Darwin – Descent with
Modification
Ch 22
• The Origin of Species
– Focused biologists’ attention on the great
diversity of organisms
Figure 22.1
• 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
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
Linnaeus (classification)
Hutton (gradual geologic change)
Lamarck (species can change)
• The historical context of Darwin’s life and
ideas
Malthus (population limits)
Cuvier (fossils, extinction)
Lyell (modern geology)
Darwin (evolution, nutural selection)
Mendel (inheritance)
American Revolution
1750
Wallace (evolution, natural selection)
French Revolution
U.S. Civil War
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.
1831–1836 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.
Figure 22.2
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
• 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”
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
• 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
Theories of Gradualism
• Gradualism
– Is the idea that profound change can take
place through the cumulative effect of slow
but continuous processes
• 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
– Lyell – uniformitarianism – same geologic
processes are operating today as in the past
and at same rate
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
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
• 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
England
NORTH
AMERICA
EUROPE
ATLANTIC
OCEAN
PACIFIC
OCEAN
Galápagos
Islands
AUSTRALIA
Cape of
Good Hope
Cape Horn
Figure 22.5
HMS Beagle in port
SOUTH
AMERICA
Andes
Darwin in 1840,
after his return
AFRICA
Tierra del Fuego
Tasmania
New
Zealand
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
• From studies made years after Darwin’s
voyage
– Biologists have concluded that this is indeed
what happened to the Galápagos finches
(a) Cactus eater. The long,
sharp beak of the cactus
ground finch (Geospiza
scandens) helps it tear
and eat cactus flowers
and pulp.
Figure 22.6a–c
(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.
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
• If an environment changes over time
– Natural selection may result in adaptation to
these new conditions
• Natural selection can produce an increase over
time
– In the adaptation of organisms to their
environment
(a) A flower mantid
in Malaysia
(b) A stick mantid
in Africa
Figure 22.11
• 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
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
• 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
Sirenia
Hyracoidea (Manatees
(Hyraxes) and relatives)
Elephas Loxodonta Loxodonta
maximus africana
cyclotis
(Africa)
(Asia)
(Africa)
Figure 22.7
Natural Selection and
Adaptation
• Evolutionary biologist Ernst Mayr
– Has dissected the logic of Darwin’s theory into
three inferences based on five observations
• Observation #1: For any species,
population sizes would increase
exponentially
– If all individuals that are born reproduced
successfully
Figure 22.8
• 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
• Observation #4: Members of a population
vary extensively in their characteristics
– No two individuals are exactly alike
Figure 22.9
• 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
• 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
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
EXPERIMENT
and measured the average age and size of guppies at maturity over an 11-year period (30 to
60 generations).
Pools with killifish,
but not guppies prior
to transplant
Predator: Killifish; preys
mainly on small guppies
Experimental
transplant of
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
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.
185.6
161.5
85.7 92.3
48.5
58.2
Control Population: Guppies
from pools with pike-cichlids
as predators
67.5 76.1
Males
Females
Males
Females
Experimental Population:
Guppies transplanted to
pools with killifish as
predators
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.
The Evolution of Drug-Resistance
• 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
• Drugs do not create resistant pathogens,
they select for resistant individuals already
in the population
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
Anatomical Homologies
• Homologous structures between
organisms
– Are anatomical resemblances that represent
variations on a structural theme that was
present in a common ancestor
Human
Figure 22.14
Cat
Whale
Bat
• Comparative embryology
– Reveals additional anatomical homologies
not visible in adult organisms
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Figure 22.15
Human embryo
• Vestigial organs
– Are some of the most intriguing homologous
structures
– Are remnants of structures that served
important functions in the organism’s
ancestors
• Analogous structures
– Similar solution to similar problems; but
doesn’t show relatedness
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
• Anatomical resemblances among species
– Are generally reflected in their molecules,
their genes, and their gene products
Species
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
100%
Human
Rhesus monkey
95%
Mouse
87%
Chicken
69%
Frog
Figure 22.16
Lamprey
54%
14%
Homologies and the Tree of Life
• The Darwinian concept of an evolutionary
tree of life
– Can explain the homologies that researchers
have observed
Biogeography
• Darwin’s observations of the geographic distribution of
species, biogeography
– Formed an important part of his theory of evolution
• Some similar mammals that have adapted to similar
environments
– Have evolved independently from different ancestors
Sugar
glider
NORTH
AMERICA
AUSTRALIA
Flying
squirrel
Figure 22.17
Summarizing
• Evolution is a change in species over time
• Heritable variations exist within a
population
• These variations can result in a differential
reproductive success
• Over generations, this can result in
changes in the genetic composition of the
population