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

3. The Origin of Species
Focused biologists’ attention on the great diversity of organisms
Figure 22.1

4. Darwin made two major points in his book
He presented evidence that the many species of organisms presently inhabiting the Earth are descendants of ancestral species
He proposed a mechanism for the evolutionary process, natural selection
The basic idea of natural selection is that a population can change over time if individuals that possess certain heritable traits leave more offspring than other individuals.
Natural selection results in evolutionary adaptation, an accumulation of inherited characteristics that increase the ability of an organism to survive and reproduce in its environment.

5. In order to understand why Darwin’s ideas were revolutionary
Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species
In order to understand why Darwin’s ideas were revolutionary
We need to examine his views in the context of other Western ideas about Earth and its life

6. 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
• The Greek philosopher Aristotle (384–322 B.C.E.) opposed any concept of evolution and viewed species as fixed and unchanging.
° Aristotle believed that all living forms could be arranged on a ladder of increasing complexity (scala naturae) with perfect, permanent species on every rung.

7. The Scale of Nature and Classification of Species
The Greek philosopher Aristotle
Viewed species as fixed and unchanging
The Old Testament of the Bible
Holds that species were individually designed by God and therefore perfect
• The Old Testament account of creation held that species were individually designed by God and, therefore, perfect.
• In the 1700s, natural theology viewed the adaptations of organisms as evidence that the Creator had designed each species for a purpose.

8. Carolus Linnaeus
Interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose
Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”
• Carolus Linnaeus (1707–1778), a Swedish physician and botanist, founded taxonomy, a system for naming species and classifying species into a hierarchy of increasingly complex categories.
° Linnaeus developed the binomial system of naming organisms according to genus and species.
° In contrast to the linear hierarchy of the scala naturae, Linnaeus adopted a nested classification system, grouping similar species into increasingly general categories.
° For Linnaeus, similarity between species did not imply evolutionary kinship but rather the pattern of their creation.

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
Figure 22.3
• Darwin’s views were influenced by fossils, remains or traces of organisms from the past mineralized in sedimentary rocks.
° Sedimentary rocks form when mud and sand settle to the bottom of seas, lakes, and marshes.
° New layers of sediment cover older ones, creating layers of rock called strata.
° Erosion may later carve through sedimentary rock to expose older strata at the surface.
° Fossils within layers of sedimentary rock show that a succession of organisms have populated Earth throughout time.

10. Paleontology, the study of fossils
Was largely developed by French scientist Georges Cuvier
Cuvier opposed the idea of gradual evolutionary change
And instead advocated catastrophism, speculating that each boundary between strata represents a catastrophe
• Paleontology, the study of fossils, was largely developed by the French anatomist Georges Cuvier (1769–1832).
• In examining rock strata in the Paris Basin, Cuvier noted that the older the strata, the more dissimilar the fossils from modern life.
° Cuvier recognized that extinction had been a common occurrence in the history of life.
° Instead of evolution, Cuvier advocated catastrophism, speculating that boundaries between strata were due to local floods or droughts that destroyed the species then present.
° He suggested that the denuded areas were later repopulated by species immigrating from unaffected areas.

11. Theories of Gradualism
Is the idea that profound change can take place through the cumulative effect of slow but continuous processes
Theories of geologic gradualism prepared the path for evolutionary biologists.
• In contrast to Cuvier’s catastrophism, Scottish geologist James Hutton (1726–1797) proposed a theory of gradualism that held that profound geological changes took place through the cumulative effect of slow but continuous processes identical to those currently operating.
° Thus, valleys were formed by rivers flowing through rocks and sedimentary rocks were formed from soil particles that eroded from land and were carried by rivers to the sea.
• Later, geologist Charles Lyell (1797–1875) proposed a theory of uniformitarianism, which held that geological processes had not changed throughout Earth’s history.
• Hutton’s and Lyell’s observations and theories had a strong influence on Darwin.

12. Geologists Hutton and Lyell
Perceived that changes in Earth’s surface can result from slow continuous actions still operating today
Exerted a strong influence on Darwin’s thinking
• Later, geologist Charles Lyell (1797–1875) proposed a theory of uniformitarianism, which held that geological processes had not changed throughout Earth’s history.
• Hutton’s and Lyell’s observations and theories had a strong influence on Darwin.

