Darwin and Natural Selection

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Darwin and Natural Selection

• Intro Why does evolution matter now?
  http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html

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Charles Darwin (1809-1882) Ideas supported by
fossil Evidence 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 Mainland
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So what did Darwin say?

• In nature, organisms produce more offspring than
  can survive
• Example
• Fishes

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• 2. Variation exists in all populations
• Example
• fish differ in
• color,
• size and
• speed

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• 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.
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• 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
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What did he call his theory?

• Natural Selection
• What does it say?
• The environment acts on phenotypes (variations)
  in a population, causing genetic change over time.

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Self Check Quiz

• What type of evidence did Darwin use to support
  his Theories of Evolution?
• Fossil Evidence
• 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.

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There are three types of Natural Selection

1. Stabilizing Selection
2. Directional Selection
3. Disruptive Selection

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Stabilizing Selection
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Example of Stabilizing Selection Spiders

• Large spiders are easily seen and eaten by birds
• Small spiders cant compete for food
• Medium size spiders are selected for

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Directional Selection
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Example of Directional Selection Woodpeckers

• A type of insect lives
• deep within the bark of
• trees
• Woodpeckers with short
• or average-size beaks
• cant get to these insects
• Long-beaked
• woodpeckers are
• selected for

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Disruptive Selection
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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

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How is Natural Selection related to Evolution?

• It is the mechanisms by which evolution occurs

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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.