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Chapter 13
How Populations Evolve
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
REECE • TAYLOR • SIMON • DICKEY • HOGAN
© 2015 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
•  In the 1960s, the World Health Organization
(WHO) launched a campaign to eradicate malaria.
•  They focused on killing the mosquitoes that carry
the parasite from person to person by massive
spraying of the pesticide DDT.
•  Early success was followed by rebounding
mosquito populations that had evolved resistance
to the pesticide DDT.
•  Today, malaria causes more than a million deaths
and 250 million cases of illness each year.
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Introduction
•  At the same time that DDT was being celebrated in
the war against malaria, a drug called chloroquine
was hailed as the miracle cure.
•  But its effectiveness diminished as the parasite
evolved resistance to the drug.
•  In some regions chloroquine is now powerless
against the disease.
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Introduction
•  At present, the most effective antimalarial drug is
artemisinin, a compound extracted from a plant
used in traditional Chinese medicine.
•  But the effectiveness of this drug will eventually
succumb to the power of evolution, too.
•  Medical experts have found cases of malaria in
Southeast Asia that do not respond to artemisinin.
•  An understanding of evolution informs all of
biology, from exploring life’s molecules to analyzing
ecosystems.
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DARWIN’S THEORY OF EVOLUTION
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Charles Darwin is best known for his book On the
Origin of Species by Means of Natural Selection,
commonly referred to as The Origin of Species,
which launched the era of evolutionary biology.
•  Darwin’s early career gave no hint of his future
fame.
•  He enrolled in but left medical school.
•  Then he entered Cambridge University to become a
clergyman.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  The cultural and scientific context of his time
instilled Darwin with a conventional view of Earth
and its life.
•  Most scientists accepted the views of the Greek
philosopher Aristotle, who generally held that
species are fixed, permanent forms that do not
evolve.
•  Judeo-Christian culture taught a literal interpretation
of the biblical book of Genesis, that each form of life
is individually created in its present-day form.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Earlier religious scholars estimated the age of Earth
at 6,000 years.
•  Thus, the idea that all living species came into
being relatively recently and are unchanging in form
dominated the intellectual climate of the Western
world at the time.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  At the age of 22, Darwin took a position on HMS
Beagle, a survey ship preparing for a long
expedition to chart poorly known stretches of the
South American coast.
•  As the ship’s naturalist (field biologist), Darwin
•  spent most of his time on shore collecting
thousands of specimens of fossils and living plants
and animals and
•  kept detailed journals of his observations.
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Figure 13.1a-0
HMS Beagle in port
Darwin
in 1840
North
America
Great
Britain
Europe
Asia
ATLANTIC
OCEAN
Africa
Pinta
Marchena
Santiago
Pinzón
Fernandina
Isabela
0
0
40 km
Galápagos
Islands
Genovesa
Equator
Daphne Islands
Santa
Cruz
Florenza
40 miles
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Santa
Fe
San
Cristobal
Española
Equator
South
America
PACIFIC
OCEAN
Andes
PACIFIC
OCEAN
PACIFIC
OCEAN
Australia
Cape of
Good Hope
Cape Horn
Tierra del Fuego
Tasmania
New
Zealand
13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Many of Darwin’s observations indicated that
geographic proximity is a better predictor of
relationships among organisms than similarity of
environment.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Darwin was particularly intrigued by the geographic
distribution of organisms on the Galápagos
Islands, including
•  marine iguanas and
•  giant tortoises.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  While on his voyage, Darwin was strongly
influenced by the newly published Principles of
Geology, by Scottish geologist Charles Lyell.
•  The book presented the case for an ancient Earth
sculpted over millions of years by gradual geologic
processes that continue today.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  By the time Darwin returned to Great Britain five
years after the Beagle first set sail, he had begun
to seriously doubt that Earth and all its living
organisms had been specially created only a few
thousand years earlier.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  As he reflected on his observations, analyzed his
collections, and discussed his work with
colleagues, he concluded that the evidence was
better explained by the hypothesis that present-day
species are the descendants of ancient ancestors
that they still resemble in some ways.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  He hypothesized that as the descendants of a
remote ancestor spread into various habitats over
millions and millions of years, they accumulated
diverse modifications, or adaptations, that fit them
to specific ways of life in their environment.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  By the early 1840s, Darwin had composed a long
essay describing the major features of his theory of
evolution by natural selection.
