Darwin and Natural Selection

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Darwin and Natural Selection
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Charles Darwin 1809-1882

• Charles attended boarding school as a boy, where
  he developed an interest in Natural History.
• Father sent him to study medicine in Edinburgh.
  Turned off by barbaric surgery practices and dull
  lectures.
• Joined Natural History society, read Buffon,
  Cuvier, Lamarck, others.

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• Darwin left Edinburgh determined not to be a
  physician. Attended Cambridge, thinking he would
  be a clergyman.
• At Cambridge, he read Herschel (philosophy of
  science), Humboldt (geological distribution of
  organisms), Lyell.
• After graduation in 1831, Darwin was encouraged
  by friends to apply to be a captains companion
  and naturalist on board the Beagle.
• December 27, 1831 Joins Captain Fitzroy on board
  the Beagle for a five-year journey.

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Voyage of the H.M.S. Beagle

• Official purpose of the voyage was to survey the
  South American coast.
• During the voyage, Darwin observed a volcanic
  eruption, an earthquake, and noted geologic
  features suggesting an ancient earth.
• In South America, found fossils of ancient
  sloths, observed living rheas (ostrich-like
  birds) and llamas, many other animals not
  described by naturalists. Collected huge numbers
  of specimens.

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The Galapagos

• In September of 1835, the Beagle reached the
  Galapagos Islands.
• Fascinated by the giant tortoises. Brought three
  back alive.
• Made detailed observations of finches,
  mockingbirds. Realized that they resembled birds
  on the mainland, yet were different. Where did
  they come from?
• Besides the Galapagos, Darwin observed organisms
  on Tahiti and other Pacific islands. Later
  developed theories of coral reef building.

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Darwins questions

• Why the replacement of species geographically
  that is, why are the species on islands not
  exactly the same as those on the nearby mainland?
• Why the replacement of species in time?
• Where do species come from in the first place?
• Change in species appears to be the answer but
  how do species change?

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Natural Selection

• Darwin rejected the idea that all members of a
  population are shaped equally by the environment
  and that all change together. Selection does NOT
  look like this

Generation 1
Generation 100
Generation 200
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Natural Selection

• Rather, selection begins with variation. Some
  variants in a population may be at an advantage,
  some at a disadvantage. Those with a strong
  advantage are more likely to have offspring.
• Fitness is the ability to survive.
• Success is passing ones traits to the next
  generation.

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Natural Selection

• Natural Selection looks more like this. Suppose
  in a population of horses, black horses get
  hotter in the sun and are more likely to die of
  heat exhaustion.

Generation 1
Generation 2
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Natural Selection

• But suppose global climate changes alter the
  local environment. It becomes cooler and
  cloudier. Now the black horses have an advantage
  because their black coats help them stay warmer.

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Natural Selection

• Notice the difference
• Buffon and Lamarck would say, The horses need to
  be darker, so they will get blacker and blacker
  each generation.
• Darwin says, Black horses are better able
  survive and reproduce each generation, so there
  will be more and more black horses each
  generation.

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Darwins New Questions

• Darwin noted two difficulties in his original
  theory of Natural Selection
• Where did variation come from in the first place?
• How did favorable traits get passed on to the
  offspring, if traits of each parent are blended
  in the offspring (as was commonly believed at the
  time)?

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In the 20th Century...

• Darwins questions led other scientists to
  explore heredity, which led to the science of
  genetics.
• Genetic research in the 20th century supported
  natural selection as a plausible mechanism, and
  raised new questions and new possibilities.

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Darwin, Buffon, or Lamarck?

1. Members of the dog family that migrated to North
   America were shaped during fetal development by
   the American environment. They became wolves,
   which were best adapted to the environment.
2. The original wolf ancestors that arrived in
   North America had a wide variety of traits. Some
   traits were favorable, some were not. Those with
   most favorable traits, such as heavy fur, sharp
   fangs, and pack hunting behaviors, tended to
   survive better, and these survivors gave rise to
   modern wolves.
3. Small dog-like ancestors had to run after prey
   and survive in the cold North American winters.
   Running exercised their legs, which grew longer
   each generation. Exposure to cold caused them to
   grow thicker coats generation by generation.
   Slowly, the dog-like creatures became modern
   wolves.

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Thinking question

• In groups, discuss and answer this question
  Buffon, Lamarck, and Darwin all thought that
  species changed over time, and each developed a
  theory that explained how that change might
  occur. Why is Darwins mechanism, Natural
  Selection, still accepted (though modified) by
  todays biologists, but Buffons and Lamarcks
  theories are not?