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
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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
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Fig. 22-1
To understand why Darwin’s ideas were revolutionary, we must examine them in relation to other Western ideas about Earth and its life
Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species
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Fig. 22-2
American Revolution
French Revolution
U.S. Civil War
1900
1850
1800
1750
1795
1809
1798
1830
1831–1836
1837
1859
1837
1844
1858
The Origin of Species is published.
Wallace sends his hypothesis to Darwin.
Darwin begins his notebooks.
Darwin writes essay on descent with modification.
Darwin travels around the world on HMS Beagle.
Malthus publishes “Essay on the Principle of Population.”
Lyell publishes Principles of Geology.
Lamarck publishes his hypothesis of evolution.
Hutton proposes his theory of gradualism.
Linnaeus (classification)
Cuvier (fossils, extinction)
Malthus (population limits)
Lamarck (species can change)
Hutton (gradual geologic change)
Lyell (modern geology)
Darwin (evolution, natural selection)
Wallace (evolution, natural selection)
Scala Naturae and Classification of Species
The Greek philosopher Aristotle viewed species as fixed and arranged them on a scala naturae
The Old Testament holds that species were individually designed by God and therefore perfect
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Carolus Linnaeus interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose
Linnaeus was the founder of taxonomy, the branch of biology concerned with classifying organisms
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Ideas About Change over Time
The study of fossils helped to lay the groundwork for Darwin’s ideas
Fossils are remains or traces of organisms from the past, usually found in sedimentary rock, which appears in layers or strata
Video: Grand Canyon
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Fig. 22-3
Younger stratum
with more recent
fossils
Layers of deposited
sediment
Older stratum
with older fossils
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
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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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Lamarck’s Hypothesis of Evolution
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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Fig. 22-4
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
Concept 22.2: Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life
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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
Darwin was influenced by Lyell’s Principles of Geology and thought that the earth was more than 6000 years old
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His interest in geographic distribution of species was kindled by a stop at the Galápagos Islands near the equator west of South America
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Fig. 22-5
NORTH
AMERICA
EUROPE
AFRICA
AUSTRALIA
GREAT
BRITAIN
SOUTH
AMERICA
ATLANTIC
OCEAN
PACIFIC
OCEAN
Cape of
Good Hope
Tierra del Fuego
Cape Horn
Tasmania
New
Zealand
Andes
Equator
The
Galápagos
Islands
Pinta
Marchena
Genovesa
Santiago
Daphne
Islands
Pinzón
Fernandina
Isabela
San
Cristobal
Santa
Fe
Santa
Cruz
Florenza
Española
Fig. 22-5a
Darwin in 1840
Fig. 22-5b
The
Galápagos
Islands
Pinta
Marchena
Genovesa
Santiago
Daphne
Islands
Pinzón
Fernandina
Isabela
San
Cristobal
Santa
Fe
Santa
Cruz
Florenza
Española
Video: Galápagos Marine Iguana
Video: Galápagos Tortoises
Video: Galápagos Sea Lion
Video: Galápagos Islands Overview
Video: Blue-footed Boobies Courtship Ritual
Video: Albatross Courtship Ritual
Video: Soaring Hawk
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Darwin’s Focus on Adaptation
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
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Fig. 22-6
(a) Cactus-eater
(c) Seed-eater
(b) Insect-eater
Fig. 22-6a
(a) Cactus-eater
Fig. 22-6b
(b) Insect-eater
Fig. 22-6c
(c) Seed-eater
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 quickly finished The Origin of Species and published it the next year
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The Origin of Species
Darwin developed two main ideas:
Descent with modification explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
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Descent with Modification
Darwin never used the word evolution in the first edition of The Origin of Species
The phrase descent with modification summarized Darwin’s perception of the unity of life
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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In the Darwinian view, the history of life is like a tree with branches representing life’s diversity
Darwin’s theory meshed well with the hierarchy of Linnaeus
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Fig. 22-7
Fig. 22-8
Hyracoidea
(Hyraxes)
Sirenia
(Manatees
and relatives)
Moeritherium
Barytherium
Deinotherium
Mammut
Elephas maximus
(Asia)
Stegodon
Mammuthus
Loxodonta
africana
(Africa)
Loxodonta cyclotis
(Africa)
0
104
2
5.5
24
34
Millions of years ago
Years ago
Platybelodon
Fig. 22-8a
Elephas maximus
(Asia)
Stegodon
Mammuthus
Loxodonta
africana
(Africa)
Loxodonta cyclotis
(Africa)
0
104
2
5.5
24
34
Millions of years ago
Years ago
Platybelodon
Artificial Selection, Natural Selection, and Adaptation
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
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Fig. 22-9
Kale
Kohlrabi
Brussels sprouts
Leaves
Stem
Wild mustard
Flowers
and stems
Broccoli
Cauliflower
Flower
clusters
Cabbage
Terminal
bud
Lateral
buds
Observation #1: Members of a population often vary greatly in their traits
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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
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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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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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Natural Selection: A Summary