•  But he delayed publishing his essay, continued to
compile evidence in support of his hypothesis, and
finally released his essay to the scientific
community when learning of the work of another
British naturalist, Alfred Wallace, who had a nearly
identical hypothesis.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  The next year, Darwin published The Origin of
Species, a book that supported his hypothesis with
immaculate logic and hundreds of pages of
evidence drawn from observations and
experiments in biology, geology, and paleontology.
•  The hypothesis of evolution generated predictions
that have been tested and verified by more than
150 years of research.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Consequently, scientists regard Darwin’s concept
of evolution by means of natural selection as a
theory, a widely accepted explanatory idea that
•  is broader in scope than a hypothesis,
•  generates new hypotheses, and
•  is supported by a large body of evidence.
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13.1 A sea voyage helped Darwin frame his
theory of evolution
•  Next we examine lines of evidence for Darwin’s
theory of evolution, the idea that living species are
descendants of ancestral species that were
different from present-day ones and that natural
selection is the mechanism for evolutionary
change.
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13.2 The study of fossils provides strong
evidence for evolution
•  Fossils
•  are the imprints or remains of organisms that lived
in the past,
•  document differences between past and present
organisms, and
•  reveal that many species have become extinct.
•  For example, the fossilized skull of one of our early
relatives, Homo erectus, represents someone who
lived 1.5 million years ago in Africa.
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Figure 13.2a
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13.2 The study of fossils provides strong
evidence for evolution
•  Some fossils are not the actual remnants of
organisms.
•  The 375-million-year-old fossils in Figure 13.2B are
casts of ammonites, shelled marine animals
related to the present-day nautilus.
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Figure 13.2b
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13.2 The study of fossils provides strong
evidence for evolution
•  The sequence in which fossils appear within
strata, layers of sedimentary rocks, is a historical
record of life on Earth.
•  Paleontologists (scientists who study fossils)
sometimes gain access to very old fossils when
erosion carves through upper (younger) strata,
revealing deeper (older) strata that had been
buried.
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Figure 13.2c
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13.2 The study of fossils provides strong
evidence for evolution
•  The fossil record is the chronicle of evolution over
millions of years of geologic time engraved in the
order in which fossils appear in rock strata.
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13.2 The study of fossils provides strong
evidence for evolution
•  The fossil record is incomplete because
•  many of Earth’s organisms did not live in areas that
favor fossilization,
•  fossils that did form were in rocks later distorted or
destroyed by geologic processes, and
•  not all fossils that have been preserved are
accessible to paleontologists.
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  In The Origin of Species, Darwin predicted the
existence of fossils of transitional forms linking very
different groups of organisms.
•  For example, if his hypothesis that whales evolved
from land-dwelling mammals was correct, then
fossils should show a series of changes in a
lineage of mammals adapted to a fully aquatic
habitat.
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  Many fossils link early extinct species with species
living today.
•  A series of fossils traces the gradual modification of
jaws and teeth in the evolution of mammals from a
reptilian ancestor.
•  A series of fossils documents the evolution of
whales from a group of land mammals.
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  Thousands of fossil discoveries have since shed
light on the evolutionary origins of many groups of
plants and animals, including
•  the transition of fish to amphibian,
•  the origin of birds from a lineage of dinosaurs, and
•  the evolution of mammals from a reptilian ancestor.
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  Whales are cetaceans, a group that also includes
dolphins and porpoises.
•  They have forelimbs in the form of flippers but lack
hind limbs.
•  If cetaceans evolved from four-legged land animals,
then transitional forms should have reduced hind
limb and pelvic bones.
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Figure 13.3a
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  Beginning in the late 1970s, paleontologists
unearthed an extraordinary series of transitional
fossils in Pakistan and Egypt.
