The Darwinian Revolution

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The Darwinian Revolution
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Archbishop James Ussher of Ireland (1581-1656)
The earth was created on October 22, 4004 BC.
Ushers History of the World
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 " ... we find no vestige of a beginning, no
prospect of an end."  James Hutton, Theory of
the Earth (1795)
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Table Mountain Cape Town, South Africa
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• Conclusions
• The earth is much older than we thought.
• Different creatures have inhabited the earth at
  different times.
• Problem How did this happen?

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• Two Theories
• Catastrophism
• Descent with Modification (Evolution)

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Darwins Three Observations
The geometrical increase of populations. If left
unchecked, the size of the population of a single
animal or plant species would increase until the
world is overrun.
Variation. Individuals within the same species
are not always exactly alikethey differ in their
particular characteristics.
Inheritance. Individuals tend to pass on their
own particular characteristics to their
descendants.
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Darwins Argument(Created from Animals, pp. 35-6)

• (1) Organisms tend to reproduce in such numbers
  that, if all survived to reproduce again, they
  would soon overrun the earth.
• This does not (and could not) happen. No species
  can continue to multiply unchecked.
• (3) It follows that a high percentage of
  organisms must die before they are able to
  reproduce.

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• Therefore, there will be a "struggle for
  existence" to determine which individuals live
  and which die. What determines the outcome of
  this struggle? What determines which live and
  which die?
• There are two possibilities it could be the
  result of random causes or the reason could be
  related to the differences between particular
  individuals.

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(5) Darwin admits that sometimes it is random
that is, the reason one organism survives to
reproduce, while another does not, may sometimes
be attributable to causes that have nothing to do
with their particular characteristics. One may
be struck by lightning, while another is not, and
this may be mere luck.
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• (6) But sometimes it is a matter of differences
  between individual organisms. Consider
• There are differences ("variations") between
  members of species.
• Some of these differences will affect the
  organism's relation to its environment, in ways
  that are helpful or harmful to its chances for
  survival.
• Therefore, because of their particular
  characteristics, some individuals will be more
  likely to survive (and reproduce) than others.

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(7) Organisms pass on their characteristics to
their descendants. (8) Therefore, the
characteristics that have "survival value" are
passed on, and tend to be more widely represented
in future generations, while other
characteristics tend to be eliminated from the
species.
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(9) In this way, a species will be
modified--the descendants of the original stock
will come to have different characteristics than
their forebears. (10) When enough of these
modifications have accumulated, we call the
result a new species.
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Alfred Russel Wallace (1823-1913)