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The Evolution of
Evolutionary Theory
Theory vs Fact
 Scientific Theory: “A well-substantiated
explanation of some aspect of the natural
world, based on a body of facts that have
been repeatedly
confirmed through observation and
experiment.“
Theory vs Fact
 Scientific Fact: Scientists most often use
the word "fact" to describe an
observation. But scientists can also use
fact to mean something that has been
tested or observed so many times that
there is no longer a compelling reason to
keep testing or looking for examples.
Theory? Fact? Both?
 "Darwin continually emphasized the
difference between his two great and
separate accomplishments: establishing
the fact of evolution, and proposing a
theory—natural selection—to explain the
mechanism of evolution.“
 Scientists continue to argue about
particular explanations or mechanisms at
work in specific instances of evolution,
but the fact that evolution has occurred
and is still occurring is undisputed.
James Hutton (1726-1797) and
Charles Lyell (1797-1875)
 Studied the forces of wind, water,
earthquakes and volcanoes.
 They concluded the earth is very old and
has changed slowly over time due to
natural processes
Erasmus Darwin(1731-1802)
 Suggested that competition between
individuals could lead to changes in
species.
 He was also Charles Darwin’s
grandfather.
Jean Baptiste Lamarck
(1744-1829)
 The first scientist to propose a system of
evolution.
 Noticed that fossils became more
complex in earlier rock strata, therefore a
drive towards complexity.
 Each species came from less complex
ones
Lamarck (continued)

Believed that evolution is based
on 2 principles:
1. Law of use and disuse
The more an animal uses a
certain part of the body, the
stronger and better developed
it becomes
2. Inheritance of acquired
characteristics
Characteristics an organism
developed through use and
disuse could be passed on to
its offspring
Thomas Malthus (1766-1834)
 Observed that human
populations cannot keep
growing indefinitely.
 If the birth rate continued
to exceed the death rate,
eventually humans
would run out of space
and food.
 Famine, disease and
war prevented endless
population growth.
Charles Darwin
(1809-1882)
 Formulated a theory of evolution
by natural selection based on observations
made during his voyage on the Beagle, and of
selective breeding of farm animals, plants and
pets.
 He drafted manuscripts outlining his theory in
the 1840s but hesitated to release them to the
public.
 His most famous work On the Origin of Species
by Means of Natural Selection was published
in 1859.
Darwin’s Theory:
 Overproduction: more offspring are produced
by an organism than can possibly survive.
 Competition: high birth rates cause a shortage
of life’s necessities leading to competition
between organisms.
 Variation: each individual differs from all other
members of its species; some differ more than
others.
Darwin’s Theory (continued)
 Adaptation: allows organisms to become
better suited to their environment.
 Natural Selection: the most fit (best
adapted) organisms survive and
reproduce.
 Speciation: formation of new species
from ancestral species by means of
natural selection.
Alfred Russell Wallace
(1823-1913)
 Proposed a theory of evolution
by natural selection similar to that
of Darwin.
 He wrote a paper and sent it to Darwin to
review. This paper spurred Darwin on to finally
agree to the release of his theory.
 In 1858, Charles Lyell presented Darwin’s
1844 essay and Wallace’s paper to the public.
Darwin SONG!
http://www.youtube.com/watch?v=Pt2gHpq
fZNA