13. Lamarck’s Theory of Evolution
Lamarck hypothesized that species evolve
Through use and disuse and the inheritance of acquired traits
But the mechanisms he proposed are unsupported by evidence
Figure 22.4
• He explained his observations with two principles: use and disuse of parts and the inheritance of acquired characteristics.
° Use and disuse was the concept that body parts that are used extensively become larger and stronger, while those that are not used deteriorate.
° The inheritance of acquired characteristics stated that modifications acquired during the life of an organism could be passed to offspring.
° A classic example is the long neck of the giraffe. Lamarck reasoned that the long, muscular neck of the modern giraffe evolved over many generations as the ancestors of giraffes reached for leaves on higher branches and passed this characteristic to their offspring.

14. As the 19th century dawned
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, but a major change would challenge this thinking
through natural selection
• Charles Darwin (1809–1882) was born in western England.
° As a boy, he developed a consuming interest in nature.
° When Darwin was 16, his father sent him to the University of Edinburgh to study medicine.
• Darwin left Edinburgh without a degree and enrolled at Cambridge University with the intent of becoming a clergyman.

15. As a boy and into adulthood, Charles Darwin
Darwin’s Research
As a boy and into adulthood, Charles Darwin
Had a consuming interest in nature
Soon after Darwin received his B.A. degree
He was accepted on board the HMS Beagle, which was about to embark on a voyage around the world
° At that time, most naturalists and scientists belonged to the clergy and viewed the world in the context of natural theology.
• Darwin received his B.A. in 1831.
• After graduation Darwin joined the survey ship HMS Beagle as ship naturalist and conversation companion to Captain Robert FitzRoy.

16. The Voyage of the Beagle
During his travels on the Beagle
Darwin observed and collected many specimens of South American plants and animals
Darwin observed various adaptations of plants and animals
That inhabited many diverse environments

17. Darwin’s interest in the geographic distribution of species
Was kindled by the Beagle’s stop at the Galápagos Islands near the equator west of South America
Figure 22.5
England
EUROPE
NORTH
AMERICA
Galápagos
Islands
Darwin in 1840,
after his return
SOUTH
Cape of
Good Hope
Cape Horn
Tierra del Fuego
AFRICA
HMS Beagle in port
AUSTRALIA
Tasmania
New
Zealand
PACIFIC
OCEAN
Andes
ATLANTIC
• Darwin had the freedom to explore extensively on shore while the crew surveyed the coast.
• He collected thousands of specimens of the exotic and diverse flora and fauna of South America.
° Darwin explored the Brazilian jungles, the grasslands of the Argentine pampas, the desolation of Tierra del Fuego near Antarctica, and the heights of the Andes.

18. Darwin’s Focus on Adaptation
As Darwin reassessed all that he had observed during the voyage of the Beagle
He began to perceive adaptation to the environment and the origin of new species as closely related processes

19. From studies made years after Darwin’s voyage
Biologists have concluded that this is likely what happened to the Galápagos finches
Figure 22.6a–c
(a) Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp.
(c) Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground.
(b) Insect eater. The green warbler finch (Certhidea olivacea) uses its narrow, pointed beak to grasp insects.
• While on the Beagle, Darwin read Lyell’s Principles of Geology.

20. In June 1858 Darwin received a manuscript from Alfred Russell Wallace
In 1844, Darwin wrote a long essay on the origin of species and natural selection
But he was reluctant to introduce his theory publicly, anticipating the uproar it would cause
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
• In June 1858, Alfred Russel Wallace (1823–1913), a young naturalist working in the East Indies, sent Darwin a manuscript containing a theory of natural selection essentially identical to Darwin’s.

21. Darwin developed two main ideas
The Origin of Species
Darwin developed two main ideas
Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
• Darwin quickly finished The Origin of Species and published it the next year.
• While both Darwin and Wallace developed similar ideas independently, the theory of evolution by natural selection is attributed to Darwin because he developed his ideas earlier and supported the theory much more extensively.
° The theory of evolution by natural selection was presented in The Origin of Species with logic and supporting evidence.

22. Descent with Modification
The phrase descent with modification
Summarized Darwin’s perception of the unity of life
States that all organisms are related through descent from an ancestor that lived in the remote past
° Descent with modification.
􀂃 All organisms are related through descent from a common ancestor that lived in the remote past.
􀂃 Over evolutionary time, the descendents of that common ancestor have accumulated diverse modifications, or adaptations, that allow them to survive and reproduce in specific habitats.