•  The new fossil discoveries were consistent with the
earlier hypothesis, and paleontologists became
more firmly convinced that whales did indeed arise
from a wolf-like carnivore.
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Figure 13.3b
Pakicetus
Rodhocetus
Dorudon
Living
cetaceans
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Key
Pelvis
Femur
Tibia
Foot
13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  But molecular biologists
•  found a close relationship between whales and
hippopotamuses and
•  hypothesized that whales and hippos were both
descendants of a cloven-hoofed ancestor.
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13.3 SCIENTIFIC THINKING: Fossils of
transitional forms support Darwin’s theory of
evolution
•  Paleontologists were taken aback by the
contradictory results, but openness to new
evidence is a hallmark of science.
•  Two fossils discovered in 2001 provided the
answer.
•  Both Pakicetus and Rodhocetus had the distinctive
ankle bone of a cloven-hoofed mammal.
•  Thus, as is often the case in science, scientists are
becoming more certain about the evolutionary origin
of whales as mounting evidence from different lines
of inquiry converge.
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13.4 Homologies provide strong evidence for
evolution
•  Evolution is a process of descent with modification.
•  Characteristics present in an ancestral organism
are altered over time by natural selection as its
descendants face different environmental
conditions.
•  Evolution is a remodeling process.
•  Related species can have characteristics that have
an underlying similarity yet function differently.
•  Similarity resulting from common ancestry is known
as homology.
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13.4 Homologies provide strong evidence for
evolution
•  Darwin cited the anatomical similarities among
vertebrate forelimbs as evidence of common
ancestry.
•  As Figure 13.4A shows, the same skeletal
elements make up the forelimbs of humans, cats,
whales, and bats, but the functions of these
forelimbs differ.
•  Biologists call such anatomical similarities in
different organisms homologous structures.
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Figure 13.4a
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
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Cat
Whale
Bat
13.4 Homologies provide strong evidence for
evolution
•  Molecular comparisons between diverse
organisms have allowed biologists to develop
hypotheses about the evolutionary divergence of
major branches on the tree of life.
•  Darwin’s boldest hypothesis was that all life-forms
are related. Molecular biology provides strong
evidence for this claim.
•  All forms of life use the same genetic language of
DNA and RNA.
•  The genetic code—how RNA triplets are translated
into amino acids—is essentially universal.
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13.4 Homologies provide strong evidence for
evolution
•  An understanding of homology helps explain why
•  early stages of development in different animal
species reveal similarities not visible in adult
organisms and
•  at some point in their development, all vertebrate
embryos have
•  a tail posterior to the anus, and
•  structures called pharyngeal (throat) pouches.
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Figure 13.4b-0
Pharyngeal
pouches
Post-anal
tail
Chick embryo
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Human embryo
13.4 Homologies provide strong evidence for
evolution
•  Some of the most interesting homologies are
“leftover” structures that are of marginal or perhaps
no importance to the organism.
•  These vestigial structures are remnants of
features that served important functions in the
organism’s ancestors.
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13.5 Homologies indicate patterns of descent
that can be shown on an evolutionary tree
•  Darwin was the first to view the history of life as a
tree, with multiple branches from a common
ancestral trunk to the descendant species at the
tips of the twigs.
•  Today, biologists represent these patterns of
descent with an evolutionary tree, although today
they often turn the trees sideways.
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13.5 Homologies indicate patterns of descent
that can be shown on an evolutionary tree
•  Homologous structures can be used to determine
the branching sequence of an evolutionary tree.
•  These homologies can include
•  anatomical structure and/or
•  molecular structure.
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Figure 13.5
Each branch point represents the
common ancestor of the lineages
beginning there and to the right of it
Lungfishes
Amniotes
Mammals
2
Tetrapod
limbs
Amnion
Lizards
and snakes
3
4
5
Ostriches
6
Feathers
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Hawks and
other birds
Birds
A hatch mark represents
a homologous character
shared by all the groups
to the right of the mark
Crocodiles
Tetrapods
Amphibians
1
13.6 Darwin proposed natural selection as
the mechanism of evolution
•  Darwin’s greatest contribution to biology was his
explanation of how life evolves.
