Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 17
Processes of Evolution
(Sections 17.1 - 17.5)
Albia Dugger • Miami Dade College
17.1 Rise of the Super Rats
• Rats that carry pathogens and parasites associated with
infectious diseases thrive wherever people do
• Fighting rats with poisons such as warfarin usually doesn’t
exterminate rat populations – instead, it selects for rats that
are genetically resistant to the poisons
Rats as Pests
• Rats infesting rice
fields in the
Philippine Islands
ruin more than
20% of the crop
17.2 Individuals Don’t Evolve,
Populations Do
• Evolution starts with mutations in individuals, which
introduces new alleles into a population
• Sexual reproduction can quickly spread a mutation through a
population
• population
• A group of organisms of the same species who live in a
specific location and breed with one another more often
than they breed with members of other populations
Variation in Populations
• Individuals of a population share morphological, physiological,
and behavioral traits with a heritable basis
• Variations within a population arise from different alleles of
shared genes: A trait with only two forms is dimorphic; traits
with more than two distinct forms are polymorphic
• Traits that vary continuously often arise by interactions among
alleles of several genes, and may be influenced by
environmental factors
Phenotypic Variation in Humans
Sources of Variation in Traits
Genetic Event
• Mutation
• Crossing over
• Independent
assortment
• Fertilization
• Changes in
chromosome
number or structure
Effect
>Source of new alleles
>Introduces new combinations of
alleles into chromosomes
>Mixes maternal and paternal
chromosomes
>Combines alleles from two parents
>Transposition, duplication, or
loss of chromosomes
An Evolutionary View of Mutations
• Mutations are the original source of new alleles; many are
lethal or neutral mutations
• lethal mutation
• Mutation that drastically alters phenotype
• Causes death
• neutral mutation
• A mutation that has no effect on survival or reproduction
Adaptive Mutations
• Occasionally, a change in the environment favors a mutation
that had previously been neutral or even somewhat harmful
• Through natural selection, a beneficial mutation tends to
increase in frequency in a population over generations
• Mutations are the source of Earth’s staggering biodiversity
Allele Frequencies
• All alleles in a population form a gene pool
• Microevolution (changes in the allele frequencies of a
population) occurs constantly by processes of mutation,
natural selection, genetic drift, and gene flow
Key Terms
• gene pool
• All of the alleles of all of the genes in a population; a pool
of genetic resources
• microevolution
• Change in allele frequencies in a population or species
• allele frequency
• Abundance of a particular allele among members of a
population
Genetic Equilibrium
• A theoretical reference point, genetic equilibrium, occurs
when the allele frequencies of a population do not change
• It requires five conditions that are never met in nature, so
natural populations are never in genetic equilibrium
• genetic equilibrium
• Theoretical state in which a population is not evolving
Conditions of Genetic Equilibrium
• Five theoretical conditions of genetic equilibrium:
(1) Mutations never occur
(2) Population is infinitely large
(3) Population is isolated from all other populations of the
species (no gene flow)
(4) Mating is random
(5) All individuals survive and produce the same number of
offspring
Key Concepts
• Microevolution
• Individuals of a population inherit different alleles, and so
they differ in phenotype
• Over generations, any allele may increase or decrease in
frequency in a population
• Such change is called microevolution
ANIMATION: Antibiotic resistance
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17.3 A Closer Look
at Genetic Equilibrium
• Researchers know whether a population is evolving by
tracking deviations from a baseline of genetic equilibrium
• We use deviations from genetic equilibrium to study how a
population is evolving
The Hardy–Weinberg Formula
• Gene pools can remain stable only when the five theoretical
conditions of genetic equilibrium are being met
• Hardy and Weinberg developed a simple formula that can be
used to track whether a population of any sexually
reproducing species is in a state of genetic equilibrium
• The following example illustrates how the Hardy-Weinberg
formula is used
Allele Frequencies in Butterflies (1)
• Consider a hypothetical gene that encodes a blue pigment in
butterflies:
• Two alleles of this gene, B and b, are codominant
• A butterfly homozygous for the B allele (BB) has dark-blue
wings
• A butterfly homozygous for the b allele (bb) has white
wings
• A heterozygous butterfly (Bb) has medium-blue wings
Allele Frequencies in Butterflies (2)
• At genetic equilibrium, the proportions of the wing-color
genotypes are:
p2(BB) + 2pq(Bb) + q2(bb) = 1.0
where p and q are the frequencies of alleles B and b
• This is the Hardy–Weinberg equilibrium equation; it defines
the frequency of a dominant allele (B) and a recessive allele
(b) for a gene that controls a particular trait in a population
Allele Frequencies in Butterflies (3)
• The frequencies of B and b must add up to 1.0
• Example: If B occupies 90% of the loci, then b must occupy
the remaining 10 percent (0.9 + 0.1 = 1.0)
• No matter what the proportions:
p + q = 1.0
Allele Frequencies in Butterflies (4)
• The Punnett square below shows the genotypes possible in
the next generation (BB, Bb, and bb)
