Science 20 Unit D: Living
Systems
Scientific Theory
• Scientific theories develop over time
• They are formed from many observations, experiments, and models
“Theory of Evolution by
Natural Selection”
• A well-supported and
widely accepted
explanation
of the
diversity
and
Developing the Theory
• Plato and Aristotle (~300 - 400 BC) were two of the most influential Greek philosophers
• “All life existed in a perfected and
unchanging form”
• This view of life dominated
for over 2000
years
Some Greek philosophers believed that life evolved gradually, but they did not have the influence of Plato or
Georges-Louis Leclerc, Comte de
Buffon (1707 - 1788)
• Naturalist
• 1749 - published
Historie Naturelle
(the
first public challenge of the Greek
school of thought)
• Speculated that humans and apes could
have a
common ancestor
Georges Cuvier (1769 - 1832)
• Paleontologist
• His observations:
– older rock strata show show more dissimilar species from modern life
– species have become extinct over time
• Cuvier proposed that the Earth had
Charles Lyell (1797 - 1875)
• Geologist
• Lyell proposed that the Earth has been
shaped by slow and continuous processes, rather than sudden catastrophic events
• The processes that shaped history are still occurring today
Buffon
Lyell Cuvier
These individuals contributed ideas and observations to the theory of evolution, but they did not propose a mechanism
Jean-Baptiste Lamarck
(1744 - 1829)
• Compared current species with fossil forms
• Observed a “line of descent” - fossils became more complex over time
• The first scientist to propose a theory of evolution with a
Lamarck’s Theories
(1) New, simple species are continually
created by
spontaneous generation
(arising from non-living material) and
gradually become more complex
•
Organisms have a “force” or “desire”
within that leads them to become
(2)
The term “Lamarckism” refers to the
concept of
inheritance of acquired
characteristics
• The use and disuse of certain structures
could be passed on to offspring
•
(eg)
A giraffe must continually stretch
its neck to reach its food, and so
develops an elongated neck. This long
neck is inherited by its offspring.
Evaluating Lamarck’s Theories
• “pros” - he recognized the role of
environment in driving evolutionary change
• “cons” - we know that acquired traits
cannot be inherited. His theory did not
consider populations or variation - change
could be accomplished by the individual.
• By the end of the 1800’s, as biologists
Darwin’s Journey
• In 1831, 22 year-old Charles Darwin (1809 - 1882) left England on the HMS Beagle, a
British survey ship.
• Darwin made many observations over the 5 year journey - he did not initially understand their significance, but they would later
(1)
The flora and fauna of South America
were very distinct from those Darwin
studied in Europe and England
Why were there distinctive clusters of similar organisms in different parts of the world? Why weren’t they
(2)
Darwin observed fossils of extinct
animals that looked very similar to living
animals
Why would living and fossilized organisms that looked similar be found within the same region? Why would
(3)
The finches that Darwin observed on
the Galapagos Islands closely
resembled birds from
the South American
mainland.
Why was there such a close resemblance? Could these Galapagos species have originated from the mainland
(4) Galapagos species looked identical at first, but actually varied between islands. For example, each finch was adapted to eating a different type of food based on beak size and shape.
(5) Darwin observed fossil deposits of coral at elevations of 3000 m. A severe earthquake lifted the coastline by 2m.
• When Darwin returned to England, he
began to organize these observations
• He also questioned experts from other
fields of study
• He became very interested in
artificial
selection
and wondered if the
environment could have the same affect
• He gained more confidence in his
theory, but was aware of how
Theory of Evolution by Natural
Selection
• Alfred Russel Wallace (1823 - 1913) reached similar conclusions to Darwin through his
work in Malaysia
• Both papers were made public at an event organized by Charles Lyell in 1858
• Darwin published On the Origin of Species in 1859, complete with a mechanism to explain
how evolution occurred (a process he called
Darwin’s theory was based on 3 key
observations and inferences
(1)
There is a great deal of
variation
within
a species
(2)
Every generation produces more
offspring than can survive to reproduce
–Individuals are in a constant struggle for survival. Resources are not
infinite.
(3)
Populations tend to remain stable in
size
Because individuals with more
favourable variations contribute the highest proportion of offspring to
succeeding generations, their traits will become more common. This is
Example
• Imagine an ancient population of giraffes, each with different neck lengths
• Giraffes with long necks have a more
consistent food supply and stay healthier
• These giraffes lead longer reproductive lives and are able to better care for and protect their young
On the Origin of Species
• In his book, Darwin assembled a group
of facts and observation that had
previously seemed unrelated
• He proposed that
– present forms of life have arisen by
descent and modification from an ancestral species
In Darwin’s words…
• As many more individuals of each species are born than can possibly survive; and as, consequently,
there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally
