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Chapter 13
How Populations Evolve
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Persistent Pests
• Mosquitoes and malaria
– In the 1960s, the World Health Organization (WHO) began a campaign to
eradicate the mosquitoes that transmit malaria.
– It used DDT, to which some mosquitoes have evolved resistance.
© 2010 Pearson Education, Inc.
• The evolution of pesticide-resistant insects is just one of the ways
that evolution affects our lives.
• An understanding of evolution informs every field of biology, for
example:
– Agriculture
– Medicine
– Biotechnology
– Conservation biology
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• Charles Darwin published On the Origin of Species by Means of
Natural Selection, November 24, 1859.
• Darwin presented two main concepts:
– Life evolves
– Change occurs as a result of “descent with modification,” with natural
selection as the mechanism
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• Natural selection is a process in which organisms with certain
inherited characteristics are more likely to survive and reproduce
than are individuals with other characteristics.
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A Trinidad tree mantid that mimics dead leaves
Figure 13.1a
A leaf mantid in Costa Rica
Figure 13.1b
A flower mantid in Malaysia
Figure 13.1c
• Natural selection leads to:
– A population (a group of individuals of the same species living in the
same place at the same time) changing over generations
– Evolutionary adaptation
• In one modern definition of evolution, the genetic composition of
a population changes over time.
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1837
Darwin begins analyzing his
specimens and writing his
notebooks on the origin
of species.
1844
Darwin writes his essay
1865
on the origin of species.
Mendel publishes
papers on genetics.
1870
1800
1809
Lamarck
publishes
1830
his theory
Lyell publishes
of evolution. Principles of Geology.
1809
Charles Darwin
is born.
1831–36
Darwin travels
around the world
on the HMS Beagle.
1858
Wallace sends an
account of his
theory to Darwin.
1859
Darwin publishes
The Origin of Species.
Green sea turtle in the
Galápagos Islands
Figure 13.2
Green sea turtle in the Galápagos Islands
Figure 13.2a
Figure 13.2b
Figure 13.2c
Figure 13.2d
Darwin’s Cultural and Scientific Context
• The Origin of Species challenged the notion that the Earth was:
– Relatively young
– Populated by unrelated species
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The Idea of Fixed Species
• The Greek philosopher Aristotle held the belief that species are
fixed and do not evolve.
• The Judeo-Christian culture fortified this idea with a literal
interpretation of the Bible and suggested the Earth may only be
6,000 years old.
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Lamarck and Evolutionary Adaptations
• In the mid-1700s, the study of fossils began to take form as a
branch of science.
• Naturalist Georges Buffon noted that:
– The Earth may be more than 6,000 years old
– There are similarities between fossils and living species
– Fossil forms might be ancient versions of similar living species
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• Jean Baptiste Lamarck suggested that organisms evolved by the
process of adaptation by the inheritance of acquired
characteristics, now known to be incorrect.
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The Voyage of the Beagle
• Darwin was born on February 12, 1809, the same day that
Abraham Lincoln was born.
• In December 1831 Darwin left Great Britain on the HMS Beagle
on a five-year voyage around the world.
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Darwin in 1840
Figure 13.3a
HMS Beagle
Figure 13.3b
Galápagos
Islands
PACIFIC
OCEAN
Pinta
Genovesa
Marchena
Equator
Santiago
Daphne Islands
Pinzón
Fernandina
Isabela
0
0
40 km
Santa
Santa
Cruz
Fe
Florenza
San
Cristobal
Española
40 miles
Figure 13.3c
• On his journey on the Beagle, Darwin:
– Collected thousands of specimens
– Observed various adaptations in organisms
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• Darwin was intrigued by:
– The geographic distribution of organisms on the Galápagos Islands
– Similarities between organisms in the Galápagos and those in South
America
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Figure 13.4a
Figure 13.4b
• Darwin was strongly influenced by the writings of geologist
Charles Lyell.
• Lyell suggested that the Earth:
– Is very old
– Was sculpted by gradual geological processes that continue today
• Darwin applied Lyell’s principle of gradualism to the evolution of
life on Earth.
