Download Ch 22

Chapter 22
Descent with Modification: A
Darwinian View of Life
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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Theories of Gradualism
• Gradualism is the idea that profound change can
take place through the cumulative effect of slow
but continuous processes
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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
• 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
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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: For any species, population sizes
would increase exponentially if all individuals that
are born reproduced successfully
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• 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
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• Observation #4: Members of a population vary
extensively in their characteristics; no two
individuals are exactly alike
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• 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: 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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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
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Natural Selection in Action
• Two examples provide evidence for natural
selection: the effect of differential predation on
guppy populations and the evolution of drugresistant HIV
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Homology, Biogeography, and the Fossil Record
• Evolutionary theory provides 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 are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
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• Comparative embryology reveals anatomical
homologies not visible in adult organisms
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• Vestigial organs are remnants of structures that
served important functions in the organism’s
ancestors
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Molecular Homologies
• Examples of homologies at the molecular level are
genes shared among organisms inherited from a
common ancestor
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Biogeography
• Darwin’s observations of biogeography, the
geographic distribution of species, formed an
important part of his theory of evolution
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• Some similar mammals that have adapted to
similar environments have evolved independently
from different ancestors
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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
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