Transcript Darwin

Darwin and Natural Selection
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.
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
captain’s companion and naturalist on
board the Beagle.
December 27, 1831: Joins Captain
Fitzroy on board the Beagle for a fiveyear journey.
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.
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.
Darwin’s 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?
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
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 one’s traits to the next
generation.
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
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.
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.”
Darwin’s 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)?
In the 20th Century...
• Darwin’s 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.
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.
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 Darwin’s
mechanism, Natural Selection, still accepted
(though modified) by today’s biologists, but
Buffon’s and Lamarck’s theories are not?