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Central Question to Early
what explains the vast diversity
and surprising similarity of life?
Unity – All organisms share
some traits.
Diversity – Organisms are
dramatically different.
• >1.5 million named species
• Life in all corners of the word
• Most species seem to ‘fit’ in with their
• Some species have adaptations for seemingly
different habitats
• Why are there so many different species?
A small sample of biological diversity
How can we explain the
diversity of life on this planet?
History of evolution
• History of science is important to
understand the philosophy of science
• How did scientists and society explain
adaptations and how did this
understanding shape their view of the
Fit of structure to function
Fit of structure to function
Similar features of different species
Adapted to their environment
Adapted to their environment
Changing view of science through
• philosophy of science swung between
rationalism and empiricism
• Rationalism
– ideas first, data later. Similar to mathematics
based on ideal principles. Used by the Greeks
• Empiricism
– gather data then develop ideas. Developed by
the British late 1700’s
The Greeks
• Were first to formalize
• Others were doing
science at the time but
the Greeks codified
these ideas
Greek explanation of variation
• Socrates/Plato:
variance doesn't
matter, only essences
of forms
• Different tables are
simply variants of an
ideal table.
– Ideal member of the species
– Some where there is a perfect tiger or human
or bird
– All other organisms strive to approach this
– Variation of species are mistakes of this
– Do we think this way?
Scala Naturae: great chain of being
• Need to rank objects,
organisms and
objects on a scale of
• Humans (land owning
Greek men) at the top
rung, dirt and moss
on the bottom rung
• Does our culture have
similar ideals?
Middle ages in Europe
• No human could
approach the genius
of the Greeks
• Scholars copied texts
• New ideas were not
looked on favorably
Middle ages in Arab countries
• Mathematics and
sciences continued
• More empirical than
Greek or Roman
• Scientific method
similar to modern
• European countries
were able to import
these ideas
• Printing press
– Allowed information to
be quickly transmitted
• Global travel
– Brought new plants
and animals to
European attention
– Maps expanded world
Accepted views in Europe
• Scala Naturae: great
chain of being
("ladder of life")
• Inanimate objects
• Plants
• Lower animals
• Man
• Angels
• God
A new view of biologic variation
• A Natural perspective
viewed biologic
variation not as a
corruption of the ideal
form but as
• A species could have
some or all of these
And scientists were finding ‘new’
Figure 22-03
• Georges Cuvier an early 19th century French anatomist
• Attempted to explain the patterns of extinction and
succession observed in the fossil record
• Thought Earth was young and species unchanging
• Miracles are an agent of change
• Gradualism
– Slow series of natural process
– Laws of nature were not disrupted (no miracles)
– Disturbances such as earthquakes and volcanoes were
the result of natural not supernatural processes
• Uniformitarianism
– You can understand the past by looking at what is
happening today.
– The past is a multiplication of current processes
Snake River Canyon
Grand Canyon
• James Hutton (‘father
of geology’)
gradualism (1788)
and Charles Lyell:
• wrote the principles of
geology that Darwin
took with him on his
Jean Baptiste de Lamarck
• Failed novelist
• Botanist
• Father of
Paleontology (study
of fossils)
• Proposed that
species changed
through time
Theory of Acquired Characteristics
• Lamarck suggested a
mechanism, inheritance
of acquired
• Children inherit traits that
their parents acquire
through life.
• So a giraffe got its long
neck by stretching it and
passing that longer on to
its offspring.
• Organisms strove to
ascend the ladder of life
plants strove to become
animals, ect
Two problems with Lamarck’s theory
• It’s goal-oriented:
suggests that evolution
occurs to satisfy a need
that a species has.
• Inheritance of acquired
– How could we test this?
Mid 1800’s
Rationalism to Empiricism
• Understanding that the Earth was many
million of years old
• Similarity of fossils and living organism
• Many agreed with common decent
• Evolution (change over time) was
generally accepted
• English and American scientists wanted a
mechanism to explain diversity
Charles Darwin
• As a young man
dropped out of
medical school.
• Entered the seminary
• Convinced his father
to let him travel for 5
years around the
Darwin’s Voyage on the Beagle
Evidence Darwin found
Geographic variation of species
• Galapagos Islands: The resemblance of
the different finches (a variety of bird) on
the individual islands caused him to
question the “fixed” nature of species.
