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Chapter 22
Descent with Modification:
A Darwinian View of Life
Overview: Endless Forms Most Beautiful
• A new era of biology began in 1859 when
Charles Darwin published The Origin of
Species
• The Origin of Species focused biologists’
attention on the great diversity of organisms
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-1
• Darwin noted that current species are
descendants of ancestral species
• Evolution can be defined by Darwin’s phrase
descent with modification
• Evolution can be viewed as both a pattern and
a process
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Concept 22.1: The Darwinian revolution challenged
traditional views of a young Earth inhabited by
unchanging species
• To understand why Darwin’s ideas were
revolutionary, we must examine them in
relation to other Western ideas about Earth
and its life
Fig. 22-2
Linnaeus (classification)
Hutton (gradual geologic change)
Lamarck (species can change)
Malthus (population limits)
Cuvier (fossils, extinction)
Lyell (modern geology)
Darwin (evolution, natural selection)
American Revolution
1750
Wallace (evolution, natural selection)
French Revolution
U.S. Civil War
1800
1900
1850
1795 Hutton proposes his theory of gradualism.
1798 Malthus publishes “Essay on the Principle of Population.”
1809 Lamarck publishes his hypothesis of evolution.
1830 Lyell publishes Principles of Geology.
1831–1836 Darwin travels around the world on HMS Beagle.
1837 Darwin begins his notebooks.
1844 Darwin writes essay on descent with modification.
1858 Wallace sends his hypothesis to Darwin.
1859 The Origin of Species is published.
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1
Scala Naturae and Classification of Species
• The Greek philosopher Aristotle viewed
species as fixed and arranged them on a scala
naturae
• Carolus Linnaeus interpreted organismal
adaptations as evidence that the Creator had
designed each species for a specific purpose
• The Old Testament holds that species were
individually designed by God and therefore
perfect
• Linnaeus was the founder of taxonomy, the
branch of biology concerned with classifying
organisms
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Ideas About Change over Time
Fig. 22-3
• The study of fossils helped to lay the
groundwork for Darwin’s ideas
Layers of deposited
sediment
• Fossils are remains or traces of organisms
from the past, usually found in sedimentary
rock, which appears in layers or strata
Younger stratum
with more recent
fossils
Video: Grand Canyon
Older stratum
with older fossils
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• Paleontology, the study of fossils, was largely
developed by French scientist Georges Cuvier
• Cuvier advocated catastrophism, speculating
that each boundary between strata represents
a catastrophe
• Geologists James Hutton and Charles Lyell
perceived that changes in Earth’s surface can
result from slow continuous actions still
operating today
• Lyell’s principle of uniformitarianism states
that the mechanisms of change are constant
over time
• This view strongly influenced Darwin’s thinking
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Lamarck’s Hypothesis of Evolution
Fig. 22-4
• Lamarck hypothesized that species evolve
through use and disuse of body parts and the
inheritance of acquired characteristics
• The mechanisms he proposed are
unsupported by evidence
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Concept 22.2: Descent with modification by natural
selection explains the adaptations of organisms and
the unity and diversity of life
• As the 19th century dawned, it was generally
believed that species had remained unchanged
since their creation
• However, a few doubts about the permanence
of species were beginning to arise
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Darwin’s Research
• As a boy and into adulthood, Charles Darwin
had a consuming interest in nature
• Darwin first studied medicine (unsuccessfully),
and then theology at Cambridge University
• After graduating, he took an unpaid position as
naturalist and companion to Captain Robert
FitzRoy for a 5-year around the world voyage
on the Beagle
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The Voyage of the Beagle
• During his travels on the Beagle, Darwin
collected specimens of South American plants
and animals
• He observed adaptations of plants and animals
that inhabited many diverse environments
• His interest in geographic distribution of
species was kindled by a stop at the
Galápagos Islands near the equator west of
South America
• Darwin was influenced by Lyell’s Principles of
Geology and thought that the earth was more
than 6000 years old
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Fig. 22-5
Fig. 22-5a
GREAT
BRITAIN
EUROPE
NORTH
AMERICA
ATLANTIC
OCEAN
The
Galápagos
Islands
AFRICA
Pinta
Santiago
Fernandina
Genovesa
Daphne
Islands
Pinzón
Isabela
Santa Santa
Cruz
Fe
Florenza
Equator
SOUTH
AMERICA
PACIFIC
OCEAN
San
Cristobal
AUSTRALIA
Andes
Marchena
Cape of
Good Hope
Tasmania
Española
Cape Horn
New
Zealand
Tierra del Fuego
Darwin in 1840
Fig. 22-5b
The
Galápagos
Islands
Video: Galápagos Islands Overview
Pinta
Marchena
Genovesa
Video: Blue-footed Boobies Courtship Ritual
Video: Albatross Courtship Ritual
Santiago
Fernandina
Pinzón
Isabela
Daphne
Islands
Video: Soaring Hawk
Santa Santa
Cruz
Fe
Florenza
Video: Galápagos Sea Lion
Video: Galápagos Tortoises
San
Cristobal
Video: Galápagos Marine Iguana
Española
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Darwin’s Focus on Adaptation
Fig. 22-6
• In reassessing his observations, Darwin
perceived adaptation to the environment and
the origin of new species as closely related