•  Because he thought that species formed gradually
over long periods of time, he knew that he would
not be able to study the evolution of new species
by direct observation.
•  But insights into how incremental change occurs
could be seen in examples of artificial selection,
in which humans have modified species through
selective breeding.
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Figure 13.6-0
Frillback
Fantail
Rock pigeon
Old Dutch Capuchine
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Trumpeter
13.6 Darwin proposed natural selection as
the mechanism of evolution
•  Darwin knew that individuals in natural populations
have small but measurable differences.
•  But what forces in nature played the role of the
breeder, choosing which individuals became the
breeding stock for the next generation?
•  Darwin found inspiration in an essay written by
economist Thomas Malthus, who contended that
much of human suffering—disease, famine, and
war—was the consequence of human populations
increasing faster than food supplies and other
resources.
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13.6 Darwin proposed natural selection as
the mechanism of evolution
•  Darwin deduced that the production of more
individuals than the limited resources can support
leads to a struggle for existence, with only some
offspring surviving in each generation.
•  The essence of natural selection is this unequal
reproduction.
•  Individuals whose traits better enable them to
obtain food or escape predators or tolerate physical
conditions will survive and reproduce more
successfully, passing these adaptive traits to their
offspring.
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13.6 Darwin proposed natural selection as
the mechanism of evolution
•  Darwin reasoned that if artificial selection can bring
about so much change in a relatively short period
of time, then natural selection could modify species
considerably over hundreds or thousands of
generations.
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13.6 Darwin proposed natural selection as
the mechanism of evolution
•  It is important to emphasize three key points about
evolution by natural selection.
1.  Although natural selection occurs through
interactions between individual organisms and the
environment, individuals do not evolve. Rather, it
is the population, the group of organisms, that
evolves over time.
2.  Natural selection can amplify or diminish only
heritable traits.
3.  Evolution is not goal directed; it does not lead to
perfectly adapted organisms.
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13.7 Scientists can observe natural selection
in action
•  Another example of natural selection in action is the
evolution of pesticide resistance in insects.
•  A relatively small amount of poison initially kills most
of the insects, but subsequent applications are less
and less effective.
•  The few survivors are individuals that are genetically
resistant, carrying an allele that somehow enables
them to survive the chemical attack.
•  When these resistant insects reproduce, the
percentage of the population resistant to the
pesticide increases.
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Figure 13.7-0
Pesticide
application
Chromosome with
allele conferring
resistance to pesticide
Survivors
Additional applications of the
same pesticide will be less effective,
and the frequency of resistant insects
in the population will grow
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13.7 Scientists can observe natural selection
in action
•  These examples of evolutionary adaptation
highlight two important points about natural
selection.
1.  Natural selection is more of an editing process
than a creative mechanism.
2.  Natural selection is contingent on time and place,
favoring those heritable traits in a varying
population that fit the current, local environment.
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THE EVOLUTION OF POPULATIONS
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  Organisms typically show individual variation.
•  However, in The Origin of Species, Darwin could
not explain
•  the cause of variation among individuals or
•  how variations were passed from parents to
offspring.
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  Just a few years after the publication of The Origin
of Species, Gregor Mendel wrote a groundbreaking
paper on inheritance in pea plants.
•  Although the significance of Mendel’s work was not
recognized during his or Darwin’s lifetime, its
rediscovery in 1900 set the stage for
understanding the genetic differences on which
evolution is based.
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  Each person has a unique genome, reflected in
individual phenotypic variations, such as
appearance and other traits.
•  Indeed, individual variation occurs in all species.
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Figure 13.8
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  Mutations are
•  changes in the nucleotide sequence of DNA and
•  the ultimate source of new alleles.
•  Thus, mutation is the ultimate source of the genetic
variation that serves as raw material for evolution.
•  A change as small as a single nucleotide in a
protein-coding gene can have a significant effect
on phenotype, as in sickle-cell disease.
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  On rare occasions, however, a mutated allele may
actually improve the adaptation of an individual to
its environment and enhance its reproductive
success.
