Download Natural Selection

Chapter 22-2 & 22-3
Descent with Modification: A
Darwinian View of Life
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
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Evolution Explained
Concept 22.2: Natural Selection
• 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 observed various adaptations of plants
and animals
– That inhabited many diverse environments
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Darwin’s Research
• 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
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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
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Darwin’s Research
• 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.
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The Origin of Species
• Darwin developed two main ideas
– Evolution explains life’s unity and diversity
– Natural selection is a cause of adaptive
evolution
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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
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Descent with Modification
• 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
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Natural Selection and Adaptation
• Evolutionary biologist Ernst Mayr
– Has dissected the logic of Darwin’s theory into
three inferences based on five observations
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Observation #1
• Observation #1: For any species, population
sizes would increase exponentially
– If all individuals that are born reproduced
successfully
Figure 22.8
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Observation # 2 & 3
• 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
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Observation #4
• Observation #4: Members of a population vary
extensively in their characteristics
– No two individuals are exactly alike
Figure 22.9
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Observation #5
• 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
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Inference #3
• 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
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Artificial Selection
• In the process of artificial selection
– Humans have modified other species over
many generations by selecting and breeding
individuals that possess desired traits
Terminal
bud
Lateral
buds
Brussels sprouts
Cabbage
Flower
cluster
Leaves
Cauliflower
Kale
Flower
and
stems
Broccoli
Stem
Wild mustard
Figure 22.10
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Kohlrabi
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
Ex.
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Natural Selection
• 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
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Concept 22.3: Darwin’s Theory
• Darwin’s theory attempts to explain 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
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Natural Selection in Action
• Two examples
– Provide evidence for natural selection
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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
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
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Differential Predation in Guppy Populations
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.
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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
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The Evolution of Drug-Resistant HIV
• 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
Patient No. 3
Weeks
Figure 22.13
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Homology, Biogeography, and the Fossil Record
• Evolutionary theory
– Attempts to provide a cohesive explanation for
many kinds of observations
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Homology
• Homology
– Is similarity resulting from common ancestry
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Anatomical Homologies
• Homologous structures between organisms
– Are anatomical resemblances that represent
variations on a structural theme that was
present in a common ancestor
Human
Cat
Figure 22.14
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Whale
Bat
Comparative embryology
• Comparative embryology
– Reveals additional anatomical homologies not
visible in adult organisms
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Figure 22.15
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Human embryo
Vestigial organs
• Vestigial organs
– Are some of the most intriguing homologous
structures
– Are remnants of structures that served
important functions in the organism’s
ancestors
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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
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Homologies and the Tree of Life
• The Darwinian concept of an evolutionary tree
of life
– Attempts to explain the homologies that
researchers have observed
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Anatomical Resemblances
• Anatomical resemblances among species
– Are generally assumed to be reflected in their
molecules, their genes, and their gene
products
Percent of Amino Acids That Are
Species
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
100%
Human
Rhesus monkey
95%
Mouse
87%
Chicken
69%
Frog
Figure 22.16
Lamprey
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54%
14%
Biogeography
• Darwin’s observations of the geographic
distribution of species, biogeography
– Formed an important part of his theory of
evolution
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Biogeography
• Some similar mammals that have adapted to
similar environments
– Have evolved independently from different ancestors
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel
Figure 22.17
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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
• The Cambrian explosion
Cambrian Explosion
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Darwinian’s View of Life
• The Darwinian view of life
– Predicts that evolutionary transitions should
leave signs in the fossil record
• Paleontologists
– Have discovered few fossils of such transitional
forms
Figure 22.18
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