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Chapter 13
How Populations Evolve
PowerPoint® Lectures for
Campbell Essential Biology, Fifth Edition, and
Campbell Essential Biology with Physiology,
Fourth Edition
– Eric J. Simon, Jean L. Dickey, and Jane B. Reece
Lectures by Edward J. Zalisko
© 2013 Pearson Education, Inc.
Biology and Society:
Mosquitoes, Microbes, and Malaria
•  In the 1960s, the World Health Organization
(WHO) launched a campaign to eradicate the
mosquitoes that transmit malaria.
•  It used DDT, to which some mosquitoes have
evolved resistance.
© 2013 Pearson Education, Inc.
Figure 13.0
Biology and Society:
Mosquitoes, Microbes, and Malaria
•  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,
–  medicine,
–  agriculture,
–  biotechnology, and
–  conservation biology.
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  Biology came of age on November 24, 1859.
Charles Darwin published On the Origin of Species
by Means of Natural Selection, an assemblage of
facts about the natural world.
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  Darwin made three observations from these facts.
1.  Life shows rich diversity.
2.  There are similarities in life that allow the
classification of organisms into groups nested
within broader groups.
3.  Organisms display many striking ways in which
they are suited for their environments.
© 2013 Pearson Education, Inc.
Figure 13.1
(b) Patterns of similarities
(a) The diversity of life
(c) An insect suited to
its environment
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  In The Origin of Species, Darwin
–  proposed a hypothesis, a scientific explanation for
his observations,
–  used hundreds of pages in his book to describe the
evidence supporting his hypothesis,
–  made testable predictions, and
–  reported the results of numerous experiments he
had performed.
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  Darwin hypothesized that
–  present-day species are the descendents of
ancient ancestors that they still resemble in some
ways and
–  change occurs as a result of “descent with
modification,” with natural selection as the
mechanism.
© 2013 Pearson Education, Inc.
Figure 13.2
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  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.
•  As a result of natural selection, a population, a
group of individuals of the same species living in
the same place at the same time, changes over
generations.
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  Natural selection leads to evolutionary adaptation,
a population’s increase in the frequency of traits
suited to the environment.
•  Natural selection thus leads to evolution, seen
either as
–  a change in the genetic composition of a population
over time or
–  on a grander scale, the entire biological history, from
the earliest microbes to the enormous diversity of
organisms that live on Earth today.
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CHARLES DARWIN AND THE ORIGIN OF
SPECIES
•  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 and
–  evolutionary adaptation.
•  In one modern definition of evolution, the genetic
composition of a population changes over time.
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Darwin’s Cultural and Scientific Context
•  The Origin of Species was fundamentally different
from the prevailing scientific and cultural views of
Darwin’s time.
•  Let’s place Darwin’s ideas in their historical
context.
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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 biblical book of
Genesis and
–  the suggestion that Earth may only be 6,000 years
old.
•  Naturalists were grappling with the interpretation of
fossils, imprints or remains of organisms that lived
in the past.
© 2013 Pearson Education, Inc.
Figure 13.3
(a) “Snakestone”
(b) Ichthyosaur skull and paddle-like forelimb
Lamarck and Evolutionary Adaptations
•  Naturalists compared fossil forms with living
species and noted patterns of similarities and
differences.
•  In the early 1800s, French naturalist Jean Baptiste
Lamarck suggested that life evolves, and explained
this evolution as the refinement of traits that equip
organisms to perform successfully in their
environment.
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Lamarck and Evolutionary Adaptations
•  Lamarck suggested a mechanism that we now
know is wrong.
•  Lamarck proposed that by using or not using its
body parts, an individual may develop certain traits
that it passes on to its offspring, thus, acquired
traits are inherited.
•  Lamarck helped set the stage for Darwin by
proposing that species evolve as a result of
interactions between organisms and their
environment.
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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.
Video: Galápagos Islands Overview
© 2013 Pearson Education, Inc.