•  This kind of effect is more likely when the
environment is changing such that mutations that
were once disadvantageous are favorable under
new conditions.
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  In organisms that reproduce sexually, most of the
genetic variation in a population results from the
unique combination of alleles that each individual
inherits.
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13.8 Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
•  Fresh assortments of existing alleles arise every
generation from three random components of
sexual reproduction:
1.  crossing over,
2.  independent orientation of homologous
chromosomes at metaphase I of meiosis, and
3.  random fertilization.
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13.9 Evolution occurs within populations
•  A population is a group of individuals of the same
species, that live in the same area, and interbreed.
•  We can measure evolution as a change in the
prevalence of certain heritable traits in a population
over a span of generations.
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13.9 Evolution occurs within populations
•  Different populations of the same species may be
geographically isolated from each other to such an
extent that an exchange of genetic material never
occurs, or occurs only rarely.
•  Such isolation is common in populations confined
to different lakes.
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Figure 13.9
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13.9 Evolution occurs within populations
•  A gene pool consists of all copies of every type of
allele, at every locus, in all members of the
population.
•  Microevolution is
•  change in the relative frequencies of alleles in a
population over a number of generations and
•  evolution occurring on its smallest scale.
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13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  To understand how microevolution works, let’s first
examine a simple population in which evolution is
not occurring and thus the gene pool is not
changing.
•  Consider an imaginary population of iguanas with
individuals that differ in foot webbing.
•  Let’s assume that
•  foot webbing is controlled by a single gene and
•  the allele for nonwebbed feet (W) is completely
dominant to the allele for webbed feet (w).
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Figure 13.10a
No webbing
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Webbing
13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  The shuffling of alleles that accompanies sexual
reproduction does not alter the genetic makeup of
the population.
•  No matter how many times alleles are segregated
into different gametes, and united in different
combinations by fertilization, the frequency of each
allele in the gene pool will remain constant unless
other factors are operating.
•  This equilibrium is the Hardy-Weinberg principle,
named for the two scientists who derived it
independently in 1908.
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13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  To test the Hardy-Weinberg principle, let’s look at
two generations of our imaginary iguana
population.
•  Figure 13.10B shows the frequencies of alleles in
the gene pool of the original population.
•  From these genotype frequencies, we can
calculate the frequency of each allele in the
population.
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Figure 13.10b
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320
500
160
500
20
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800
1,000
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= 0.32
160 W + 160 w
= 0.8 W
200
1,000
= 0.04
40 w
= 0.2 w
13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  Figure 13.10C shows a Punnett square that uses
these gamete allele frequencies and the rule of
multiplication to calculate the frequencies of the
three genotypes in the next generation.
•  Because the genotype frequencies are the same
as in the parent population, the allele frequencies p
and q are also the same.
•  Thus, the gene pool of this population is in a state
of equilibrium—Hardy-Weinberg equilibrium.
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Figure 13.10c
Gametes reflect allele
frequencies of parental
gene pool
W
p = 0.8
Eggs
w
q = 0.2
Sperm
W
p = 0.8
w
q = 0.2
WW
p2 = 0.64
Ww
pq = 0.16
wW
qp = 0.16
ww
q2 = 0.04
Next generation:
Genotype frequencies 0.64 WW
Allele frequencies
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0.32 Ww
0.8 W
0.04 ww
0.2 w
13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  If a population is in Hardy-Weinberg equilibrium,
allele and genotype frequencies will remain
constant, generation after generation.
•  The Hardy-Weinberg principle tells us that
something other than the reshuffling processes of
sexual reproduction is required to change allele
frequencies in a population.
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13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  For a population to be in Hardy-Weinberg
equilibrium, it must satisfy five main conditions.
There must be
1. 
2. 
3. 
4. 
5. 
a very large population,
no gene flow between populations,
no mutations,
random mating, and
no natural selection.
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13.10 The Hardy-Weinberg equation can test
whether a population is evolving
•  Rarely are all five conditions met in real
populations, and thus allele and genotype
frequencies often do change.
•  The Hardy-Weinberg equation can be used to test
whether evolution is occurring in a population.