processes
• From studies made years after Darwin’s
voyage, biologists have concluded that this is
indeed what happened to the Galápagos
finches
(a) Cactus-eater
(c) Seed-eater
(b) Insect-eater
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The Origin of Species
• In 1844, Darwin wrote an essay on the origin of
species and natural selection but did not
introduce his theory publicly, anticipating an
uproar
• In June 1858, Darwin received a manuscript
from Alfred Russell Wallace, who had
developed a theory of natural selection similar
to Darwin’s
• Darwin developed two main ideas:
– Descent with modification explains life’s unity
and diversity
– Natural selection is a cause of adaptive
evolution
• Darwin quickly finished The Origin of Species
and published it the next year
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Descent with Modification
• Darwin never used the word evolution in the
first edition of The Origin of Species
• In the Darwinian view, the history of life is like a
tree with branches representing life’s diversity
• The phrase descent with modification
summarized Darwin’s perception of the unity of
life
• Darwin’s theory meshed well with the hierarchy
of Linnaeus
• The phrase refers to the view that all
organisms are related through descent from an
ancestor that lived in the remote past
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-7
Fig. 22-8
Hyracoidea
(Hyraxes)
Sirenia
(Manatees
and relatives)
Moeritherium
Barytherium
Deinotherium
Mammut
Platybelodon
Stegodon
Mammuthus
Elephas maximus
(Asia)
Loxodonta
africana
(Africa)
Loxodonta cyclotis
(Africa)
34
24
Millions of years ago
5.5
2 104 0
Years ago
5
Fig. 22-8a
Artificial Selection, Natural Selection, and
Adaptation
Platybelodon
Stegodon
Mammuthus
Elephas maximus
(Asia)
Loxodonta
africana
(Africa)
• Darwin noted that humans have modified other
species by selecting and breeding individuals
with desired traits, a process called artificial
selection
• Darwin then described four observations of
nature and from these drew two inferences
Loxodonta cyclotis
(Africa)
34
24
5.5
2 104 0
Years ago
Millions of years ago
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Fig. 22-9
Terminal
bud
Cabbage
Lateral
buds
Flower
clusters
Brussels sprouts
• Observation #1: Members of a population often
vary greatly in their traits
Leaves
Kale
Cauliflower
Stem
Wild mustard
Flowers
and stems
Broccoli
Kohlrabi
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Fig. 22-10
• Observation #2: Traits are inherited from
parents to offspring
• Observation #3: All species are capable of
producing more offspring than the environment
can support
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Fig. 22-11
Spore
cloud
• Observation #4: Owing to lack of food or other
resources, many of these offspring do not
survive
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• Inference #1: Individuals whose inherited traits
give them a higher probability of surviving and
reproducing in a given environment tend to
leave more offspring than other individuals
• Inference #2: This unequal ability of individuals
to survive and reproduce will lead to the
accumulation of favorable traits in the
population over generations
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Natural Selection: A Summary
• Darwin was influenced by Thomas Malthus
who noted the potential for human population
to increase faster than food supplies and other
resources
• If some heritable traits are advantageous,
these will accumulate in the population, and
this will increase the frequency of individuals
with adaptations
• This process explains the match between
organisms and their environment
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• Individuals with certain heritable characteristics
survive and reproduce at a higher rate than
other individuals
• Natural selection increases the adaptation of
organisms to their environment over time
• If an environment changes over time, natural
selection may result in adaptation to these new
conditions and may give rise to new species
Video: Seahorse Camouflage
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Fig. 22-12
(a) A flower mantid
in Malaysia
• Note that individuals do not evolve; populations
evolve over time
• Natural selection can only increase or
decrease heritable traits in a population
(b) A stick mantid
in Africa
• Adaptations vary with different environments
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Concept 22.3: Evolution is supported by an
overwhelming amount of scientific evidence
Direct Observations of Evolutionary Change
• New discoveries continue to fill the gaps
identified by Darwin in The Origin of Species
• Two examples provide evidence for natural
selection: the effect of differential predation on
guppy populations and the evolution of drugresistant HIV
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Predation and Coloration in Guppies : Scientific
Inquiry
Fig. 22-13
• John Endler has studied the effects of
predators on wild guppy populations
EXPERIMENT
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”
RESULTS
12
Number of
colored spots
12
Area of colored
spots (mm 2 )
• Guppy populations in pools with fewer
predators had more brightly colored males
Pools with
killifish,
but no
guppies prior
to transplant
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
• Brightly colored males are more attractive to
females
• However, brightly colored males are more
vulnerable to predation
Experimental
transplant of
guppies
10
8
6
4
2
0
Source
population
Transplanted
population
10
8
6
4
2
0
Source
population
Transplanted
population
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The Evolution of Drug-Resistant HIV
• Endler transferred brightly colored guppies
(with few predators) to a pool with many
predators
• As predicted, over time the population became
less brightly colored
• Endler also transferred drab colored guppies
(with many predators) to a pool with few
predators
• The use of drugs to combat HIV selects for
viruses resistant to these drugs
• HIV uses the enzyme reverse transcriptase to
make a DNA version of its own RNA genome
• The drug 3TC is designed to interfere and
cause errors in the manufacture of DNA from
the virus
• As predicted, over time the population became
more brightly colored
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Some individual HIV viruses have a variation
that allows them to produce DNA without errors
• These viruses have a greater reproductive
success and increase in number relative to the
susceptible viruses
• The population of HIV viruses has therefore
developed resistance to 3TC
• The ability of bacteria and viruses to evolve
rapidly poses a challenge to our society
Percent of HIV resistant to 3TC
Fig. 22-14
100
Patient
No. 1
Patient No. 2
75
50
Patient No. 3
25
0
0
2
4
6
Weeks
8
10
12
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The Fossil Record
• Natural selection does not create new traits,
but edits or selects for traits already present in
the population
• The fossil record provides evidence of the
extinction of species, the origin of new groups,
and changes within groups over time
• The local environment determines which traits
will be selected for or selected against in any
specific population
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Fig. 22-15
0
2
4
Depth (meters)
4
3
6
4 Bristolia insolens
8
3 Bristolia bristolensis
10
12
14
2 Bristolia harringtoni
16
18 1 Bristolia mohavensis
• The Darwinian view of life predicts that
evolutionary transitions should leave signs in
the fossil record
• Paleontologists have discovered fossils of
many such transitional forms
3
2
1
Latham Shale dig site, San
Bernardino County, California
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Fig. 22-16
Fig. 22-16ab
(a) Pakicetus (terrestrial)
(b) Rhodocetus (predominantly aquatic)
(a) Pakicetus (terrestrial)
Pelvis and
hind limb
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)
Fig. 22-16cd
(b) Rhodocetus (predominantly aquatic)
Homology
Pelvis and
hind limb
• Homology is similarity resulting from common
ancestry
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)
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Anatomical and Molecular Homologies
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
Fig. 22-17
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
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Fig. 22-18
• Comparative embryology reveals anatomical
homologies not visible in adult organisms
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
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Homologies and “Tree Thinking”
• Vestigial structures are remnants of features
that served important functions in the
organism’s ancestors
• Examples of homologies at the molecular level
are genes shared among organisms inherited
from a common ancestor
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• The Darwinian concept of an evolutionary
tree of life can explain homologies
• Evolutionary trees are hypotheses about the
relationships among different groups
• Evolutionary trees can be made using different
types of data, for example, anatomical and
DNA sequence data
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Fig. 22-19
Convergent Evolution
Branch point
(common ancestor)
Lungfishes
Mammals
2
Amnion
Lizards
and snakes
3
4
Homologous
characteristic
Amniotes
Tetrapod limbs
Crocodiles
Ostriches
Feathers
6
Birds
5
Tetrapods
Amphibians
1
• Convergent evolution is the evolution of
similar, or analogous, features in distantly
related groups
• Analogous traits arise when groups
independently adapt to similar environments in
similar ways
• Convergent evolution does not provide
information about ancestry
Hawks and
other birds
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Fig. 22-20
Biogeography
Sugar
glider
• Darwin’s observations of biogeography, the
geographic distribution of species, formed an
important part of his theory of evolution
NORTH
AMERICA
• Islands have many endemic species that are
often closely related to species on the nearest
mainland or island
AUSTRALIA
Flying
squirrel
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What Is Theoretical About Darwin’s View of Life?
• Earth’s continents were formerly united in a
single large continent called Pangaea, but
have since separated by continental drift
• In science, a theory accounts for many
observations and data and attempts to explain
and integrate a great variety of phenomena
• An understanding of continent movement and
modern distribution of species allows us to
predict when and where different groups
evolved
• Darwin’s theory of evolution by natural
selection integrates diverse areas of biological
study and stimulates many new research
questions
• Ongoing research adds to our understanding of
evolution
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You should now be able to:
1. Describe the contributions to evolutionary
theory made by Linnaeus, Cuvier, Lyell,
Lamarck, Malthus, and Wallace
2. Describe Lamarck’s theories, and explain why
they have been rejected
5. Explain why an individual organism cannot
evolve
6. Describe at least four lines of evidence for
evolution by natural selection
3. Explain what Darwin meant by “descent with
modification”
4. List and explain Darwin’s four observations
and two inferences
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