• The frequencies of the three genotypes add up to 1.0:
p2 + 2pq + q2 = 1.0
Allele Frequencies in Butterflies (4)
p
B
q
b
p
B
BB (p2 )
Bb (pq)
q
b
Bb (pq)
bb (q2)
p. 260
Allele Frequencies in Butterflies (5)
• If 1,000 individuals each produces two gametes:
• 490 BB individuals make 980 B gametes
• 420 Bb individuals make 420 B and 420 b gametes
• 90 bb individuals make 180 b gametes
• The frequency of alleles B and b among 2,000 gametes is:
B = (980 + 420)÷ 2,000 alleles = 1,400 ÷ 2,000 = 0.7 = p
b = (180 + 420) ÷ 2,000 alleles = 600 ÷2,000 = 0.3 = q
Allele Frequencies in Butterflies (6)
• At fertilization, gametes combine at random and start a new
generation
• If the population size stays constant at 1,000, there will be
490 BB, 420 Bb, and 90 bb individuals
• Allele frequencies for dark-blue, medium-blue, and white
wings are the same as they were in the original gametes – the
population is not evolving
Frequencies of Wing-Color Alleles
Starting Population
Frequencies
of Wing-Color
Alleles
490 BB butterflies 420 Bb butterflies 90 bb butterflies
dark-blue wings medium-blue wings white wings
2nd Generation
490 BB butterflies 420 Bb butterflies 90 bb butterflies
dark-blue wings medium-blue wings white wings
3rd Generation
490 BB butterflies 420 Bb butterflies 90 bb butterflies
dark-blue wings medium-blue wings white wings
Fig. 17.3, p. 260
ANIMATION: How to find out if a population
is evolving
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Applying the Rule
• In the real world, researchers can use the Hardy–Weinberg
formula to estimate the frequency of carriers of alleles that
cause genetic traits and disorders
• Example: Hereditary hemochromatosis (HH) in Ireland
• If the frequency of the autosomal recessive allele that
causes HH is q = 0.14, then p = 0.86
• The carrier frequency (2pq) is calculated to be about 0.24
• Such information is useful to doctors and to public health
professionals
17.4 Patterns of Natural Selection
• Natural selection occurs in three different patterns,
depending on the organisms involved and their environment
• natural selection
• Process in which environmental pressures result in
differential survival and reproduction of individuals of a
population who vary in details of shared, heritable traits
Three Patterns of Natural
Selection
• Directional selection shifts
the range of variation in
traits in one direction
• Stabilizing selection favors
intermediate forms of a trait
• Disruptive selection favors
forms at the extremes of a
range of variation
Three
Patterns of
Natural
Selection
population
before selection
directional
selection
stabilizing
selection
disruptive
selection
Fig. 17.4, p. 261
17.5 Directional Selection
• Directional selection shifts an allele’s frequency in a
consistent direction, so forms at one end of a range of
phenotypic variation become more common over time
• directional selection
• Mode of natural selection in which phenotypes at one end
of a range of variation are favored
Directional Selection
• Bell-shaped curves
indicate continuous
variation in a butterfly
wing-color trait
• Red arrows show which
forms are being
selected against; green,
forms that are being
favored
Directional Selection
Fig. 17.5a, p. 262
Number of individuals
in population
Directional Selection
Time 1
Range of values for the trait
Fig. 17.5a, p. 262
Directional Selection
Fig. 17.5b, p. 262
Directional Selection
Time 2
Fig. 17.5b, p. 262
Directional Selection
Fig. 17.5c, p. 262
Directional Selection
Time 3
Fig. 17.5c, p. 262
Time 1
Number of individuals
in population
Directional
Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 17.5, p. 262
ANIMATION: Directional selection
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The Peppered Moth
• The peppered moth’s coloration camouflages it from
predatory birds
• When the air was clean, trees were light-colored, and so were
most peppered moths
• When smoke from coal-burning factories changed the
environment, predatory birds ate more white moths –
selection pressure favored darker moths
Directional Selection: Peppered Moth
Directional Selection: Peppered Moth
Fig. 17.6a, p. 262
Directional Selection: Peppered Moth
Fig. 17.6a, p. 262
Directional Selection: Peppered Moth
Fig. 17.6b, p. 262
Directional Selection: Peppered Moth
Fig. 17.6b, p. 262
ANIMATION: Change in moth population
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Rock Pocket Mice
• Directional selection also affects the color of rock pocket mice
in Arizona’s Sonoran Desert
• Mice with light fur are more common in areas with lightcolored granite; mice with dark fur are more common in areas
with dark basalt
• Mice with coat colors that do not match their surroundings are
more easily seen by predators, so they are preferentially
eliminated from the populations
Directional Selection: Rock Pocket Mice
Directional Selection: Rock Pocket Mice
Fig. 17.7a, p. 263
Directional Selection: Rock Pocket Mice
Fig. 17.7b, p. 263
Directional Selection: Rock Pocket Mice
Fig. 17.7c.1, p. 263
Directional Selection: Rock Pocket Mice
Fig. 17.7c.2, p. 263
Directional Selection: Rock Pocket Mice
Fig. 17.7d.1, p. 263
Directional Selection: Rock Pocket Mice
Fig. 17.7d.2, p. 263
Antibiotic Resistance
• Antibiotics have been used in humans since the 1940s, but
they are also fed daily to cattle, pigs, chickens, fish, and other
animals raised on factory farms
• Bacteria that survive this selection pressure are antibioticresistant – an increasing problem in hospitals and schools
• This trend is bad news for millions of people each year who
contract cholera, tuberculosis, or another dangerous bacterial
disease