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NATURAL SELECTION
The ideas that shaped Darwin…
Hutton, Lyell, Lamarck and Malthus
Evidence of a Changing Earth
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Scientists Hutton and Lyell
studied the organisms
found in fossils.
The older the fossil, the
deeper it was buried
Changes in the
appearance of the
organisms could be seen
between layers.
Lamarck’s Evolution Hypothesis
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In 1809, Lamarck
proposed that an animal’s
behavior could change the
traits passed down to their
babies.
He thought, the more you
use it, the more it grows.
He did not know about
how genes were passed
down.
Malthus’ Population Growth
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Population growth is
limited by the
resources available,
such as food and living
space.
Once the food or the
space starts to run out,
then the growth rate
will have to slow and
then stop.
Darwin’s Work
Inherited Variation
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Slight mutations in the
genetic code happen
Before Darwin, scientists
did not think this was
important
Darwin claimed that
differences matter and
can change the direction
of a species
Artificial Selection
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Nature provides genetic
variation
Humans often select for
traits that they find most
useful.
Example: We like larger,
sweeter tomatoes, so we
will selectively breed
plants for those traits.
Struggle for Existence
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Species have to
compete for the
best food, living
space and mates
This drives the
selection process
Survival of the Fittest
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Organisms with the best adaptations for the
environment will survive.
These adaptations are passed down through
the genes from parents to babies.
The others will die off.
Darwin called this,
“Natural Selection.”
Evidence for Evolution
The Fossil Record
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There are many marine
life fossils
They saw many species
with shells
The shells got
increasingly complex,
suggesting evolution
took place over time
Homologous Body Structures
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Same bones, but
different uses
Mammal Examples:
 Human Arm
w/Hand
 Horse Leg w/Hoof
 Whale Flipper
 Bat Wing
Similarities in Embryos
Types of Selection
Directional Selection
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Individuals at one end of the curve are better
fitted for the situation, so all shift.
Stabilizing Selection
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Individuals in the middle of the curve are best
fitted for the situation, so ends drop off.
Disruptive Selection

Individuals at either extreme are better fitted
for the situation than an average individual.
Genetic Drift

Certain traits many become more common in a
population by chance, rather than by a type of
purposeful selection.
Hardy-Weinberg Principle
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1.
2.
3.
4.
5.
There are 5 was to have a population at
equilibrium (where there is no change)
Random Mating
Large Population
No Movement into or
out of the population
No Mutations
No Natural Selection
The Process of Speciation
Reproductive Isolation

Only individuals of the same species can mate
?
Behavioral Isolation

Individuals could breed, but their styles of
behavior or courtship make them not attracted
to one another.
Geographic Isolation

Individuals could mate, but they are
separated by distance
Temporal Isolation

Individuals could
mate, but they
naturally mate at
different time
periods.
Process for the Evolution of Darwin’s Finches
How Speciation Occurred…
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Founders Arrive: A few finches flew to one of the
islands from South America and mated
Separation of Populations: Some of these
finches flew to a neighboring island
Changes in Gene Pool: Adapting to new island
Reproductive Isolation: Now the finches on the
different islands are too different to mate!
Ecological Competition: need food & nests
Continued Evolution: natural selection continues
Patterns of Macro-Evolution
Extinction

Species that cannot adapt to their environment
will die out.
Adaptive Radiation
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Many species evolve from a single species
Convergent Evolution
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Some species look
similar
However, they are not
closely related
genetically
Similar environmental
factors cause the
species to evolve in
similar ways.
Beaver
Coypu
Coevolution
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The process when two species evolve in
response to changes in each other over time
Example: Flowers change to keep attracting
the bees needed to pollinate them.