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13.11 CONNECTION: The Hardy-Weinberg
equation is useful in public health science
•  Public health scientists use the Hardy-Weinberg
equation to estimate how many people carry
alleles for certain inherited diseases.
•  One out of 10,000 babies born in the United States
has phenylketonuria (PKU), an inherited inability to
break down the amino acid phenylalanine.
•  The health problems associated with PKU can be
prevented by strict adherence to a diet that limits
the intake of phenylalanine.
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Figure 13.11
INGREDIENTS: SORBITOL,
MAGNESIUM STEARATE,
ARTIFICIAL FLAVOR,
ASPARTAME† (SWEETENER),
ARTIFICIAL COLOR
(YELLOW 5 LAKE, BLUE 1
LAKE), ZINC GLUCONATE.
†PHENYLKETONURICS:
CONTAINS PHENYLALANINE
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13.11 CONNECTION: The Hardy-Weinberg
equation is useful in public health science
•  PKU is a recessive allele.
•  The frequency of the recessive allele for PKU in
the population, q, equals the square root of 0.0001,
or 0.01.
•  The frequency of the dominant allele would equal
1 – q, or 0.99.
•  The frequency of carriers = 2pq = 2 × 0.99 × 0.01 =
0.0198 = 1.98% of the U.S. population.
•  Thus, the equation tells us that about 2% (actually
1.98%) of the U.S. population are carriers of the
PKU allele.
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MECHANISMS OF MICROEVOLUTION
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13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  If the five conditions for the Hardy-Weinberg
equilibrium are not met in a population, the allele
frequencies may change. However,
•  mutations are rare and random and have little effect
on the gene pool, and
•  nonrandom mating may change genotype
frequencies but usually has little impact on allele
frequencies.
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13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  The three main causes of evolutionary change are
1.  natural selection,
2.  genetic drift, and
3.  gene flow.
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13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
1.  Natural selection
•  If individuals differ in their survival and reproductive
success, natural selection will alter allele frequencies.
•  Consider the imaginary iguana population. Individuals
with webbed feet (genotype ww) might survive better
and produce more offspring because they are more
efficient at swimming and catching food than individuals
that lack webbed feet.
•  Genetic equilibrium would be disturbed as the frequency
of the w allele increased in the gene pool from one
generation to the next.
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13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
2.  Genetic drift
•  In a process called genetic drift, chance events
can cause allele frequencies to fluctuate
unpredictably from one generation to the next.
•  The smaller the population, the more impact genetic
drift is likely to have.
© 2015 Pearson Education, Inc.
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  Catastrophes such as hurricanes, floods, or fires
may kill large numbers of individuals, leaving a
small surviving population that is unlikely to have
the same genetic makeup as the original
population.
•  The bottleneck effect leads to a loss of genetic
diversity when a population is greatly reduced.
© 2015 Pearson Education, Inc.
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  Analogous to shaking just a few marbles through a
bottleneck, certain alleles may be present at higher
frequency in the surviving population than in the
original population, others may be present at lower
frequency, and some (orange marbles) may not be
present at all.
•  After a population is drastically reduced, genetic
drift may continue for many generations until the
population is again large enough for fluctuations
due to chance to have less of an impact.
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Animation: Causes of Evolutionary Change
© 2015 Pearson Education, Inc.
Figure 13.12a-1
Original
population
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Figure 13.12a-2
Original
population
© 2015 Pearson Education, Inc.
Bottlenecking
event
Figure 13.12a-3
Original
population
© 2015 Pearson Education, Inc.
Bottlenecking
event
Surviving
population
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  One reason it is important to understand the
bottleneck effect is that human activities such as
overhunting and habitat destruction may create
severe bottlenecks for other species.
•  Examples of species affected by bottlenecks
include the endangered Florida panther, the African
cheetah, and the greater prairie chicken.
© 2015 Pearson Education, Inc.
Figure 13.12b
© 2015 Pearson Education, Inc.
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  Genetic drift is also likely when a few individuals
colonize an island or other new habitat, producing
what is called the founder effect.
•  The smaller the group, the less likely the genetic
makeup of the colonists will represent the gene pool
of the larger population they left.
