Transcript 16.3_Darwin_Presents_His_Case

1 Review What happens in the process of natural
selection
Explain Why do organisms with greater fitness
generally leave more offspring than organisms that
are less fit
Compare and Contrast How are natural selection
and artificial selection similar and how are they
different
2 Review Why were Hutton’s and Lyell’s ideas
important to Darwin
Visual Thinking Think about the teeth in a lions
mouth. How is the structure of the lion’s teeth an
adaptation
CH 16 DARWIN’S THEORY OF EVOLUTION
16.3 Darwin Presents His Case
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Darwin wrote up a complete draft of his ideas
about natural selection, but he put the work aside
and didn’t publish it for another 20 years
His theory was very radical, so he wanted to gather
as much evidence to support his ideas before he
made them public
He wrote on barnacles during that time.
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1858, Darwin reviewed an essay containing similar
ideas about evolution by Alfred Russel Wallace
Darwin decided to move forward with his own
work
Wallace’s essay was presented together with some
of Darwin’s observations at a scientific meeting in
1858.
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“On the Origin of Species by Means
of Natural Selection, or the
Preservation of Favored Races in
the Struggle for Life” was published
in November of 1859
First printing was 1250 copies, 15
schillings each sold out on the first
day
Never been out of print since.
Natural Selection
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Occurs when:
More individuals are born than can survive (the
struggle for existence)
There is natural heritable variation (variation and
adaptation)
There is variable fitness among individuals
(survival of the fittest).
The Struggle for Existence
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If more individuals are produced than can survive,
members of a population must compete to obtain
food, living space, and other limited necessities of
life
Came from reading Malthus.
Variation and Adaptation
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Individuals have natural variations among their
heritable traits
Some of those variants are better suited to life in
their environment than others
Adaptation
 Heritable
characteristic that increases an organism’s
ability to survive and reproduce in its environment.
Adaptations
Survival of the Fittest
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Fitness
 How
well an organism can survive and reproduce in its
environment
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Individuals with adaptations that are well-suited to
their environment can survive and reproduce
Individuals with characteristics that are not wellsuited to their environment either die without
reproducing or leave few offspring.
Natural Selection
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Process by which organisms with variations most
suited to their local environment survive and leave
more offspring
Darwin’s mechanism for evolution
The environment - not a farmer or animal breeder
- influences fitness.
Steps of Natural Selection
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There is a population with variation
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not only cup lip, but body size, eye color, etc…
Some variations prove favorable.
Steps of Natural Selection
3. Those with favorable variation produce more
offspring (why), and their offspring share their
favorable trait (why)
4. Over time, each new generation has a greater
percentage of individuals with the favorable trait
until the whole population has it.
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From generation to generation, populations
continue to change as they become better
adapted, or as their environment changes
Natural selection acts only on inherited traits
because those are the only characteristics that
parents can pass on to their offspring.
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Grasshoppers can lay
more than 200 eggs at a
time, but only a small
fraction of these
offspring survive to
reproduce.
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This population includes
yellow and green body
color.
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Green is camouflaged,
green grasshoppers
have higher fitness and
so survive and
reproduce more often
than yellow
grasshoppers do.
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Green grasshoppers
become more common
than yellow
grasshoppers in this
population over time
because more
grasshoppers are born
than can survive.
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Natural selection does not make organisms
“better”
Adaptations don’t have to be perfect
Natural selection also doesn’t move in a fixed
direction.
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If local environmental conditions change, some
traits that were once adaptive may no longer be
useful, and different traits may become adaptive
If environmental conditions change faster than a
species can adapt to those changes, the species
may become extinct.
Common Descent
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All species - living and extinct are descended from ancient
common ancestors
Every organism alive today is
descended from parents who
survived and reproduced.