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
(b) A stick mantid
in Africa
(a) A flower mantid
in Malaysia
Fig. 22-12a
(a) A flower mantid
in Malaysia
Fig. 22-12b
(b) A stick mantid
in Africa
Note that individuals do not evolve; populations evolve over time
Natural selection can only increase or decrease heritable traits in a population
Adaptations vary with different environments
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Concept 22.3: Evolution is supported by an overwhelming amount of scientific evidence
New discoveries continue to fill the gaps identified by Darwin in The Origin of Species
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Direct Observations of Evolutionary Change
Two examples provide evidence for natural selection: the effect of differential predation on guppy populations and the evolution of 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
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Fig. 22-13
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”
Source
population
Transplanted
population
Source
population
Transplanted
population
Number of
colored spots
Area of colored
spots (mm2)
12
12
10
10
8
8
6
6
4
4
2
2
0
0
RESULTS
EXPERIMENT
Fig. 22-13a
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”
EXPERIMENT
Fig. 22-13b
Source
population
Transplanted
population
Source
population
Transplanted
population
Number of
colored spots
Area of colored
spots (mm2)
12
12
10
10
8
8
6
6
4
4
2
2
0
0
RESULTS
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
As predicted, over time the population became more brightly colored
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The Evolution of Drug-Resistant HIV
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
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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
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Fig. 22-14
Weeks
Patient No. 3
Patient No. 2
Patient
No. 1
Percent of HIV resistant to 3TC
0
0
25
50
75
100
2
4
6
8
10
12
Natural selection does not create new traits, but edits or selects for traits already present in the population
The local environment determines which traits will be selected for or selected against in any specific population
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The Fossil Record
The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time
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Fig. 22-15
Bristolia insolens
Bristolia bristolensis
Bristolia harringtoni
Bristolia mohavensis
Latham Shale dig site, San
Bernardino County, California
Depth (meters)
0
2
4
6
8
10
12
14
16
18
1
2
3
3
3
1
2
4
4
Fig. 22-15b
Bristolia harringtoni
Bristolia mohavensis
Latham Shale dig site, San
Bernardino County, California
12
14
16
18
2
Depth (meters)
1
1
2
Fig. 22-15c
Bristolia insolens
10
8
0
Depth (meters)
4
Bristolia bristolensis
2
4
6
3
4
3
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
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Fig. 22-16
(a) Pakicetus (terrestrial)
(b) Rhodocetus (predominantly aquatic)
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)
Fig. 22-16ab
(a) Pakicetus (terrestrial)
(b) Rhodocetus (predominantly aquatic)
Fig. 22-16cd
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)
Homology
Homology is similarity resulting from common ancestry
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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
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Fig. 22-17
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Whale
Cat
Bat
Comparative embryology reveals anatomical homologies not visible in adult organisms
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Fig. 22-18
Human embryo
Chick embryo (LM)
Pharyngeal
pouches
Post-anal
tail
Fig. 22-18a
Chick embryo (LM)
Pharyngeal
pouches
Post-anal
tail
Fig. 22-18b
Human embryo
Pharyngeal
pouches
Post-anal
tail
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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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
Hawks and
other birds
Ostriches
Crocodiles
Lizards
and snakes
Amphibians
Mammals
Lungfishes
Tetrapod limbs
Amnion
Feathers
Homologous
characteristic
Branch point
(common ancestor)
Tetrapods
Amniotes
Birds
6
5
4
3
2
1
Convergent Evolution
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
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Fig. 22-20
Sugar
glider
Flying
squirrel
AUSTRALIA
NORTH
AMERICA
Biogeography
Darwin’s observations of biogeography, the geographic distribution of species, formed an important part of his theory of evolution
Islands have many endemic species that are often closely related to species on the nearest mainland or island
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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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What Is Theoretical About Darwin’s View of Life?
In science, a theory accounts for many observations and data and attempts to explain and integrate a great variety of phenomena
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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Fig. 22-UN1
Observations
Over time, favorable traits
accumulate in the population.
Inferences
and
Individuals in a population
vary in their heritable
characteristics.
Organisms produce more
offspring than the
environment can support.
Individuals that are well suited
to their environment tend to leave
more offspring than other individuals
Fig. 22-UN2
Fig. 22-UN3
You should now be able to:
Describe the contributions to evolutionary theory made by Linnaeus, Cuvier, Lyell, Lamarck, Malthus, and Wallace
Describe Lamarck’s theories, and explain why they have been rejected
Explain what Darwin meant by “descent with modification”
List and explain Darwin’s four observations and two inferences
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Explain why an individual organism cannot evolve
Describe at least four lines of evidence for evolution by natural selection
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