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Chapter 22

Descent with Modification: A Darwinian View of Life

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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

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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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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)

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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

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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

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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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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

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Fig. 22-5a

Darwin in 1840

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Fig. 22-5b

The

Galápagos

Islands

Pinta

Marchena

Genovesa

Santiago

Daphne

Islands

Pinzón

Fernandina

Isabela

San

Cristobal

Santa

Fe

Santa

Cruz

Florenza

Española

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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

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Fig. 22-6a

(a) Cactus-eater

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Fig. 22-6b

(b) Insect-eater

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Fig. 22-6c

(c) Seed-eater

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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

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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

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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

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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

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Observation #1: Members of a population often vary greatly in their traits

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Fig. 22-10

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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

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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

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Fig. 22-12a

(a) A flower mantid

in Malaysia

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Fig. 22-12b

(b) A stick mantid

in Africa

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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

10

8

6

4

2

0

RESULTS

EXPERIMENT

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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

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Fig. 22-13b

Source

population

Transplanted

population

Source

population

Transplanted

population

Number of

colored spots

Area of colored

spots (mm²)

12

10

8

6

4

2

0

RESULTS

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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

25

50

75

100

2

4

6

8

10

12

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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

1

2

4

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Fig. 22-15b

Bristolia harringtoni

Bristolia mohavensis

Latham Shale dig site, San

Bernardino County, California

12

14

16

18

2

Depth (meters)

1

2

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Fig. 22-15c

Bristolia insolens

10

8

0

Depth (meters)

4

Bristolia bristolensis

2

4

6

3

4

3

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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)

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Fig. 22-16ab

(a) Pakicetus (terrestrial)

(b) Rhodocetus (predominantly aquatic)

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Fig. 22-16cd

(c) Dorudon (fully aquatic)

Pelvis and

hind limb

Pelvis and

hind limb

(d) Balaena

(recent whale ancestor)

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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

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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

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Fig. 22-18a

Chick embryo (LM)

Pharyngeal

pouches

Post-anal

tail

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Fig. 22-18b

Human embryo

Pharyngeal

pouches

Post-anal

tail

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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

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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

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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

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Fig. 22-UN2

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Fig. 22-UN3

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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