1. Descent with Modification: A Darwinian View of Life
Chapter 22
Descent with Modification: A Darwinian View of Life

2. Overview: Endless Forms Most Beautiful
A new era of biology began in 1859 when Charles Darwin published On The Origin of Species by Means of Natural Selection
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3. Darwin noted that current species are descendants of ancestral species
Evolution can be defined by Darwin’s phrase descent with modification (change in species over time)
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4. Fig. 22-1
Figure 22.1 How can this beetle survive in the desert, and what is it doing?

5. 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
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6. 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)
Wallace (evolution, natural selection)
American Revolution
French Revolution
U.S. Civil War
1750
1800
1850
1900
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.
Figure 22.2 The historical context of Darwin’s life and ideas
1831–1836
Darwin travels around the world on HMS Beagle.
1837
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.

7. Scala Naturae and Classification of Species
The Greek philosopher Aristotle viewed species as fixed and arranged them on a scala naturae (ladder of complexity)
The Old Testament holds that species were individually designed by God and therefore perfect
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8. 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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9. 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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10. Layers of deposited sediment Younger stratum with more recent fossils
Fig. 22-3
Layers of deposited
sediment
Younger stratum
with more recent
fossils
Figure 22.3 Formation of sedimentary strata with fossils
Older stratum
with older fossils

11. 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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12. This view strongly influenced Darwin’s thinking
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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13. 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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14. Fig. 22-4
Figure 22.4 Acquired traits cannot be inherited

15. 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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16. 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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17. 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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18. Fig. 22-5 Figure 22.5 The voyage of HMS Beagle
GREAT
BRITAIN
EUROPE
NORTH
AMERICA
ATLANTIC
OCEAN
The
Galápagos
Islands
AFRICA
Pinta
Genovesa
Marchena
Equator
SOUTH
AMERICA
Santiago
Daphne
Islands
Pinzón
AUSTRALIA
Fernandina
PACIFIC
OCEAN
Isabela
Figure 22.5 The voyage of HMS Beagle
For the Discovery Video Charles Darwin, go to Animation and Video Files.
Santa
Cruz
Andes
Cape of
Good Hope
Santa Fe
San
Cristobal
Tasmania
Florenza
Española
Cape Horn
New
Zealand
Tierra del Fuego

19. 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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20. (a) Cactus-eater (c) Seed-eater (b) Insect-eater Fig. 22-6
Figure 22.6 Beak variation in Galápagos finches
(b) Insect-eater

21. 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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22. Darwin developed two main ideas:
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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23. 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
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24. Darwin’s theory meshed well with the hierarchy of Linnaeus
Fig. 22-7
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
Figure 22.7 “I think . . .”

25. Fig. 22-8 Figure 22.8 Descent with modification Millions of years ago
Hyracoidea
(Hyraxes)
Sirenia
(Manatees
and relatives)
Moeritherium
Barytherium
Deinotherium
Mammut
Platybelodon
Stegodon
Mammuthus
Figure 22.8 Descent with modification
Elephas maximus
(Asia)
Loxodonta
africana
(Africa)
Loxodonta cyclotis
(Africa) 34 24
5.5 2
104
Millions of years ago
Years ago

26. Artificial Selection, Natural Selection, and Adaptation
Darwin noted that humans have modified other species
selecting and breeding individuals with desired traits, a process called artificial selection
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27. Terminal bud Lateral buds Cabbage Brussels sprouts Flower clusters
Fig. 22-9
Terminal
bud
Lateral
buds
Cabbage
Brussels sprouts
Flower
clusters
Leaves
Kale
Cauliflower
Figure 22.9 Artificial selection
Stem
Wild mustard
Flowers
and stems
Broccoli
Kohlrabi

28. Darwin described four observations of nature and from these drew two inferences
Observation #1: Members of a population often vary greatly in their traits
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29. Fig
Figure Variation in a population

30. 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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31. Fig
Spore
cloud
Figure Overproduction of offspring

32. Observation #4: Owing to lack of food or other resources, many of these offspring do not survive
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33. 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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34. 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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35. 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
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36. 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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37. (a) A flower mantid in Malaysia Fig. 22-12a
Figure Camouflage as an example of evolutionary adaptation
(a) A flower mantid
in Malaysia

38. (b) A stick mantid in Africa Fig. 22-12b
Figure Camouflage as an example of evolutionary adaptation
(b) A stick mantid
in Africa

39. 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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40. 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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41. 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 but are more vulnerable to predation
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42. EXPERIMENT Experimental transplant of guppies Fig. 22-13a
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Figure Can predation result in natural selection for color patterns in guppies?
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”

43. Area of colored spots (mm2) colored spots Number of
Fig b
RESULTS 12 12 10 10 8 8
Area of colored
spots (mm2)
colored spots
Number of 6 6 4 4 2 2
Figure Can predation result in natural selection for color patterns in guppies?
Source
population
Transplanted
population
Source
population
Transplanted
population

44. As predicted, over time the population became less brightly colored
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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45. The Evolution of Drug-Resistant HIV
The use of drugs to combat HIV selects for viruses resistant to these drugs
The drug 3TC is designed to interfere and cause errors in the manufacture of DNA from the virus
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46. 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
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47. Fig
100
Patient
No. 1
Patient No. 2 75
Percent of HIV resistant to 3TC 50
Patient No. 3
Figure Evolution of drug resistance in HIV 25 2 4 6 8 10 12
Weeks

48. 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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49. 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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50. Fig. 22-15 2 4 4 6 4 Bristolia insolens 8 3 Bristolia bristolensis 102 4 4 6 4
Bristolia insolens 8 3
Bristolia bristolensis 10 12 3
Depth (meters) 2
Bristolia harringtoni 14 16 18 1
Bristolia mohavensis 3
Figure Fossil evidence of evolution in a group of trilobites 2 1
Latham Shale dig site, San
Bernardino County, California

51. 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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52. Fig. 22-16 (a) Pakicetus (terrestrial)
(b) Rhodocetus (predominantly aquatic)
Figure The transition to life in the sea
Pelvis and
hind limb
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor) /

53. Homology is similarity resulting from common ancestry
Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor
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54. Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat
Fig
Humerus
Radius
Ulna
Carpals
Metacarpals
Figure Mammalian forelimbs: homologous structures
Phalanges
Human
Cat
Whale
Bat

65. Comparative embryology reveals anatomical homologies not visible in adult organisms
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66. Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo
Fig
Pharyngeal
pouches
Post-anal
tail
Figure Anatomical similarities in vertebrate embryos
Chick embryo (LM)
Human embryo

67. 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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68. 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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69. Homologous characteristic
Fig
Branch point
(common ancestor)
Lungfishes 1
Amphibians
Tetrapods 2
Mammals
Tetrapod limbs
Amniotes 3
Lizards
and snakes
Amnion 4
Crocodiles
Figure Tree thinking: information provided in an evolutionary tree
Homologous
characteristic 5
Ostriches
Birds 6
Feathers
Hawks and
other birds

70. Convergent evolution does not provide information about ancestry
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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71. NORTH AMERICA Sugar glider AUSTRALIA Flying squirrel Fig. 22-20
Figure Convergent evolution
Flying
squirrel

72. 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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73. 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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74. 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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