•  The founder effect explains the relatively high
frequency of certain inherited disorders among
some human populations established by small
numbers of colonists.
© 2015 Pearson Education, Inc.
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
3.  Gene flow
•  Allele frequencies in a population can also change
as a result of gene flow, where a population may
gain or lose alleles when fertile individuals move
into or out of a population or when gametes (such
as plant pollen) are transferred between
populations.
•  Gene flow tends to reduce differences between
populations.
© 2015 Pearson Education, Inc.
13.12 Natural selection, genetic drift, and
gene flow can cause microevolution
•  To counteract the lack of genetic diversity in the
remaining Illinois greater prairie chickens,
researchers added 271 birds from neighboring
states to the Illinois populations, which successfully
introduced new alleles.
•  This strategy worked. New alleles entered the
population, and the egg-hatching rate improved to
more than 90%.
© 2015 Pearson Education, Inc.
13.13 Natural selection is the only
mechanism that consistently leads to
adaptive evolution
•  Genetic drift, gene flow, and mutations could each
result in microevolution, but only by chance could
these events improve a population’s fit to its
environment.
•  In natural selection, only the genetic variation
produced by mutation and sexual reproduction results
from random events.
•  The process of natural selection, in which some
individuals are more likely than others to survive and
reproduce, is not random.
•  Because of this sorting, only natural selection
consistently leads to adaptive evolution.
© 2015 Pearson Education, Inc.
13.13 Natural selection is the only
mechanism that consistently leads to
adaptive evolution
•  The adaptations of organisms include many striking
examples.
•  Consider some of the features that make the bluefooted booby suited to its home on the Galapagos
Islands.
•  The bird’s large, webbed feet make great flippers,
propelling the bird through the water at high speeds.
•  Its body and bill are streamlined, minimizing friction as it
dives from heights up to 24 m (over 75 feet) into the
shallow water below.
•  To pull out of this high-speed dive once it hits the water,
the booby uses its large tail as a brake.
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Figure 13.13
© 2015 Pearson Education, Inc.
13.13 Natural selection is the only
mechanism that consistently leads to
adaptive evolution
•  Let’s take a closer look at natural selection.
•  The commonly used phrases “struggle for
existence” and “survival of the fittest” are
misleading if we take them to mean direct
competition between individuals. Reproductive
success is generally more subtle and passive.
•  Relative fitness is the contribution an individual
makes to the gene pool of the next generation
relative to the contributions of other individuals.
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13.14 Natural selection can alter variation in a
population in three ways
•  Natural selection can affect the distribution of
phenotypes in a population.
•  Stabilizing selection favors intermediate
phenotypes.
•  Directional selection shifts the overall makeup of
the population by acting against individuals at one
of the phenotypic extremes.
•  Disruptive selection typically occurs when
environmental conditions vary in a way that favors
individuals at both ends of a phenotypic range over
individuals with intermediate phenotypes.
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Figure 13.14
Frequency of
individuals
Original population
Original
population
Evolved
population
Stabilizing selection
© 2015 Pearson Education, Inc.
Phenotypes (fur color)
Directional selection
Disruptive selection
13.15 Sexual selection may lead to
phenotypic differences between males and
females
•  Sexual selection is a form of natural selection in
which individuals with certain characteristics are
more likely than other individuals to obtain mates.
•  In many animal species, males and females may
have secondary sexual characteristics, noticeable
differences not directly associated with
reproduction or survival, called sexual
dimorphism.
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Figure 13.15a
© 2015 Pearson Education, Inc.
13.15 Sexual selection may lead to
phenotypic differences between males and
females
•  In some species, individuals compete directly with
members of the same sex for mates.
•  This type of sexual selection is called intrasexual
selection (within the same sex, most often the
males).
•  Contests may involve physical combat, but are
more often ritualized displays.
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Figure 13.15b
© 2015 Pearson Education, Inc.
13.15 Sexual selection may lead to
phenotypic differences between males and
females
•  In a more common type of sexual selection, called
intersexual selection (between sexes) or mate
choice, individuals of one sex (usually females) are
choosy in selecting their mates.
