Download Theories of Evolution


Evolution
Origins
When was evolution first discussed?
 Geological record challenged traditional
thought
 Key evidence: Earth’s crust was
stratified

Interpretation?
 Earth is very old!
 The fossil record also showed
organisms thought to not be in existence
 Questions arose, and the answers were
debated
 General conclusion: life has existed on
Earth for much longer that initially
thought

Jean-Baptiste Lamarck
As evolution gained popularity, question
now became, “How?”
 Enter the soldier-turned-invertebrate
taxonomist

Released numerous papers between
1800 and 1809, summed up in
Zoological Philosophy (1809)
 Why the change of heart?
 Paris sits on a rather large fossil bed
 Most scientists speculated that species
found in shallow Paris basin now lived in
other parts of the world
 But, by Lamarck’s time, inventory of
species had gone so far it seemed
unlikely

Extinction?
 Again, Lamarck says, “Nay, nay”
 Lamarck clung to notions of benevolent
Mother Nature
 He postulated that species had evolved,
and set out to determine the mechanism

Two-Factor Mechanism
Lamarck was a believer in the “great
chain of being” (similar to Aristotle)
 Believed simplest organisms
continuously being spontaneously
generated (again Aristotle)
 All species can be traced back to simple
ancestors
 Humans, and other “higher” species
have just been around longer


However, this does not address how
animals change habits (and he believed,
form) to adapt
Georges Cuvier
Lamarck’s ideas certainly were not
universally accepted
 Cuvier noted a greater number of extinct
species in older strata
 Suggested a “progression” of life on
Earth
 Yet he did not see his findings as
evidence of evolution

Three Key Arguments
Great chain of being is false
1.
•
Living things are too complex and diverse
Mass extinction in the past is a reality
2.
•
Majority of species that had ever existed
had gone extinct
The correlation of parts makes
evolution impossible
3.
•
Animals in the process of change would be
less suited for their environment and would
become extinct
Catastrophic Geology
Cuvier noted the catastrophic events
that punctuated Earth’s history
 After each event, new species of plants
and animals had arisen
 Somewhat avoided where they had
come from

Thomas Malthus
Yet another key figure in the evolution
story before Darwin
 Wrote Essay on the Principle of
Population
 Basically stated that birth rates
exceeded death
 Applies even more so in nature

Charles Darwin
Born in 1809
 Father was a physician (and
evolutionist), mother came from a
wealthy family
 1825 went to University of Edinburgh to
study medicine
 After, went to Cambridge to become….
 …a clergymen

While there, studied natural history,
caught the attention of prominent
professors
 Set out to serve as a naturalist on the
HMS Beagle
 Sent many specimens back to London –
this gained him respect
 Read Principles of Geology by Charles
Lyell – converted to uniformitarianism
 Opposed catastrophism (Cuvier)

Views of uniformism coincided with
tranformism brought up by Lemarck
 Saw many things while on the Beagle
that indicated evolution was happening
 His next move?
 Nothing. (at least with regards to
evolution)
 Mechanism was still an unknown
 Moved back to England, received
money from his father, and…married his
cousin

Moved to Downe House in 1842
 This is where he would compose one of
the most ground-breaking and
controversial books in history, On the
Origin of Species by Means of Natural
Selection

Developing Natural Selection
Privately, Darwin was very enthusiastic
about Lemarck’s theories
 However, it seemed limited to him
 Thought key was variation – observed
domestic animals
 Read work by Malthus – thought
struggles may apply in nature as well
 Key idea: competition
 Also focus on struggles within a species

Delays
Darwin was ready to publish his findings
in 1844, but waited until 1859
 Why??
 A book by a colleague, Robert
Chambers outlined his views on
evolution
 Not well received
 Works out well for Darwin though

New Ideas
After witnessing Thomas Huxley tear
apart Chamber’s book, Darwin reevaluates some of his ideas
 Initially theorized evolution was
intermittent – abandons this
 Also drops idea of perfect adaptation
(the correlation of parts idea)
 Viewed evolution as two types

Linear view
1.
•
One species continues to change
Speciation
2.
•
Species diverge off into different species to
find other niches
Based on what he saw in the
Galapagos, speciation seemed to make
the most sense
 Stated that natural selection acted as “a
thousand tiny wedges, driving evolution”
 Adaptation to the environment
(Lemarck) was the why, natural
selection was the how
 So why 1859?
 Alfred Russell Wallace writes letters to
Darwin in favour of evolution

Sends a manuscript – shockingly similar
to Darwin’s ideas
 Both release their work in 1859, Darwin
gets recognition
 This, yet again, sparked vigorous
debates about evolution
 Darwin wants no part, Huxley steps in to
defend his ideas
 Refers to himself as “Darwin’s Bulldog”
 Unfortunately, Darwin dies in 1882,
when his work experienced a drop in
support

Experiences a revival in early 1900’s
with advent of Mendelian genetics
 Two theories combine in what was
referred to as the “forging of the modern
synthesis”