Figure 13.4
HMS Beagle
Darwin
in 1840
North
America
Great
Britain
Europe
Asia
ATLANTIC
OCEAN
Africa
Galápagos
Islands
PACIFIC
OCEAN
Pinta
Marchena
South
America
Genovesa
Equator
Santiago
Daphne Islands
Pinzón
Fernandina
Isabela
0
0
Equator
40 km
Santa
Cruz
Santa
Fe
Florenza
San
Cristobal
Australia
PACIFIC
OCEAN
Cape of
Good Hope
Cape Horn
Española
40 miles
Tierra del Fuego
Tasmania
New Zealand
Figure 13.4a
Darwin in 1840
Figure 13.4b
HMS Beagle
Figure 13.4c
Galápagos
Islands
PACIFIC
OCEAN
Pinta
Marchena
Genovesa
Equator
Santiago
Daphne Islands
Pinzón
Fernandina
Isabela
0
0
40 km
Santa
Cruz
Santa
Fe
Florenza
40 miles
San
Cristobal
Española
The Voyage of the Beagle
•  On his journey on the Beagle, Darwin
–  collected thousands of specimens and
–  observed various adaptations in organisms.
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The Voyage of the Beagle
•  Darwin was intrigued by
–  the geographic distribution of organisms on the
Galápagos Islands and
–  similarities between organisms in the Galápagos
and those in South America.
Video: Galápagos Marine Iguana
Video: Galápagos Sea Lion
Video: Galápagos Tortoise
© 2013 Pearson Education, Inc.
Figure 13.5
Figure 13.5a
Figure 13.5b
The Voyage of the Beagle
•  Darwin was strongly influenced by the writings of
geologist Charles Lyell.
•  Lyell suggested that Earth
–  is very old and
–  was sculpted by gradual geological processes that
continue today.
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The Voyage of the Beagle
•  Darwin reasoned that the extended time scale
would allow for gradual changes to occur
–  in species and
–  in geologic features.
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Descent with Modification
•  Darwin made two main points in The Origin of
Species.
1.  Organisms inhabiting Earth today descended from
ancestral species.
2.  Natural selection is the mechanism for descent
with modification.
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EVIDENCE OF EVOLUTION
•  Evolution leaves observable signs.
•  We will examine five of the many lines of evidence
in support of evolution:
1.  the fossil record,
2.  biogeography,
3.  comparative anatomy,
4.  comparative embryology, and
5.  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
•  The fossil record
–  is the ordered sequence of fossils as they appear
in rock layers,
–  reveals the appearance of organisms in a historical
sequence, and
–  fits with the molecular and cellular evidence that
prokaryotes are the ancestors of all life.
Video: Grand Canyon
© 2013 Pearson Education, Inc.
Figure 13.6
The Fossil Record
•  Paleontologists (scientists who study fossils) have
discovered many transitional forms that link past
and present.
•  Transitional fossils include evidence that
–  birds descended from one branch of dinosaurs and
–  whales descended from four-legged land
mammals.
© 2013 Pearson Education, Inc.
Figure 13.7-3
Biogeography
•  Biogeography, the study of the geographic
distribution of species, first suggested to Darwin
that today’s organisms evolved from ancestral
forms.
•  Darwin noted that Galápagos animals resembled
species of the South American mainland more than
they resembled animals on similar but distant
islands.
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Biogeography
•  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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Figure 13.8
Australia
Common
ringtail
possum
Koala
Common wombat
Red kangaroo
Comparative Anatomy
•  Vestigial structures
–  are remnants of features that served important
functions in an organism’s ancestors and
–  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 and
–  form parts of the ear and throat in humans.
–  Comparative embryology of vertebrates supports
evolutionary theory.
© 2013 Pearson Education, Inc.
Figure 13.10
Pharyngeal
pouches
Post-anal
tail
Chicken embryo
Human embryo
Molecular Biology
•  The hereditary background of an organism is
documented in
–  its DNA and
–  the proteins encoded by the DNA.
•  Evolutionary relationships among species can be
determined by comparing
–  genes and
–  proteins of different organisms.
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Figure 13.11
Primate
Percent of selected DNA
sequences that match a
chimpanzee’s DNA
92%
Chimpanzee
Human
Gorilla
Orangutan
Gibbon
Old World
monkey
96%
100%
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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Figure 13.12
(a) The large ground finch
(b) The warbler finch
(c) The woodpecker finch
Darwin’s Theory of Natural Selection
•  Darwin based his theory of natural selection on two
key observations.
