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 The Origin of Species
The Origin of Species focused biologists’ attention on the great diversity of organisms
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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
Evolution can be viewed as both a pattern and a process
survivorship at extreme conditions
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4. 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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5. Scala Naturae and Classification of Species
The Greek philosopher Aristotle ( B.C.) 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
Carolus Linnaeus ( ) saw the adaptations as evidence that God had designed each species for a specific purpose. He created taxonomy.
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6. 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
the higher the strata the newer the sample
the lower the strata the older the sample
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7. This view strongly influenced Darwin’s thinking
Georges Cuvier developed Paleontology, he 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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8. 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
Bonsai trees are "trained" to be
dwarf, a seed will produce a
normal sized tree
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9. Concept 22.2: Descent with Modifications by Natural Selection explains 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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10. Darwin’s Research
Charles Darwin had a consuming interest in nature through out his life
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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11. 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
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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12. 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

13. The Galápagos Islands Pinta Genovesa Marchena Santiago Daphne Islands
Fig. 22-5b
The
Galápagos
Islands
Pinta
Genovesa
Marchena
Santiago
Daphne
Islands
Pinzón
Fernandina
Isabela
Santa
Cruz
Santa Fe
Figure 22.5 The voyage of HMS Beagle
San
Cristobal
Florenza
Española

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

16. 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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17. 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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18. Darwin’s theory meshed well with the hierarchy of Linnaeus
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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19. 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

20. 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
Observation #1: Members of a population often vary greatly in their traits
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21. Terminal bud Lateral buds Cabbage Brussels sprouts Flower clusters
Fig. 22-9
Terminal
bud
Lateral
buds
Cabbage
Brussels sprouts
var. capitata
Flower
clusters
var. gemmifera
Leaves
Kale
Cauliflower
var. acephala
var. botrytis
Figure 22.9 Artificial selection
Stem
Wild mustard
Brassica olerasia
Flowers
and stems
Broccoli
var. italica
Kohlrabi
var. gogylodes

22. Observation #2: Traits are inherited from parents to offspring
Observation #3: All species are capable of producing more offspring than the environment can support
Observation #4: Owing to lack of food or other resources, many of these offspring do not survive
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23. "The higher chances of survival, the more offspring they have"
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 higher chances of survival, the more offspring they have"
example: fishes, frogs, birds, insects
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24. "Those that survive will bring the favorable traits with them"
Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations
"Those that survive will bring the favorable traits with them"
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25. 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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26. 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
Direct Observations of Evolutionary Change
evolution of drug-resistant TB and MRSA
color change in moths, from light to dark due to pollution
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27. 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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28. 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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29. 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

30. 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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31. 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)

32. Homology Homology is similarity resulting from common ancestry
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33. Anatomical and Molecular Homologies
Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor
Human
Cat
Whale
Bat
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
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34. Comparative embryology reveals anatomical homologies not visible in adult organisms
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
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35. 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
tail in human embryo
teeth in birds
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36. 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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37. 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

38. 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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39. NORTH AMERICA Sugar glider AUSTRALIA Flying squirrel Fig. 22-20
Figure Convergent evolution
Flying
squirrel

40. 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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41. 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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42. 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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43. Individuals in a population vary in their heritable characteristics.
Fig. 22-UN1
Observations
Individuals in a population
vary in their heritable
characteristics.
Organisms produce more
offspring than the
environment can support.
Inferences
Individuals that are well suited
to their environment tend to leave
more offspring than other individuals
and
Over time, favorable traits
accumulate in the population.
The End