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•Intro: Why does
evolution matter
Darwin and Natural Selection
Charles Darwin (1809-1882)
• Ideas supported by fossil
• On HMS Beagle he
studied and collected
biological specimens In
the Galapagos Islands
• Noticed that species
were unique to the
islands they were from,
but similar to species on
So what did Darwin say?
1. In nature, organisms produce more
offspring than can survive
2. Variation exists in all populations
fish differ in
size and
3. Individuals with useful variations survive and pass
those traits (alleles) on to their offspring
“Survival of the Fittest”
Any useful variation that helps an organism to
survive and reproduce is called an adaptation.
Example: fast fish
escape predators,
survive and
produce more
fast fish.
4. Over time, organisms with certain
variations become the majority and the
population may look very different from the
ancestral population
For Example, the
Galapagos Finches
studied by Darwin
What did he call his theory?
• Natural Selection
• What does it say?
• The environment
acts on
(variations) in a
causing genetic
change over time.
Self Check Quiz
What type of evidence did Darwin use to support his
Theories of Evolution?
Fossil Evidence
2. Why is variation important to the survival of a population?
Useful variations (adaptaions) survive and the traits (alleles)
get passed on to their offspring
3. Natural selection acts on the ___________ of populations.
Phenotypes (variations)
4. What does “Survival of the Fittest” mean?
Organisms with the most useful traits for reproduction and
survival are able to be successful and pass their traits on to
their offspring.
There are three types of Natural
1. Stabilizing Selection
2. Directional Selection
3. Disruptive Selection
Stabilizing Selection
Example of Stabilizing Selection:
• Large spiders are easily seen and eaten by birds
• Small spiders can’t compete for food
• Medium size spiders are selected for
Directional Selection
Example of Directional Selection:
• A type of insect lives
deep within the bark of
• Woodpeckers with short
or average-size beaks
can’t get to these insects
• Long-beaked
woodpeckers are
selected for
Disruptive Selection
Example of Disruptive Selection: Limpets
• Limpets are snails
that live attached to
rocks in the tidepools
• Limpets range in
color from white to tan
to dark brown
• White colored limpets
have the advantage on
light-colored rocks
• Dark brown limpets
have the advantage on
dark-colored rocks
How is Natural Selection related to
• It is the mechanisms by which evolution
Do the following:
For each of the following examples, tell me which type of
selection it would be analogous to.
My uncle Ted has a real sweet tooth. He will
only eat the sweetest of candies.
On scantron forms, some types of lead are too
soft while others are too hard.
In a basketball obstacle course game, really
large people are good at making baskets and
really small people are best at getting through
the obstacle course, so teams are usually a
mix of very big people with very small people.