© 2015 Pearson Education, Inc.
13.15 Sexual selection may lead to
phenotypic differences between males and
females
•  What is the advantage to females of being choosy?
•  One hypothesis is that females prefer male traits
that are correlated with “good genes.”
•  In several bird species, research has shown that
traits preferred by females, such as bright beaks or
long tails, are related to overall male health.
•  The “good genes” hypothesis was also tested in
gray tree frogs. Female frogs prefer to mate with
males that give long mating calls.
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Figure 13.15c
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13.16 EVOLUTION CONNECTION: The
evolution of drug-resistant microorganisms
is a serious public health concern
•  Antibiotics are drugs that kill infectious
microorganisms.
•  During the 1950s, some medical experts even
thought the age of human infectious diseases
would soon be over.
•  Why didn’t that optimistic forecast come true?
•  Answer: It did not take into account the force of
evolution.
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13.16 EVOLUTION CONNECTION: The
evolution of drug-resistant microorganisms
is a serious public health concern
•  In the same way that pesticides select for resistant
insects, antibiotics select for resistant bacteria.
•  Again we see both the random and nonrandom
aspects of natural selection: the random genetic
mutations in bacteria and the nonrandom selective
effects as the environment favors the antibioticresistant phenotype.
© 2015 Pearson Education, Inc.
13.16 EVOLUTION CONNECTION: The
evolution of drug-resistant microorganisms
is a serious public health concern
•  In what ways do we contribute to the problem of
antibiotic resistance?
•  Doctors may overprescribe antibiotics.
•  Patients may misuse prescribed antibiotics by
prematurely stopping the medication because they
feel better.
•  Livestock producers add antibiotics to animal feed
as a growth promoter and to prevent illness.
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13.16 EVOLUTION CONNECTION: The
evolution of drug-resistant microorganisms
is a serious public health concern
•  Each year in the United States nearly 100,000
people die from infections they contract in the
hospital, often because the bacteria are resistant to
multiple antibiotics.
•  A formidable “superbug” known as MRSA
(methicillin-resistant Staphylococcus aureus) can
cause “flesh-eating disease” and potentially fatal
systemic (whole-body) infections.
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Figure 13.16
© 2015 Pearson Education, Inc.
13.17 Diploidy and balancing selection
preserve genetic variation
•  What prevents natural selection from eliminating all
variation as it selects against unfavorable
genotypes?
•  Why aren’t less adaptive alleles eliminated as the
“best” alleles are passed to the next generation?
•  It turns out that the tendency for natural selection
to reduce variation in a population is countered by
mechanisms that maintain variation.
© 2015 Pearson Education, Inc.
13.17 Diploidy and balancing selection
preserve genetic variation
•  In some cases, genetic variation is preserved
rather than reduced by natural selection.
•  Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population.
© 2015 Pearson Education, Inc.
13.17 Diploidy and balancing selection
preserve genetic variation
•  Heterozygote advantage is a type of balancing
selection in which heterozygous individuals have
greater reproductive success than either type of
homozygote, with the result that two or more
alleles for a gene are maintained in the population.
© 2015 Pearson Education, Inc.
13.17 Diploidy and balancing selection
preserve genetic variation
•  Frequency-dependent selection is a type of
balancing selection that maintains two different
phenotypic forms in a population.
•  In this case, selection acts against either
phenotypic form if it becomes too common in the
population.
•  An example is a scale-eating fish in Lake
Tanganyika, Africa, which attacks other fish from
behind, darting in to remove a few scales from the
side of its prey.
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Figure 13.17
“Left-mouthed”
“Right-mouthed”
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13.18 Natural selection cannot fashion
perfect organisms
•  The evolution of organisms is constrained.
1.  Selection can act only on existing variations. New,
advantageous alleles do not arise on demand.
2.  Evolution is limited by historical constraints.
Evolution co-opts existing structures and adapts
them to new situations.
3.  Adaptations are often compromises. The same
structure often performs many functions.
4.  Chance, natural selection, and the environment
interact. Environments often change
unpredictably.
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