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Chapter 22
Descent with Modification:
A Darwinian View of Life
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
Overview: Endless Forms Most Beautiful
• Charles Darwin changed biology when he
published The Origin of Species in 1859
– Fringe idea  mainstream acceptance
How? Decades of evidence
– People had talked before about new
species forming. Darwin had a method and
years of observations to support it.
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The “Somebody would have figured it out” principle
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• 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
I know nothing about
this bug 
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Concept 22.1: Darwin built on earlier ideas of
intellectuals
• Difference? He said how, and had evidence
that difference changed everything
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Scala Naturae and Classification of Species
• Aristotle: arranged species on scala naturae
(natural scale)
– Thought species never changed
• Linnaeus: taxonomy (classifying organisms)
• Each species created individually for a
specific purpose
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Fossils: life now different from life in past
• Fossils: remains or traces of past organisms
• Found in strata (layers)
• The study of fossils helped to lay the
groundwork for Darwin’s ideas
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• Paleontology: the study of fossils
• Georges Cuvier advocated catastrophism
– The idea strata boundaries are formed by
catastrophies
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• James Hutton and Charles Lyell: earth shaped
by slow gradual processes
– Uniformitarianism: the mechanisms of
change are constant over time
• Lyell’s book Principles of Geology strongly
influenced Darwin’s thinking
If the processes are slow, the earth must be old
Lyell’s book convinced many scientists, not just Darwin
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Lamarck: Evolution due to use and disuse
• Lamarck: evolution = use,
disuse, and inheritance of
acquired characteristics
• Not much evidence for this
method (we think lamarck
was wrong)
• The book says “no” evidence
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Concept 22.2: Descent with modification by natural
selection
• Early 1800’s: most people still thought species
never changed
• However, a few scientists were beginning to
discuss the idea of new species forming
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Darwin: liked to study nature (child adult)
• Darwin studied @ Cambridge University:
– started in medicine
– switched to theology
• Graduated took position on the Beagle as a
naturalist, around the world,1831-1836
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The Voyage of the Beagle: what did darwin do?
• Collected South American plants and animals
– Saw adaptations to many diverse environments
• He read a lot
– Lyell’s Principles of Geology: earth > 6000 years old
– Malthus: people compete, some win, some loose
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• Galápagos Islands: many unique species
He saw that these species
had features that seem
to be useful in their
environment and their
lifestyle
Darwin explained this as
adaptation due to
“Natural Selection”
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Fig. 22-6
(a) Cactus-eater
(c) Seed-eater
(b) Insect-eater
Vampire Finch!!!!
OK, it’s really called the “Sharp beaked ground finch”
Darwin had the idea, but would not publish
• 1836: the Beagle trip ends
• 1844: Darwin writes essay on species’ origin
and natural selection
– won’t publish or discuss publicly
(probably wanted more evidence: fringemainstream)
Discussed privately with other scientists
• Showed essay to Joseph Hooker in 1847
• Wrote a letter to Asa Grey about it in 1857
• Wrote to Alfred Russell Wallace about it in 1857
Two times
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Darwin had the idea, but would not publish
So what was Darwin doing from
1836 until 1859?
1. Writing books on books on coral reefs,
barnacles, geology
became a famous scientist for this
2. Collects info and writes notes
Gathering evidence for natural selection.
(plans to publish someday)
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Alfred Russell Wallace finds same thing
1855: Wallace sent essay to Darwin
"On the Law which has regulated the Introduction of New
Species“
Darwin wrote back saying that they were thinking similarly,
and that he’d have a publication on the same topic within a
few years
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Alfred Russell Wallace and natural selection
1857: Wallace sent another essay to Darwin
“On the Tendency of Varieties to Depart Indefinitely From
the Original Type”
Same idea’s as Darwin’s “Natural Selection” but didn’t use that term
• Darwin quickly finished The Origin of Species
and published it the next year
– Didn’t want to get scooped
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Alfred Russell Wallace had the same idea
June 1858: Darwin gets this second paper on
speciation from Alfred Russell Wallace
Wallace asks Darwin to review, give to Lyell
Darwin tells Lyell and Hooker that it is basically
the same as what he was going to write
Lyell, Hooker and Darwin present Wallace’s
paper at meeting of scientific society
(along with letters showing Darwin had the same idea earlier)
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Why do we talk about Darwin, not Wallace?