1.  All species tend to produce excessive numbers of
offspring.
2.  Organisms vary, and much of this variation is
heritable.
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Darwin’s Theory of Natural Selection
•  Observation 1: Overproduction and
competition
–  All species have the potential to produce many
more offspring than the environment can support.
–  This leads to inevitable competition among
individuals.
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Darwin’s Theory of Natural Selection
•  Observation 2: Individual variation
–  Variation exists among individuals in a population.
–  Much of this variation is heritable.
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Figure 13.14
Darwin’s Theory of Natural Selection
•  Inference: Unequal 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, and
–  drug-resistant strains of HIV.
Blast Animation: Evidence for Evolution: Antibiotic Resistance in Bacteria
Blast Animation: Natural Selection
© 2013 Pearson Education, Inc.
Figure 13.15-3
Insecticide application
Chromosome with gene
conferring resistance
to pesticide
Survivors
Reproduction
Figure 13.16
(a) A flat-tailed horned lizard
Length (mm)
Killed
Live
20
10
0
Killed Live
Rear horns
Side horns
(tip to tip)
(b) The remains of a lizard impaled by a shrike (c) Results of measurement of lizard horns
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.
–  The tips of millions of twigs represent the species
living today.
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Figure 13.17
Common ancestor of
lineages to the right
Lungfishes
Tetrapods
Amniotes
Amphibians
1
Mammals
2
Tetrapod
limbs
Lizards
and snakes
3
Amnion
Crocodiles
4
Homologous trait
shared by all groups
to the right
Ostriches
6
Feathers
Hawks and
other birds
Birds
5
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 and
–  the smallest biological unit that can evolve.
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Populations as the Units of Evolution
•  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.
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Genetic Variation in Populations
•  Individual variation abounds in all species.
–  Not all variation in a population is heritable.
–  Only the genetic component of variation is relevant
to natural selection.
Animation: Genetic Variation from Sexual Recombination
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Genetic Variation in Populations
•  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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Sources of Genetic Variation
•  Genetic variation results from processes that both
involve randomness:
1.  mutations, changes in the nucleotide sequence of
DNA, and
2.  sexual recombination, the shuffling of alleles
during meiosis.
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Sources of Genetic Variation
•  For any given gene locus, 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 mutation as the
only source of genetic variation.
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Analyzing Gene Pools
•  A gene pool
–  consists of all the alleles in a population at any one
time and
–  is a reservoir from which the next generation draws
its alleles.
•  Alleles in a gene pool occur in certain frequencies.
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Analyzing Gene Pools
•  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, and
–  p + q = 1.
•  Note that if we know the frequency of either allele
in the gene pool, we can subtract it from 1 to
calculate the frequency of the other allele.
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Analyzing Gene Pools
•  Genotype frequencies can be calculated from allele
frequencies (if the gene pool is stable = not
evolving).
•  The Hardy-Weinberg formula
–  p2 + 2pq + q2 = 1
–  can be used to calculate the frequencies of
genotypes in a gene pool from the frequencies of
alleles.
© 2013 Pearson Education, Inc.
Figure 13.20
Figure 13.21
Allele frequencies
p = 0.8
(R)
q = 0.2
(r)
Eggs
R
R
r
p = 0.8
q = 0.2
RR
p2 = 0.64
Rr
pq = 0.16
rR
pq = 0.16
rr
q2 = 0.04
p2 = 0.64
(RR)
2pq = 0.32
(Rr)
p = 0.8
Sperm
r
q = 0.2
Genotype frequencies
q2 = 0.04
(rr)
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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Population Genetics and Health Science
•  PKU
–  is a recessive allele that prevents the breakdown of
the amino acid phenylalanine and
–  occurs in about one out of every 10,000 babies
born in the United States.
•  People with PKU must strictly regulate their dietary
intake of the amino acid phenylalanine.
© 2013 Pearson Education, Inc.
Figure 13.22
INGREDIENTS: SORBITOL,
MAGNESIUM STEARATE,
ARTIFICIAL FLAVOR,
ASPARTAME† (SWEETENER),
ARTIFICIAL COLOR
(YELLOW 5 LAKE, BLUE 1
LAKE), ZINC GLUCONATE.
