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EVOLUTION
Transformation of the Biological Paradigm
Evolution
• Charles Darwin
• BUT, the history of Evolutionary Thought
pre-dates Darwin considerably…
The Aristotlean-Christian World
View
• Aristotle (350 BC)
• Observes Life’s Complexity &
• Suggests a “Hierarchy of Life”
Complexity
Naturalists begin to find problems
• Some organisms were not fitting properly
into the hierarchy.
• Due to Variations
– Viewed as imperfections of an idealized form.
– Which characteristics of an organism are
most important for placing it in the hierarchy?
– How much variation should be viewed as
normal within a species?
Jean-Baptiste de Monet de Lamarck
• Suggested that CHANGE in organisms
took place by “Inheritance of Acquired
Characteristics”.
– Characteristics acquired during an organism’s
life were passed on to offspring.
– Giraffe Necks became longer and longer as
each generation stretched higher into the
trees for food.
• Problem…
Jean-Baptiste de Monet de Lamarck
• Lamarck’s hypothesis was never
supported by experiments & has long
since been DISPROVED.
• Why? …An environmentally-induced
phenotype cannot be passed on to the
next generation…only genotype can!
• Lamarck still remembered for this
mistaken hypothesis, but still contributed
many innovative ideas.
Charles Darwin
• 5 year voyage (1831-1836)
• Lots of DATA!
1. Geologic
• He saw first hand what Hutton had suggested
(Gradualism), and agreed with Lyell’s premise that
the Earth must be very old.
• He examined many fossils – some resembling
modern (current) organisms, but not exactly.
– Saw the possibility for Descent with
Modification (modern forms having
descended from now-extinct forms), and for
organisms changing over time.
Charles Darwin
…DATA
2. Biogeography
• He found similar organisms in far-reaching places,
but in similar habitats.
– Suggested that organisms may be adapting
to the same types of environment, despite
the distance between them.
• He found slight variations in organisms found in
slightly different habitats (but separated from each
other … Finch Bills, Tortoise Necks, etc.)
– Suggested that a common ancestor gave
rise to the different varieties, which each
adapted to slightly different environments.
Charles Darwin
• Returned from the voyage
and…PUBLISHED!
• Saw that adaptation to environment was
indeed real, and eventually formulated a
mechanism for it, but…
– He did not want to jeopardize his scientific
status, so didn’t publish it.
• Along came Alfred Russel Wallace…
Alfred Russel Wallace
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(1823-1913)
Naturalist
Studied and admired Darwin’s work.
Wrote Darwin to see what he thought about
a hypothesis regarding a mechanism for
adaptation to the environment.
• This hypothesis was virtually the same as
Darwin’s (unpublished) ideas about how
change could take place!
Darwin & Wallace
• 1858 -- Presented their hypotheses at a meeting
of the Linnaean Society (London):
"The Variation of Organic Beings under
Domestication and in their Natural State"
"On the Variation of Organic Beings in the State
of Nature; on the Natural Means of Selection;
on the Comparison of Domestic Races and
true Species"
Darwin
• Later (1859) published his famous essay:
“I have called
this principle, by
which each slight
variation, if
useful, is
preserved, by the
term Natural
Selection.”
- Charles Darwin,
The Origin of
Species
Evolution by Natural Selection:
5 KEYS
1. VARIATION
•
•
Variation is inherent in populations of
organisms (members of a population are not
all the same).
This Variation is Heritable – able to be
passed from generation to generation.
Elaphe obsoleta
Evolution by Natural Selection:
5 KEYS
2. More offspring are born than survive.
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Differential reproduction and survival.
“Survive” = Live long enough to reproduce.
Evolution by Natural Selection:
5 KEYS
3. Differences occur between surviving
offspring and those not surviving.
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Advantageous Varations = Certain
characteristics found in surviving offspring that
afford them a survival advantage.
Sometimes called “Adaptations” (but this implies
a goal, and Natural selection is NOT goaloriented).
