequency of the dominant allele (A) plus the
recessive allele (a) equals 1.0
p + q = 1.0
A Population in Equilibrium
Fig. 18-3a, p. 280
Fig. 18-3b, p. 280
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
90 aa butterflies
white wings
90 aa butterflies
white wings
90 aa butterflies
white wings
Starting Population
Next Generation
Next Generation
Fig. 18-3b, p. 280
Animation: How to find out if a
population is evolving
Applying the Rule
 Example: Hereditary hemochromatosis
• Frequency of the mutated allele q = 0.14
• Frequency of the normal allele p = 0.86
• Carrier frequency 2pq = 0.24
18.1-18.2 Key Concepts
Populations Evolve
 Individuals of a population differ in which alleles
they inherit, and so they differ in phenotype
 Over generations, any allele may increase or
decrease in frequency in a population
 Such change is called microevolution
18.3 Natural Selection Revisited
 Natural selection
• The differential survival and reproduction among
individuals of a population that vary in details of
their shared traits
• A driving force of evolution
• Occurs in recognizable patterns depending on the
organisms and their environment
Three Modes of Natural Selection
population
before selection
directional selection
stabilizing selection
disruptive selection
Fig. 18-4, p. 281
18.4 Directional Selection
 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
Directional Selection
Fig. 18-5a, p. 282
Fig. 18-5b, p. 282
Fig. 18-5c, p. 282
Time 1
Number of individuals
in population
Directional Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 18-5, p. 282
Animation: Directional selection
Time 1
Number of individuals
in population
Stabilizing Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 18-8, p. 284
Animation: Stabilizing selection
Predation and Peppered Moths
 Light color is adaptive in areas of low pollution;
dark color is adaptive in areas of high pollution
Fig. 18-6a (1), p. 283
Fig. 18-6a (2), p. 283
Fig. 18-6b (1), p. 283
Fig. 18-6b (2), p. 283
Predation and 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
Directional Selection
in Rock-Pocket Mice
Fig. 18-7a, p. 283
Fig. 18-7b, p. 283
Antibiotic Resistant Bacteria
 A typical two-week course of antibiotics can exert
selection pressure on over a thousand
generations of bacteria
 Antibiotic resistant strains are now found in
hospitals and schools
18.5 Selection Against or
in Favor of Extreme Phenotypes
 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
Stabilizing Selection:
Body Weight of Sociable Weavers
Fig. 18-9a, p. 284
Fig. 18-9b, p. 284
Disruptive Selection
Fig. 18-10a, p. 285
Fig. 18-10b, p. 285
Fig. 18-10c, p. 285
Animation: Disruptive selection
Disruptive Selection:
Bill Size in African Finches
lower bill 12 mm wide
Fig. 18-11a, p. 285
lower bill 15 mm wide
Fig. 18-11b, p. 285
Animation: Adaptation to what?
Animation: Change in moth population
Animation: Disruptive selection
Animation: Disruptive selection among
African finches
Animation: Distribution of sickle-cell trait
Animation: Life cycle of Plasmodium