†PHENYLKETONURICS:
CONTAINS PHENYLALANINE
Microevolution as Change in a Gene Pool
•  How can we tell if a population is evolving?
•  A non-evolving population is in genetic equilibrium,
also known as Hardy-Weinberg equilibrium,
meaning the population’s gene pool is constant
over time.
•  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, and
–  natural selection.
•  Natural selection is the most important, because it
is the only process that promotes adaptation.
© 2013 Pearson Education, Inc.
Genetic Drift
•  Genetic drift is a change in the gene pool of a
small population due to chance.
Bioflix Animation: Mechanisms of Evolution
Animation: Causes of Evolutionary Change
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Figure 13.23-3
Only 5 of 10
plants leave
offspring
RR
RR
Only 2 of 10
plants leave
offspring
Rr
Rr
RR
rr
rr
Rr
RR
rr
Rr
Rr
Generation 1
p = 0.7
q = 0.3
Rr
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
The Bottleneck Effect
•  The bottleneck effect
–  is an example of genetic drift and
–  results from a drastic reduction in population size.
•  Passing through a “bottleneck,” a severe reduction
in population size,
–  decreases the overall genetic variability in a
population because at least some alleles are lost
from the gene pool, and
–  results in a loss of individual variation and hence
adaptability.
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Figure 13.24-1
Original
population
Figure 13.24-2
Original
population
Bottleneck
event
Figure 13.24-3
Original
population
Bottleneck
event
Surviving
population
The Bottleneck Effect
•  Cheetahs appear to have experienced at least two
genetic bottlenecks:
1.  during the last ice age, about 10,000 years ago,
and
2.  during the 1800s, when farmers hunted the
animals to near extinction.
•  With so little variability, cheetahs today have a
reduced capacity to adapt to environmental
challenges.
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Figure 13.25
The Founder Effect
•  The founder effect is likely when a few individuals
colonize an isolated habitat.
•  This represents genetic drift in a new colony.
•  The founder effect explains the relatively high
frequency of certain inherited disorders in some
small human populations.
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Figure 13.26
Africa
South
America
Tristan da Cunha
Figure 13.26a
Figure 13.26b
Africa
South
America
Tristan da Cunha
Gene Flow
•  Gene flow
–  is another source of evolutionary change,
–  is separate from genetic drift,
–  is genetic exchange with another population,
–  may result in the gain or loss of alleles, and
–  tends to reduce genetic differences between
populations.
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Natural Selection: A Closer Look
•  Of all causes of microevolution, only natural
selection promotes adaptation.
•  Evolutionary adaptation results from
–  chance, in the random generation of genetic
variability, and
–  sorting, in the unequal reproductive success
among the varying individuals.
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Figure 13.30
(a) Sexual dimorphism in a finch species (b) Competing for mates
Evolution Connection:
An Evolutionary Response to Malaria
•  We can see the results of past natural selection in
present-day humans.
•  Malaria first emerged as a serious threat to people
in Africa just 10,000 years ago,
–  long after humans had established populations
around the globe,
–  therefore only producing evolutionary responses in
malarial regions.
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Evolution Connection:
An Evolutionary Response to Malaria
•  Sickle hemoglobin
–  is a mutation that denies the malarial parasite
essential access to human hemoglobin and
–  distorts the shape of red blood cells.
•  Individuals with one copy of this sickle allele
(heterozygotes) are relatively resistant to malaria.
•  Individuals with two copies (homozygotes) are
usually fatally ill.
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Evolution Connection:
An Evolutionary Response to Malaria
•  In the African tropics,
–  malaria is most common and
–  the frequency of the sickle-cell allele is highest.
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Colorized SEM
Figure 13.31
Asia
Africa
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%
Colorized SEM
Figure 13.31b
Figure 13.UN02
Frequency of
homozygotes
for one allele
Frequency of
heterozygotes
Frequency of
homozygotes
for alternate allele
Figure 13.UN09
Observations
Overproduction
of offspring
Individual
variation
Conclusion
Natural selection:
unequal reproductive success