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1. The ancestor giraffes had
short necks.
2. The ancestor giraffes
lengthened their necks
by stretching to reach
3. The longer neck was
passed on to offspring.
Whose theory of evolution
is this?
Lamarck’s Hypothesis – Not Supported
• According to Lamarck’s hypothesis, organisms are striving
for perfection.
– Organisms can change their features to be perfectly suited
for their environment.
Does this look like a
“perfect” organism to you?
Winter Flounder
Darwin’s Hypothesis – Well Supported
• According to Darwin’s hypothesis, organisms evolved by
a process called natural selection.
What is natural selection?
Natural Selection
What is natural selection & how does it drive evolution?
What do you think is about to happen?
Natural Selection
• The process by which organisms who are better
adapted to their environment will survive & reproduce
Some giraffes have long necks,
but some giraffes have short
Which giraffes are probably better
adapted for their environment?
The taller giraffes are more likely
to survive & pass down their
genes to offspring.
Natural Selection – “Survival of the Fittest”
• Fitness: ability to survive & reproduce
– High fitness means best adapted, not necessarily the
Some beavers have large, strong
teeth, but some beavers have
small, weak teeth.
Which beavers probably have the
higher fitness?
The beavers with strong teeth
are more likely to survive & pass
down their genes to offspring.
How does natural selection cause evolution?
Variation Within a Species
• Different individuals within a species have differences in
their morphology & physiology.
– Morphology: the form or shape of an organism
– Physiology: the mechanical, physical, & biochemical
functions of living organisms
Morphology – some of
us are tall, while some
of us are short.
Physiology – some of us have a
fast metabolism, while some of
us have a slow metabolism.
Morphology – some cats have long legs,
but some cats have short legs.
Physiology – some cats can digest
cow milk, but some cats can’t.
These differences change the ability of an
organism to survive & reproduce (fitness).
• Some peppered moths are black &
some are white.
• This variation allows some individuals
to survive better than others.
• The moths that survive will reproduce
& pass down beneficial traits.
Which moths have the higher fitness?
The black moths have a higher fitness than
the white moths. They will survive & pass
on the black gene to offspring.
• Any inherited characteristic that increases an organism’s
chance of survival & ability to reproduce
– Increases fitness
Camels store fat in their humps
to help them survive long
periods without food/water.
This is an adaptation that gets
passed down from parent to
A mouse has learned how to avoid mousetraps,
increasing its chances of survival.
Is this an adaptation?
• Learned behaviors are NOT adaptations, because they’re not
carried in genes, so they don’t get passed on to offspring.
• An adaptation is a trait that is coded for in genes, allowing
it to be passed from parent to offspring.
Fitness & Adaptations
• Those individuals that have a higher
fitness will pass on their
morphological & physiological
differences to their offspring.
Which mouse has the higher fitness?
The tan mouse blends in better, &
therefore will survive better.
The tan mouse is more likely to
reproduce & pass down the tan gene to
Selective Pressures
• Over time, populations change based on the selective
pressures of the environment.
• Selective Pressure: any factor that makes it hard for some
organisms to continue surviving, & rewards any advantage
that some organisms may have been born with
The selective pressure in an
environment is what determines which
organisms have a high fitness & which
ones have a low fitness.
Selective pressures drive populations
to evolve.
Selective Pressures
Climate Changes
Food Supply Changes
Sexual Selection
What type of selective pressure do you think
affects this moth population?
Predators make it difficult for white moths to
survive, giving them a lower fitness.
The black moths have the advantage, so they
will contribute genes to the next generation.
Antibiotic Resistance
What’s the selective pressure driving this population to evolve?
Large ground finches use their large beaks to open seeds.
• Let’s say that the area has had an ideal amount of rainfall, so the
plants are able to grow much larger.
• Since the plants are larger, the seeds are also much bigger.
• Now, only the finches with the biggest beaks can eat.
• The finches with smaller beaks don’t eat, so they don’t survive &
• The finches with large beaks survive & pass on their “good
genes” to the next generation.
What do you think the beak sizes will look like in
the next generation?
The next generation of finches will probably have
larger beaks – that is evolution by NS!
The Main Ideas of Natural Selection
• Organisms produce more offspring than can possibly
– Those that do not survive do not reproduce, & therefore do
not pass down their genes.
• Each organism has different advantages & disadvantages in
the struggle for existence.
• Individuals best suited for their environment survive &
reproduce most successfully.
The Main Ideas of Natural Selection
• “Survival of the Fittest”
– Adaptations are inherited, NOT learned!
– Adaptations may be an advantage in 1 environment, but a
disadvantage in another.
• Natural selection only acts on heritable traits.
A mouse that has learned how to
avoid mousetraps does not pass that
down to its offspring in its genes.
This is not an adaptation.
• Peppered moths in the industrial revolution