23. In the Darwinian view, the history of life is like a tree
With multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms
Figure 22.7
Hyracoidea
(Hyraxes)
Sirenia
(Manatees
and relatives)
Years ago
Millions of years ago
Deinotherium
Mammut
Stegodon
Mammuthus
Platybelodon
Barytherium
Moeritherium
Elephas
maximus
(Asia)
Loxodonta
africana
(Africa)
cyclotis

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

25. Observation #1: For any species, population sizes would increase exponentially
If all individuals that are born reproduced successfully
Figure 22.8

26. Observation #2: Nonetheless, 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

27. Observation #4: Members of a population vary extensively in their characteristics
No two individuals are exactly alike
Figure 22.9

28. 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

29. 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

30. In the process of artificial selection
Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits
Figure 22.10
Terminal
bud
Lateral
buds
Brussels sprouts
Cabbage
Flower
cluster
Leaves
Cauliflower
and
stems
Broccoli
Wild mustard
Kohlrabi
Stem
Kale

31. Summary of Natural Selection
Natural selection is differential success in reproduction
That results from the interaction between individuals that vary in heritable traits and their environment

32. In the adaptation of organisms to their environment
Natural selection can produce an increase over time
In the adaptation of organisms to their environment
Figure 22.11
(a) A flower mantid in Malaysia
(b) A stick mantid in Africa

33. If an environment changes over time
Natural selection may result in adaptation to these new conditions

34. 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

35. Natural Selection in Action
Two examples
Provide evidence for natural selection

36. Differential Predation in Guppy Populations
Researchers have observed natural selection
Leading to adaptive evolution in guppy populations
Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools
and measured the average age and size of guppies at maturity over an 11-year period (30 to
60 generations).
EXPERIMENT
Pools with killifish,
but not 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”
Figure 22.12

37. Control Population: Guppies from pools with pike-cichlids as predators
RESULTS
After 11 years, the average size and age at maturity of guppies in the transplanted
populations increased compared to those of guppies in control populations.
161.5
185.6
67.5
Weight of guppies
at maturity (mg)
Age of guppies
at maturity (days)
92.3
48.5
Control Population: Guppies
from pools with pike-cichlids
as predators
Experimental Population:
Guppies transplanted to
pools with killifish as
predators
76.1
Males
Females
85.7
58.2
CONCLUSION
Reznick and Endler concluded that the change in predator resulted in different variations
in the population (larger size and faster maturation) being favored. Over a relatively short time, this altered
selection pressure resulted in an observable evolutionary change in the experimental population.

38. The Evolution of Drug-Resistant HIV
In humans, the use of drugs
Selects for pathogens that through chance mutations are resistant to the drugs’ effects
Natural selection is a cause of adaptive evolution

39. Percent of HIV resistant to 3TC
Researchers have developed numerous drugs to combat HIV
But using these medications selects for viruses resistant to the drugs
Patient
No. 1
Patient No. 2
Percent of HIV resistant to 3TC
Patient No. 3
Weeks
Figure 22.13

40. The ability of bacteria and viruses to evolve rapidly
Poses a challenge to our society

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

42. Homology
Homology
Is similarity resulting from common ancestry

43. Anatomical Homologies
Homologous structures between organisms
Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor
Figure 22.14
Human
Cat
Whale
Bat

44. Comparative embryology
Reveals additional anatomical homologies not visible in adult organisms
Figure 22.15
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Human embryo

45. Vestigial organs Are some of the most intriguing homologous structures
Are remnants of structures that served important functions in the organism’s ancestors

46. Molecular Homologies
Biologists also observe homologies among organisms at the molecular level
Such as genes that are shared among organisms inherited from a common ancestor

47. Homologies and the Tree of Life
The Darwinian concept of an evolutionary tree of life
Can explain the homologies that researchers have observed

48. Anatomical resemblances among species
Are generally reflected in their molecules, their genes, and their gene products
Figure 22.16
Species
Human
Rhesus monkey
Mouse
Chicken
Frog
Lamprey
14%
54%
69%
87%
95%
100%
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide

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

50. Some similar mammals that have adapted to similar environments
Have evolved independently from different ancestors
Sugar
glider
AUSTRALIA
NORTH
AMERICA
Flying
squirrel
Figure 22.17

51. 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

52. 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
Figure 22.18

53. 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

54. Darwin’s theory of evolution by natural selection
Integrates diverse areas of biological study and stimulates many new research questions