Wallace wrote a paper. Darwin wrote a book.
1859: Darwin publishes Origin of the Species
What: species are different now than past
How: same method as selective breeding
Evidence: 20+ years of observations
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Was Darwin a Jerk? Probably not
It may look like it now, but probably not
Famous naturalist: already had good reputation
Wallace was an unknown
Wealthy, respected family
Wallace: a lot less money
(less likely to be listened to in Victorian England)
Could easily have taken all the credit
Later helped Wallace find a job
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The Origin of Species (1859)
• Darwin developed two main ideas:
1. “Descent with modification” explains life’s unity
and diversity
– What we now call “evolution”
– He didn’t use that term when writing book
2. Natural selection is a cause of adaptive
evolution
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Descent with Modification
• The phrase ”descent with modification”
• organisms are related:
common ancestor in past
Old, but uncommon, idea
Zoonomia, Erasmus Darwin
C. Darwin explained HOW, had EVIDENCE
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Darwinian view of natural world
• the history of life is
like a tree with
branches
representing life’s
diversity
• Darwin’s theory can
explain hierarchy of
Linnaeus
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Darwin: Natural Selection is like Artificial Selection
• artificial selection: humans modify by
selecting and breeding individuals with desired
traits
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Darwin: Natural Selection is like Artificial Selection
• Darwin then described four observations
1. Variation exists
2. Traits can be inherited from parents
3. Too many offspring for environment
4. Competition to survive, not all will
• Darwin made two inferences
Will discuss in a second
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• Observation #1: Members of a population often
vary greatly in their traits
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Observation #2: Traits
are inherited from
parents to offspring
Observation #3: All
species are capable of
producing more
offspring than the
environment can
support
If all the spores grew up we’d live in
mushroom world
• We don’t live in mushroom world!!!!
• Observation #4:
Owing to lack of food
or other resources,
many of these
offspring do not
survive
• Most spores die 
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Darwin’s two inferences
• 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
The ones that survive get to make babies!!!
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Darwin’s two inferences
• Inference #2: This unequal ability of individuals
to survive and reproduce will lead to the
accumulation of favorable traits in the
population over generations
• The ones with good traits live to breed, so good
traits accumulate
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Darwin applied Malthus to nature
• Thomas Malthus: humans might reproduce so
fast that they run out of food and resources
• Good traits accumulate, and individuals with
good traits are will become more common
• This process explains the match between
organisms and their environment
• Wallace also based his ideas on Malthus!!!
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Fig. 22-12
the match between
organisms and their
environment
(a) A flower mantid
in Malaysia
(b) A stick mantid
in Africa
Natural Selection: A Summary
• Good traits = survival = live to breed
– # of kids: how evolution keeps score
• Good traits build up, and organisms appear to
adapt to their environment
– Populations evolve, not individuals!!!!!
• natural selection + new environmental
conditions can cause new species to form
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• Note that individuals do not evolve; populations
evolve over time
• Evolution: A change in the genetic makeup of
a population over time
• Natural selection can only increase or
decrease heritable traits in a population
– Works on existing traits.
– Mutation, not natural selection, makes new traits
Adaptations vary with different environments
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Lamarck vs. Darwin/Wallace
Thought
individuals
evolved
populations
evolve
because
those bad
traits don’t
have kids
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Concept 22.3: Evolution has LOTS of evidence
• New discoveries continue add more evidence
– Guppies
– Drug resistant HIV
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Predation and Coloration in Guppies : Scientific
Inquiry
• John Endler has studied the effects of
predators on wild guppy populations
• Brightly colored males are more attractive to
females
• However, brightly colored males are more
vulnerable to predation
• Guppy populations in pools with fewer
predators had more brightly colored males
What happens to a group if you move them?
