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Chapter 22 – Descent
with Modification, A
Darwinian View of Life
Darwin
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A new era of biology began on
November 24, 1859. On this day
Charles Darwin published On the
Origin of Species by Means of
Natural Selection
Darwin made two major points in
his book:
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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
Evolutionary History  Cuvier: paleontology
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Linnaeus: taxonomy
Hutton: gradualism
Lamarck: evolution
Malthus: populations
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Lyell: uniformitarianism
Darwin: evolution
Mendel: inheritance
Wallace: evolution
Influences on Darwin
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Linneaus: classification
Cuvier: Fossils, the remains or traces of organisms from
the past; along with classification, helped to lay the
groundwork for Darwin’s theories.
Hutton and Lyell: Gradualism, the idea that profound
change can take place through the cumulative effect of
slow but continuous processes (opposite of
catastrophism); had a strong influence of Darwin’s
thinking
Lamarck: Evolution, idea that current species are
modified descendents of previous/extinct species; even
though mechanism was unsupported (through acquired
traits), but concept was valid
Darwin’s Voyage
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Naturalist on HMS Beagle
5 year voyage around the globe from 1831-1836
Job: to catalogue all of the flora and fauna they encountered
Made significant discoveries to his theory in the Galapagos
Islands
Darwin’s Publication
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, most of
scientific world did not believe in evolution or
gradualism
 In June 1858 Darwin received a manuscript from
Alfred Russell Wallace. Wallace had developed
a theory of natural selection similar to Darwin’s
 Darwin quickly finished The Origin of Species
and published it the next year, 1859
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Darwin’s Theory
Descent with Modification - all organisms are
related through descent from an ancestor that
lived in the remote past
 Modification through Natural Selection populations of organisms can change over
generations if individuals having certain heritable
traits leave more offspring than others
(differential reproductive success), organisms
better “fit” to the environment will reproduce
more successfully thereby passing on their
better adapted genes.
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Darwin’s Evidence
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Darwin’s
Finches –
looked at
beaks
adapted to
specific diets
found on
different
islands
Darwin’s View
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In the Darwinian view, the history of life is like a tree with
multiple branches from a common trunk. The tips of the
youngest twigs represent the diversity of living organisms
as they exist today
Darwin’s Theory Explained
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Evolutionary biologist Ernst Mayr – dissected
the logic of Darwin’s theory into three inferences
based on five observations
Observations:
1. Exponential fertility
2. Stable population size
3. Limited resources
4. Individuals vary
5. Heritable variation
Inferences:
1. Struggle for existence
2. Non-random survival
3. Natural selection
(differential success in
reproduction)
Darwin’s Theory Explained
Observation #1: For any species, population sizes
would increase exponentially if all individuals that
are born reproduced successfully
 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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Darwin’s Theory Explained
Observation #4: Members of
a population vary extensively
in their characteristics; no two
individuals are exactly alike
 Observation #5: Much of this
variation is heritable
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Darwin’s Theory Explained
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
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Natural Selection
Differential success in reproduction that results
from the interaction between individuals that
vary in heritable traits and their environment
 Can produce an increase over time in the
adaptation of organisms to their environment
 If an environment changes over time, natural
selection may result in adaptation to these new
conditions
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Evidence for Natural Selection
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Examined predation
pressures
Transplanted guppies
from pike-cichlid pools
to killfish pools,
measured ages and
size over 11-year
period
Average size and age
at maturity of guppies
of transplanted
populations increased
Concluded that
change in predator
resulted in some
population variations
being favored over
others
Evidence for Evolution
Homology
 Embryology
 Vestigial Structures
 Similarities in Macromolecules
 Biogeography
 Fossils
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Evolution Evidence: Homology
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Anatomical resemblances that represent
variations on a structural theme that was present
in a common ancestor
Evolution Evidence: Embryology
Comparative embryology reveals additional
anatomical homologies not visible in adult
organisms
 Shows common ancestry
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Evolution Evidence: Vestigial
Remnants of
structures that
served important
functions in the
organism’s
ancestors
 Shows
evolutionary
history of a
species and
common ancestry
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Evolution Evidence: Molecular
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Biologists also
observe homologies
among organisms at
the molecular level,
including genes that
are shared among
organisms inherited
from a common
ancestor and the
proteins encoded in
those genes.
Evolution Evidence: Biogeography
The geographic
distribution of
species
 Some mammals
that have adapted
to similar
environments
have evolved
independently
from different
ancestors
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Evolution Evidence:
Fossil Record
The succession of
forms found in the fossil
record
 Shows evolution of
current species from
ancestral species over
time
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