• On Continents: Observed differences
between “fixed” species when they were
separated by long distances on the
Adaptive radiation
• Occurs whenever there are open niches.
• Some individuals of a species expand into
unfilled niches (roles) in the environment.
• Thus, over LONG time periods through
selective reproduction and genetic
variation, the population may evolve
slightly and better fit that role.
Darwin talked to pigeon and plant
breeders about artificial selection
• By selective breeding,
you can change
appearance of birds,
dogs, cattle
Brussels sprouts
Wild mustard
Thomas Malthus
• An Economist
• Essay On the Growth
of Populations gave
Darwin the idea that
not all young can
• Malthus worried that
the Earth would soon
Received letter from Alfred Russel
• Darwin spent 20yrs
working on Origin of
• Darwin received
Wallace’s paper about
a theory of evolution
by natural selection.
• Wallace asked Darwin
to publish his paper if
he liked it
Darwin’s Principles
First Principle
• For any species,
population sizes
would increase
exponentially if all
individuals that were
born reproduced
A mushroom
produces millions
of spores
A mustard plant
¾ million seeds/yr
A sea hare produces over a million eggs/yr
• Gestation period of 22 months and
produce one calf.
– Say we start with 500 elephants.
– And each female produces 1 calf every 22
– In only 50 generations there would be how
many elephants (Hint, calculated the same as
compound interest)
143,813,294 elephants
But populations remain relatively
Limited Resources
Mating partners
Second Principle
• Survival of the fittest
– The reproductive potential of populations
exceeds the capacity of the environment to
support them. Reproductive over capacity
makes a struggle for existence among
individuals inevitable.
Reproductive Overcapacity
Third Principle
• Individuals of a population vary in their
characteristics. Much of this variability is
Population = a group of potentially interbreeding
organisms that share the same gene pool and
are geographically linked
Millions of offspring=millions of
variations that can be selected
Fourth Principle
• Individuals with heritable traits “best
suited” to the local environment leave a
disproportionately large number of
Fourth Principle
• Disproportional representation by individuals
with the “best suited” traits increases the overall
frequency of these traits in the next generation.
Disproportionate Representation
Fifth Principle
• The effects of
reproductive success
accumulate over
generations to bring
about evolutionary
changes in
LE 1-20
of organisms
and competition
Differences in
reproductive success
Evolution of adaptations
in the population
Summary of principles of evolution
by natural selection
1. Genetic variation within a population
2. Reproduction exceeds carrying capacity
(limited resources)
3. Competition for resources: Survival of
the fittest
4. Survivors increase proportion of genes
within population
Leads to evolutionary change of population
over time
Evolution is a
Population Phenomenon
Individuals do not evolve; populations do.
• Individuals are either favored or
disfavored by selection but individuals do
not evolve in a heritable way.
• However, selection can lead to changes
in populations.
• Fitness is the degree of success
measured as the relative contribution to
the next generation.
• Fitness can be determined from the rate of
survival through a selective phenomenon.
Important to Remember
• Evolutionary Success
does not always go to
the biggest or
strongest but to the
individuals who
produce the most
viable offspring.
• Why do farmers need
to re-apply pesticides
to their fields?
If a population lacks necessary
variation they may go extinct
Natural Selection
Natural Selection
• The environment acts as a filter favoring
some variations over others.
• Environmental factors vary from place to
place and from time to time.
• Therefore, the specifics of natural
selection are regional and timely.
Natural Selection
• Differential reproductive success is made
possible by variation and overproduction.
Natural Selection
• Selection does not create variations
• Natural selection works as an ‘editor’ to
existing variation
Natural Selection
• Selection is an editing process that
increases the relative frequency of
certain variations that arise randomly.
• Natural selection is the mechanism that
explains evolution. There are other
factors to evolution such as mutation
and genetic drift
Natural Selection
• Natural selection is not intentional.
– Individuals do not decide to change.
• Individuals do not change
• individuals do no decide to mutate in order
to adapt to changing environmental
LE 1-21a
Population with varied inherited traits
LE 1-21b
Elimination of individuals with certain traits
LE 1-21c
Reproduction of survivors
LE 1-21d
Increasing frequency of traits that enhance
survival and reproductive success