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Results match prediction: support for evolution
• Endler put drab
colored guppies
(many predators) in a
pool with few
predators
• As predicted, the
population changed
dullbright
More area
Covered in
Bright spots
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More
bright
spots
Results match prediction: support for evolution
• Endler transferred brightly colored guppies
(with few predators) to a pool with many
predators
• As predicted, over time the population became
less brightly colored

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Direct Observations of Evolutionary Change
• the evolution of drug-resistant HIV
• drugs to fight HIV selects for viruses resistant
to these drugs
• The drug 3TC is supposed to interfere with
HIV’s reverse transcriptase enzyme
Virus needs enzyme for RNA  DNA
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The Evolution of Drug-Resistant HIV
• there are different forms of Reverse Transcriptase
Some variations are immune to 3TC
Can make DNA without errors
•Viruses with 3TC resistant enzymes are better are
reproducing, and become more common than other
variants
•Now HIV strains have resistance to 3TC
•The ability of bacteria and viruses to evolve rapidly
poses a challenge to our society
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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
Natural Selection: like pruning shears
• Natural selection does not create new traits
• It chooses between existing traits in the phenotype
• local environment determines what is selected for
or against
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The Fossil Record – records of past living things
• Extinctions
• New groups
• Changes in groups
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Transitional Fossils Exist
• Archeopteryx
Lizard and bird features
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Transitional Fossils Exist
• 11 specimens found, various sizes
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reconstructions
Very lizard-like,
very birdlike…the only
sure thing is that
they probably
don’t look right
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Transitional Fossils: Changes in groups
• Archeopteryx is not
the only one found
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Transitional Fossils: Changes in groups
• Prediction: we
should find
intermediate or
transitional fossils
that show change
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Homology: similarity due to common ancestors
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
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Homology in Embryos
• Comparative embryology reveals anatomical
homologies not visible in adult organisms
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Molecular Homology
• Examples of homologies at the molecular level
are genes shared among organisms inherited
from a common ancestor
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Homologies and “Tree Thinking”
• 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
Branch point
(common ancestor)
Lungfishes
Amphibians
1
Mammals
2
Tetrapod limbs
Amnion
Lizards
and snakes
3
4
Homologous
characteristic
Crocodiles
Ostriches
6
Feathers
Hawks and
other birds
Birds
5
• Vestigial structures have reduced function
Python “spurs”, ING (19:39)
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Skinks – lizard with legs
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Skinks – With Reduced Legs
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Florida’s Glass lizards and Worm Lizards
“We don’t need no stinkin’ legs”
http://ufwildlife.ifas.ufl.edu/snakes/leglesslizards.shtml
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• Vestigial structures have reduced function
Whale Pelvis, ING (44:27)
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Dorudon: intemediate limbs, joints + nostrils
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Dorudon: Pelvis and legs
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Dorudon: front flippers had elbows
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Convergent Evolution: similar but NOT from ancestry
• Convergent evolution is the evolution of
similar traits because of a similar lifestyle
• Analogous traits: similar features in distantly
related organisms
• Convergent evolution does not provide
information about ancestry
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Fig. 22-20
Sugar
glider
NORTH
AMERICA
AUSTRALIA
Flying
squirrel
Biogeography: geographic distribution of species
• an important part of Charles Darwin’s theory of
evolution
• Endemic species: particular to that location
• Islands have many endemic species
often closely related to species on the
nearest mainland or island
 Seen by Darwin in Galapagos
 Seen by Wallace in Indonesia
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• Earth’s continents were formerly united in a
single large continent called Pangaea, but
have since separated by continental drift
• An understanding of continent movement and
modern distribution of species allows us to
predict when and where different groups
evolved
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Homology vs. Convergence: how to tell the difference
What scientists think about the relationships
between organisms can ALWAYS change
No belief in biology is above challenge, and
relationships are always being reexamined
The only way to separate homology from
convergence is to use EVERY method we can
to examine both the similarities and differences
Fossils
DNA
embryology morphology
protein
distribution
Math game: bias
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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
3. Explain what Darwin meant by “descent with
modification”
4. List and explain Darwin’s four observations
and two inferences
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5. Explain why an individual organism cannot
evolve
6. Describe at least four lines of evidence for
evolution by natural selection
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