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Evolution
History of Evolution
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Aristotle- all living forms arranged on a
scale of increasing complexity, species are
perfect and do not evolve
Natural theology: species are individually
designed and perfect
Linnaeus- founder of taxonomy, developed
2 part naming system (binomial
nomenclature)
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Cuvier- founder of paleontology;
advocated catastrophism
Hutton- gradualism, change happens over
time
Lyell- uniformitarianism; geological
processes have not changed over time
Lamarck- use and disuse, “felt needs”,
adaptation to the environment
Darwin
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Origin of new species and adaptation to
the environment are intertwined
(Alfred Wallace)
Descent with modification
Natural selection
Darwin’s Inferences
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Natural selection is the unequal success of
individuals when surviving and
reproducing
Natural selection comes out of the
relationship with the environment
Product of natural selection= adaptation
Tenets of Natural selection
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Populations evolve
Can only exaggerate or diminish traits that
can be inherited
Situational
Support of Evolution
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Fossil record
Comparative anatomy
Comparative embryology
Molecular biology
Population Genetics
Before Population Genetics
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Before population genetics the science of
genetics was contradictory to evolution.
Geneticists recognized “either-or” traits.
They hadn’t yet learned that most physical
characteristics are controlled by more than
one gene so having a continuum in a trait
was counterintuitive to their findings.
There was no genetic basis to support
Darwin’s findings.
Population Genetics
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Recognizes the continuum and large
variation of traits
Modern Synthesis
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First main comprehensive theory involving
population genetics
Integrated discoveries from different
disciplines
Tenets of Modern Synthesis theory
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Populations are the units of evolution
Natural selection= primary mode of
evolution
Importance of gradualism
The effect of location on evolution
Effect of location on evolution
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Species vs. populations
Populations that are isolated from others
of the same species by geographic
boundaries exchange genetic material with
others outside of their population
occasionally, if at all. This may cause a
divergence in populations of the same
species that have been separated by a
geographic barrier.
The gene pool
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All of the genes in a population at any one
time
Fixed alleles
Allelic frequency
300 dogs in population. Brown (B) is
dominant to white (b). 50 white dogs, 250
brown dogs (150 homozygous, 100
heterozygous)
Allelic frequency
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Example: 300 dogs in population. Brown
(B) is dominant to white (b). 50 white
dogs, 250 brown dogs (150 homozygous,
100 heterozygous)
Total of 600 copies of allele for fur color
Copies of dominant allele (B): 150*2
(BB)+100= 400
Frequency of B allele= 400/600=
0.667=66.7%
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So b allele frequency= 100%-66.6%=
33.3%
Genotype frequency:
BB= 150/300=0.5
Bb= 100/300=0.33
bb= 50/300=.166
Hardy Weinberg Equilibrium
Describes a non evolving population
 Allelic and genotypic frequencies remain
constant over generations unless acted on
by something other than sex
 Equation:
(p + q) x (p + q)= p2 + 2pq + q2
(allelic frequency of male x allelic frequency
of female)
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Hardy-Weinberg Equilibrium
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Assumptions
Very large population (smaller populations
mean more chance for changes in allelic
frequency)
No gene flow
No mutations
Random mating
No natural selection
Microevolution
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Generation-to-generation change in
population’s allele or genotype frequencies
If populations deviate from the baseline
provided by H-W equilibrium then
population is evolving
Practice
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Hardy-Weinberg Equilibrium problems
What causes microevolution?
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Genetic drift
Gene flow
Mutation
Nonrandom mating
Natural selection
*only natural selection adapts population to
nature
Genetic Drift
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Changes in the gene pool of a small
population from chance
Can be caused by: Bottleneck effect and
founder effect
Bottleneck Effect
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Unselective event that greatly diminishes
the population
Resulting population frequencies will not
be indicative of the original population
Bottleneck effect combined with genetic
drift reduce genetic variability
Founder Effect
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Genetic drift caused by a small number of
individuals colonizing a new area
May account for higher populations of
inherited disorders in certain places
(islands)
Gene flow
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Due to migration (immigration,
emigration)
Introduction of new genes or just
alteration of gene frequencies
Mutations
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Changes in organism’s DNA
Can alter phenotypic frequencies
Nonrandom mating
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In reality mate with closer neighbors more
often.
This eventually creates inbreeding which
alters allelic frequencies.
Assortive mating: like selects like
Natural Selection
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Differential success in reproduction
The organisms better adapted to their
environment will live longer, produce more
offspring and those offspring will have a
better chance of survival than others who
are not as well adapted
Accumulates and maintains favorable traits
in the environment
Not all variation is heritable
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Phenotype= genotype + environmental
influence
Only the genetic component of this has
evolutionary consequences (remember
Lamarck)
Variation within Populations
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Polymorphism: 2 or more forms of a
discrete (either-or) trait are present in the
population ex: freckles, blood type (4
morphs) non examples: height, weight,
hair color (no characteristics that vary
continuously)
Variation between Populations
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Geographical variation
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Natural selection
Genetic drift
clines
Genetic variation from other factors
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Mutation
Sexual recombination
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The fact that most organisms are diploid
hides many genetic variations (recessives
even harmful ones can continue on for
generations even if they don’t
physiologically manifest)
Heterozygote advantage
Frequency dependent selection
Types of selection
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Disruptive/diversifying
Directional
Stabilizing
Disruptive selection
Directional Selection
Stabilizing selection
Sexual selection
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Females choose males with certain traits
Those traits are perpetuated with greater
intensity