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Historical Context
 Organisms are unchanging
 Aristotle & Old Testament
 Georges Cuvier (1812) extinctions happen. The older the
life form, the more different to current version.
 Hutton and Lyell: geological features are produced
gradually over vast time scales
 Lamark proposes organisms change through use and
disuse and inheretance of acquired characteristics
 Wallace proposes a “natural selection” mechanism
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Figure 22.2
1809
Lamarck publishes his
hypothesis of evolution.
1798
Malthus publishes
“Essay on the Principle
of Population.”
1795
Hutton proposes
his principle of
gradualism.
Sketch of a flying
frog by Wallace
1812
Cuvier publishes his
extensive studies of
vertebrate fossils.
1830
Lyell publishes
Principles of Geology.
1858
While studying species in the
Malay Archipelago, Wallace
(shown above in 1848) sends
Darwin his hypothesis of
natural selection.
1790
1870
1809
Charles Darwin
is born.
1831–1836
Darwin travels
around the world
on HMS Beagle.
Marine iguana
in the
Galápagos
Islands
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1859
On the Origin of
Species is published.
1844
Darwin writes his
essay on descent
with modification.
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 drew two inferences from two observations
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Figure 22.UN03
Observations
Individuals in a population
vary in their heritable
characteristics.
Organisms produce more
offspring than the
environment can support.
Inferences
Individuals that are well suited
to their environment tend to leave more
offspring than other individuals.
Selection &
differential
reproduction
and
Over time, favorable traits
accumulate in the population.
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Adaptive
evolution
 Evolution explains:
 The unity of life
 Why there is so much underlying similarities in
apparently different organisms
 The diversity of life
 Why there are so many different types of living thing
 The match between organisms and their
environment (adaptations)
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Figure 22.13
Results
10
Number of individuals
Field Study
Soapberry bug with beak
inserted in balloon vine
fruit
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8
6
4
On native species,
balloon vine
(southern Florida)
Beak
2
0
Average for museum specimens
10
On introduced
species,
goldenrain tree
(central Florida)
8
6
4
2
0
6
7
8
9
Beak length (mm)
10
11
Figure 22.10
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 Natural selection does not create new traits, but
edits or selects for traits already present in the
population
 The current, local environment determines which
traits will be selected for or selected against in any
specific population
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Modern Synthesis
aka neo-darwinism
Modern Synthesis = darwin’s natural selection + modern genetics
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Microevolution is a change in allele frequencies in a population over
generations
Three mechanisms cause allele frequency change
• Natural selection (including sexual selection)
• Produces consistent adaptive evolution
• Genetic drift
• Gene flow
• Artificial selection
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Definitions
A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
A gene pool consists of all the alleles for all loci in
a population
Gene a segment of DNA coding for an
RNA/protein
Allele different versions of a gene
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Effects of Genetic Drift: A Summary
1.
2.
3.
4.
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed
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Gene Flow
Gene flow consists of the movement of alleles
among populations
Alleles can be transferred through the
movement of fertile individuals or gametes
(for example, pollen)
Gene flow tends to reduce variation among
populations over time
can be adaptive or not
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Figure 23.12
Central
population
NORTH SEA
N
Eastern
population
Vlieland,
the Netherlands
Parus major
Population in which
the surviving females
eventually bred
40
Central
Eastern
30
Survival rate (%)
50
20
10
0
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Females born in
Females born in
central population eastern population
2 km
Frequency of
individuals
Figure 23.13
Original
population
Original
Evolved
population population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
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(c) Stabilizing selection
Sexual Selection
Sexual selection is natural selection for mating
success
It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Why Natural Selection Cannot
Fashion Perfect Organisms
1.
2.
3.
4.
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment
interact
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