Evolution by Natural Selection:
5 KEYS
4. More individuals with advantageous
adaptations occur in succeeding
generations of organisms.
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Their variations are beneficial, therefore they
survive & reproduce.
Individuals without beneficial variation do not
survive to reproductive age.
Individuals
Evolution by Natural Selection:
5 KEYS
5. Results in a population that is welladapted to its environment.
•
Population as a whole is becomes fitted to
its particular environment.
NATURAL SELECTION
• “Struggle for Existence”
– Limited Resources are available for a
population, leading to Competition.
• Some survive, some don’t
• Sometimes this is random
– The “Struggle” is for the resources, not as
much direct competition between individuals.
NATURAL SELECTION
• “Survival of the Fittest”
– The most FIT organisms survive & reproduce.
– Fitness = how well an organism fits into its
environment
• Securing resources
• Avoiding predators
• Finding food
– What determines Fitness?
• Varies with organism, but ultimately….ENVIRONMENT
• This is always changing – what is most fit one month,
may not be the next.
NATURAL SELECTION
• Environment is important!
– In Artificial Selection, humans select for
survival & reproduction.
– In Natural Selection, nature selects for
survival & reproduction.
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Habitat
Climate
Food & Shelter Availability
Etc.
NATURAL SELECTION
• Not goal-oriented, no particular goal.
– Environmental conditions are always
changing.
– Organisms’ needs are always changing.
– Genotype is heritable, not phenotype.
NATURAL SELECTION
• Populations Evolve, NOT individuals.
– Groups of individuals belonging to the same
species and occupying the same given area,
and being similar in form, behavior, and
physiology.
– Inheritance of acquired characteristics
(Lamarck) does NOT occur.
Microevolution
We know….
Alleles
Genotype
Phenotype
Form
Adaptation to Environment,
Evolution
Agents of Microevolutionary
Change
•
Processes that elicit changes in allele
frequencies in populations:
1.
2.
3.
4.
5.
Mutation
Gene Flow
Non-Random Mating
Genetic Drift
Natural Selection
Agents of Microevolutionary
Change
1. Mutation: A random heritable change in
the DNA.
– Only source of NEW alleles in a population.
– Actually rather common!
– Can be:
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Harmful (most – we’re complex), these do not
remain in the gene pool and so are rare (selected
against!).
Neutral (many), masked, occur in genes not
essential for survival, often passed on.
Beneficial (rare – we’re complex), these are the
key to better-adapting to changing environments.
Agents of Microevolutionary
Change
2. Gene Flow: New individuals enter or exit
a population (Migration).
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Introduces the possibility of new allele
combinations.
Often has a homogenizing (mixing) effect on
the population – offering stability.
Agents of Microevolutionary
Change
3. Non-Random Mating: Mating &
offspring production by individuals with
specific phenotypes (genotypes).
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A.K.A. “Assortive Mating” or “Preferential
Mating” (Not random).
Results in accumulation of certain traits in
the population (sometimes reduction of
heterozygotes).
Examples: Mate selection in white-tailed
deer (“Sexual Selection”), tall Men & tall
women,
Agents of Microevolutionary
Change
4. Genetic Drift: Changes in allele frequency
due to chance.
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Often due to natural disasters or major
catastrophes.
Especially in small populations.
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Bottleneck Effect: loss of certain alleles as a
population dwindles, reduction in allelic diversity
with a smaller and smaller population.
Founder Effect: small population with a specific
and limited gene pool is separated from the main
population.
Agents of Microevolutionary
Change
5. Natural Selection: Differential survival &
reproduction of individuals in a
population.
1. Stabilizing (the extremes of the population are selected
against in favor of the average/most common)
2. Disruptive (the average/most common individuals of the
population are selected against in favor of the extremes)
3. Directional (one extreme or the other of the population are
selected against)
Agents of Microevolutionary
Change
5. Natural Selection
(Peppered Moths)