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Descent with Modification
• Darwin made two main points in The Origin of Species:
– Organisms inhabiting Earth today descended from ancestral species
– Natural selection was the mechanism for descent with modification
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EVIDENCE OF EVOLUTION
• Biological evolution leaves observable signs.
• We will examine five of the many lines of evidence in support of
evolution:
– The fossil record
– Biogeography
– Comparative anatomy
– Comparative embryology
– Molecular biology
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The Fossil Record
• Fossils are:
– Imprints or remains of organisms that lived in the past
– Often found in sedimentary rocks
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• The fossil record:
– Is the ordered sequence of fossils as they appear in rock layers
– Reveals the appearance of organisms in a historical sequence
– Fits the molecular and cellular evidence that prokaryotes are the ancestors
of all life
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Figure 13.5
• Paleontologists:
– Are scientists that study fossils
– Have discovered many transitional forms that link past and present
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Figure 13.6-3
Biogeography
• Biogeography is the study of the geographic distribution of
species that first suggested to Darwin that today’s organisms
evolved from ancestral forms.
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• Many examples from biogeography would be difficult to
understand, except from an evolutionary perspective.
• One example is the distribution of marsupial mammals in
Australia.
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Australia
Common
ringtail
possum
Koala
Common wombat
Red kangaroo
Figure 13.7
Comparative Anatomy
• Comparative anatomy
– Is the comparison of body structure between different species
– Confirms that evolution is a remodeling process
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• Homology is:
– The similarity in structures due to common ancestry
– Illustrated by the remodeling of the pattern of bones forming the
forelimbs of mammals
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Human
Cat
Whale
Bat
Figure 13.8
• Vestigial structures:
– Are remnants of features that served important functions in an organism’s
ancestors
– Now have only marginal, if any, importance
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Comparative Embryology
• Early stages of development in different animal species reveal
additional homologous relationships.
– For example, pharyngeal pouches appear on the side of the embryo’s
throat, which:
–
Develop into gill structures in fish
–
Form parts of the ear and throat in humans
– Comparative embryology of vertebrates supports evolutionary theory.
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Pharyngeal
pouches
Post-anal
tail
Chicken embryo
Human embryo
Figure 13.9
Molecular Biology
• The hereditary background of an organism is documented in:
– Its DNA
– The proteins encoded by the DNA
• Evolutionary relationships among species can be determined by
comparing:
– Genes
– Proteins of different organisms
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Primate
Percent of selected DNA sequences
that match a chimpanzee’s DNA
92%
96%
100%
Chimpanzee
Human
Gorilla
Orangutan
Gibbon
Old World
monkey
Figure 13.10
NATURAL SELECTION
• Darwin noted the close relationship between adaptation to the
environment and the origin of new species.
• The evolution of finches on the Galápagos Islands is an excellent
example.
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(a) The large
ground finch
(b) The small tree finch
(c) The woodpecker finch
Figure 13.11
Darwin’s Theory of Natural Selection
• Darwin based his theory of natural selection on two key
observations:
– All species tend to produce excessive numbers of offspring
– Organisms vary, and much of this variation is heritable
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• Observation 1: Overproduction
– All species tend to produce excessive numbers.
– This leads to a struggle for existence.
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Spore
cloud
Figure 13.12
• Observation 2: Individual variation
– Variation exists among individuals in a population.
– Much of this variation is heritable.
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Figure 13.13
• Inference: Differential reproductive success
(natural selection)
– Those individuals with traits best suited to the local environment
generally leave a larger share of surviving, fertile offspring.
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Natural Selection in Action
• Examples of natural selection include:
– Pesticide-resistant insects
– Antibiotic-resistant bacteria
– Drug-resistant strains of HIV
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Insecticide application
Chromosome with gene
conferring resistance
to pesticide
Survivors
Reproduction
Figure 13.14-3
The Process of Science: Does Predation
Drive the Evolution of Lizard Horn Length?
