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

2. Overview: Darwin Introduces a Revolutionary Theory
A new era of biology began on November 24, 1859, the day Charles Darwin published On the Origin of Species by Means of Natural Selection
The Origin of Species focused biologists’ attention on the great diversity of organisms
Video: Galápagos Marine Iguana

4. Darwin made two major points in his book:
Many current species are descendants of ancestral species
Natural selection is a mechanism for this evolutionary process

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

6. LE 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)
Mendel (inheritance)
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 theory of evolution.
1830
Lyell publishes Principles of Geology.
1831–1936
Darwin travels around the world on HMS Beagle.
1837
Darwin begins his notebooks on the origin of species.
1844
Darwin writes his essay on the origin of species.
1858
Wallace sends his theory to Darwin.
1859
The Origin of Species is published.
1865
Mendel publishes inheritance papers.

7. Resistance to the Idea of Evolution
The Origin of Species
Shook the deepest roots of Western culture
Challenged a worldview that had been prevalent for centuries

8. The Scale of Nature and Classification of Species
The Greek philosopher Aristotle viewed species as fixed and unchanging
The Old Testament holds that species were individually designed by God and therefore perfect
Carolus Linnaeus interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose
Linnaeus was a founder of taxonomy, the branch of biology concerned with classifying organisms

9. Fossils, Cuvier, and Catastrophism
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

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

12. Theories of Gradualism
Gradualism is the idea that profound change can take place through the cumulative effect of slow but continuous processes

13. Geologists Hutton and Lyell perceived that changes in Earth’s surface can result from slow continuous actions still operating today
This view strongly influenced Darwin’s thinking

14. Lamarck’s Theory of Evolution
Lamarck hypothesized that species evolve through use and disuse and the inheritance of acquired traits
The mechanisms he proposed are unsupported by evidence

16. Concept 22.2: In The Origin of Species, Darwin proposed that species change through natural selection
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

17. Darwin’s Research
As a boy and into adulthood, Charles Darwin had a consuming interest in nature
After receiving his B.A. degree, he was accepted on board the HMS Beagle, which was embarking on a voyage around the world

18. 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

19. LE 22-5 England EUROPE NORTH AMERICA PACIFIC OCEAN ATLANTIC OCEAN
AFRICA
Galápagos
Islands
HMS Beagle in port
Equator
SOUTH
AMERICA
Darwin in 1840,
after his return
AUSTRALIA
Andes
Cape of
Good Hope
Tasmania
Cape Horn
New
Zealand
Tierra del Fuego

20. Video: Albatross Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Video: Galápagos Island Overview
Video: Galápagos Sea Lion
Video: Soaring Hawk
Video: Galápagos Tortoise

21. 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

22. LE 22-6
Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp.
Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground.
Insect eater. The green warbler finch (Certhidea olivacea) used its narrow, pointed beak to grasp insects.

23. 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

24. The Origin of Species Darwin developed two main ideas:
Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution

25. Descent with Modification
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
In the Darwinian view, the history of life is like a tree with branches representing life’s diversity

26. LE 22-7 Sirenia (Manatees and relatives) Elephas maximus (Asia)
Loxodonta
africana
(Africa)
Loxodonta
cyclotis
(Africa)
Hyracoidea
(Hyraxes)
Years ago
10,000 2
5.5
Mammut
Stegodon
Mammuthus
Deinotherium
Platybelodon
Millions of years ago 24
Barytherium
Moeritherium 34

27. Natural Selection and Adaptation
Evolutionary biologist Ernst Mayr has dissected the logic of Darwin’s theory into three inferences based on five observations

28. Observation #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully

30. Observation #2: Populations tend to be stable in size, except for seasonal fluctuations
Observation #3: Resources are limited
Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

31. Observation #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike

33. Observation #5: Much of this variation is heritable
Inference #2: Survival depends in part on inherited traits; individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

34. Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

35. Artificial Selection
In artificial selection, humans have modified other species over many generations by selecting and breeding individuals with desired traits

36. LE 22-10 Terminal bud Lateral buds Cabbage Brussels sprouts Leaves
Flower
clusters
Cauliflower
Kale
Flowers
and
stems
Stem
Broccoli
Wild mustard
Kohlrabi

37. Summary of Natural Selection
Natural selection is differential success in reproduction from interaction between individuals that vary in heritable traits and their environment
Natural selection produces an increase over time in adaptation of organisms to their environment
If an environment changes over time, natural selection may result in adaptation to these new conditions
Video: Seahorse Camouflage

38. LE 22-11
A flower mantid
in Malaysia
A stick mantid
in Africa

39. Concept 22.3: Darwin’s theory explains a wide range of observations
Darwin’s theory of evolution continues to be tested by how effectively it can account for additional observations and experimental outcomes

40. Natural Selection in Action
Two examples provide evidence for natural selection: the effect of differential predation on guppy populations and the evolution of drug-resistant HIV

41. Differential Predation in Guppy Populations
Researchers have observed natural selection leading to adaptive evolution in guppy populations

42. Predator: Killifish; preys mainly on small guppies
LE 22-12a
Pools with killifish
but no guppies prior
to transplant
Experimental
transplant of
guppies
Predator: Killifish; preys
mainly on small guppies
Guppies:
Larger at
sexual maturity
than those in
“pike-cichlid pools”
Predator: Pike-cichlid; preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in “killifish pools”

43. LE 22-12b Control population: Guppies from pools with
200
185.6
100
85.7
92.3
Control population:
Guppies from pools with
pike-cichlids as predators
161.5
160 80
Mass of guppies
at maturity (mg)
Age of guppies
at maturity (days)
58.2
120 60
48.5 80
67.5
76.1 40
Experimental population:
Guppies transplanted to
pools with killifish as
predators 40 20
Males
Females
Males
Females

44. The Evolution of Drug-Resistant HIV
The use of drugs to combat HIV selects for viruses resistant to these drugs
The ability of bacteria and viruses to evolve rapidly poses a challenge to our society

45. Percent of HIV resistant to 3TC
LE 22-13
100
Patient
No. 1
Patient No. 2 75
Percent of HIV resistant to 3TC 50
Patient No. 3 25 2 4 6 8 10 12
Weeks

46. Homology, Biogeography, and the Fossil Record
Evolutionary theory provides a cohesive explanation for many kinds of observations

47. Homology
Homology is similarity resulting from common ancestry

48. Anatomical Homologies
Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor

49. LE 22-14
Human
Cat
Whale
Bat

50. Comparative embryology reveals anatomical homologies not visible in adult organisms

51. Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo
LE 22-15
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo

52. Vestigial organs are remnants of structures that served important functions in the organism’s ancestors

53. Molecular Homologies
Examples of homologies at the molecular level are genes shared among organisms inherited from a common ancestor

54. Homologies and the Tree of Life
The Darwinian concept of an evolutionary tree of life can explain homologies
Anatomical resemblances among species are generally reflected in their molecules, genes, and gene products

55. LE 22-16 Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
Species
Human
100%
Rhesus monkey
95%
Mouse
87%
Chicken
69%
Frog
54%
Lamprey
14%

56. Biogeography
Darwin’s observations of biogeography, the geographic distribution of species, formed an important part of his theory of evolution

57. Some similar mammals that have adapted to similar environments have evolved independently from different ancestors

58. NORTH AMERICA AUSTRALIA
LE 22-17
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel

59. The Fossil Record
The succession of forms observed in the fossil record is consistent with other inferences about the major branches of descent in the tree of life

60. 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

62. What Is Theoretical about the Darwinian 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