• Observation: Flat-tailed horned lizards defend against attack by:
– Thrusting their heads backward
– Stabbing a shrike with the spiked horns on the rear of their skull
• Question: Are longer horns a survival advantage?
• Hypothesis: Longer horns are a survival advantage.
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Live
(a) A flat-tailed horned lizard
Length (mm)
Killed
20
10 Killed Live
0
Rear horns
(b) The remains of a lizard impaled
by a shrike
Side horns
(tip to tip)
(c) Results of measurement of lizard horns
Figure 13.15
• Prediction: Live horned lizards have longer horn lengths than
dead ones.
• Experiment: Measure the horn lengths of dead and living lizards.
• Results: The average horn length of live lizards is about 10%
longer than that of dead lizards.
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Live
Killed
Length (mm)
20
10
Killed
Live
0
Rear horns
Side horns
(tip to tip)
(c) Results of measurement of lizard horns
Figure 13.15c
EVOLUTIONARY TREES
• Darwin saw the history of life as analogous to a tree:
– The first forms of life on Earth form the common trunk
– At each fork is the last common ancestor to all the branches extending
from that fork
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Lungfishes
Amniotes
Mammals
Tetrapod
limbs
Lizards
and snakes
Amnion
Tetrapods
Amphibians
Crocodiles
Feathers
Birds
Ostriches
Hawks and
other birds
Figure 13.16
The Modern Synthesis:
Darwinism Meets Genetics
• The modern synthesis is the fusion of genetics with evolutionary
biology.
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Populations as the Units of Evolution
• A population is:
– A group of individuals of the same species, living in the same place, at
the same time
– The smallest biological unit that can evolve
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(a) Two dense populations of
trees separated by a lake
Figure 13.17a
(b) A nighttime satellite view of North America
Figure 13.17b
• The total collection of alleles in a population at any one time is
the gene pool.
• When the relative frequency of alleles changes over a number of
generations, evolution is occurring on its smallest scale, which is
sometimes called microevolution.
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Genetic Variation in Populations
• Individual variation abounds in populations.
– Not all variation in a population is heritable.
– Only the genetic component of variation is relevant to natural selection.
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• Variable traits in a population may be:
– Polygenic, resulting from the combined effects of several genes or
– Determined by a single gene
• Polygenic traits tend to produce phenotypes that vary more or less
continuously.
• Single gene traits tend to produce only a few distinct phenotypes.
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Figure 13.18
Sources of Genetic Variation
• Genetic variation results from:
– Mutations, changes in the DNA of an organism
– Sexual recombination, the shuffling of alleles during meiosis
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• For any one gene, mutation alone has little effect on a large
population in a single generation.
• Organisms with very short generation spans, such as bacteria, can
evolve rapidly with mutations as the only source of genetic
variation.
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Analyzing Gene Pools
• The gene pool is a reservoir from which the next generation draws
its genes.
• Alleles in a gene pool occur in certain frequencies.
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• Alleles can be symbolized by:
– p for the relative frequency of the dominant allele in the population
– q for the frequency of the recessive allele in the population
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• Genotype frequencies:
– Can be calculated from allele frequencies
– Are symbolized by the expressions p2, 2pq, and q2
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Figure 13.19
Allele frequencies
p  0.8
(R)
q  0.2
(r)
Eggs
R
R
r
p  0.8
q  0.2
RR
2
p  0.64
Rr
pq  0.16
rR
qp  0.16
rr
2
q  0.04
p  0.8
Sperm
r
q  0.2
Genotype frequencies
p2  0.64
(RR)
2pq  0.32
(Rr)
q2  0.04
(rr)
Figure 13.20
• The Hardy-Weinberg formula can be used to calculate the
frequencies of genotypes in a gene pool from the frequencies of
alleles.
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Population Genetics and Health Science
• The Hardy-Weinberg formula can be used to calculate the
percentage of a human population that carries the allele for a
particular inherited disease.
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• PKU:
– Is a recessive allele that prevents the breakdown of the amino acid
phenylalanine
– Occurs in about one out of every 10,000 babies born in the United States
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INGREDIENTS: SORBITOL,
MAGNESIUM STEARATE,
ARTIFICIAL FLAVOR,
ASPARTAME† (SWEETENER),
ARTIFICIAL COLOR
(YELLOW 5 LAKE, BLUE 1
LAKE), ZINC GLUCONATE.
†PHENYLKETONURICS:
CONTAINS PHENYLALANINE
Figure 13.21
Microevolution as Change in a Gene Pool
• How can we tell if a population is evolving?
• A non-evolving population is in genetic equilibrium, called the
Hardy-Weinberg equilibrium, in which the population gene pool
remains constant over time.
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• From a genetic perspective evolution can be defined as a
generation-to-generation change in a population’s frequencies of
alleles, sometimes called microevolution.
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MECHANISMS OF EVOLUTION
• The main causes of evolutionary change are:
– Genetic drift
– Gene flow
– Natural selection
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Genetic Drift
• Genetic drift is:
– A change in the gene pool of a small population
– Due to chance
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rr
RR
RR
Only 5 of
10 plants
leave
offspring
Rr
RR
rr
Rr
rr
RR
Rr
rr
Rr
Rr
Generation 1
p (frequency of R)  0.7
q (frequency of r)  0.3
Only 2 of
10 plants
leave
offspring
RR
RR
RR
RR
RR
Rr
RR
RR
RR
RR
RR
RR
RR
Rr
Rr
Generation 2
p  0.5
q  0.5
RR
RR
Generation 3
p  1.0
q  0.0
Figure 13.22-3
The Bottleneck Effect
• The bottleneck effect:
– Is an example of genetic drift
– Results from a drastic reduction in population size
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Original
population
Bottlenecking
event
Surviving
population
Figure 13.23-3
• Bottlenecking in a population usually reduces genetic variation
because at least some alleles are likely to be lost from the gene
pool.
• Cheetahs appear to have experienced at least two genetic
bottlenecks in the past 10,000 years.
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Figure 13.24
The Founder Effect
• The founder effect is likely when a few individuals colonize an
isolated habitat and represent genetic drift in a new colony.
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• The founder effect explains the relatively high frequency of
certain inherited disorders among some small human populations.
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Africa
South
America
Tristan da
Cunha
Figure 13.25
Gene Flow
• Gene flow:
– Is genetic exchange with another population
– Tends to reduce genetic differences between populations
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Figure 13.26
Natural Selection: A Closer Look
• Of all causes of microevolution, only natural selection promotes
adaptation.
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Darwinian Fitness
• 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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Figure 13.27
Three General Outcomes of Natural Selection
• Directional selection:
– Shifts the phenotypic “curve” of a population
– Selects in favor of some extreme phenotype
• Disruptive selection can lead to a balance between two or more
contrasting phenotypic forms in a population.
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• Stabilizing selection:
– Favors intermediate phenotypes
– Is the most common
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Frequency
of individuals
Original
population
Evolved
population
(a) Directional selection
Original
population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Figure 13.28
Sexual Selection
• Sexual dimorphism is:
– A distinction in appearance between males and females
– Not directly associated with reproduction or survival
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• Sexual selection is a form of natural selection in which inherited
characteristics determine mating preferences.
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(a) Sexual dimorphism in a finch species
Figure 13.29a
(b) Competing for mates
Figure 13.29b
Evolution Connection:
The Genetics of the Sickle-Cell Allele
• Sickle-cell disease:
– Is a genetic disorder
– Affects about one out of every 400 African-Americans
• Abnormally shaped red blood cells cause painful and lifethreatening complications.
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• Heterozygous individuals for the sickle-cell allele:
– Do not develop sickle-cell anemia
– Are more resistant to malaria
• In the African tropics, where malaria is most common, the
frequency of the sickle-cell allele is highest.
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Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
Areas with high
incidence of
malaria
7.5–10.0%
10.0–12.5%
12.5%
Figure 13.30a
Colorized SEM
Figure 13.30b