Download Population Genetics

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The Gene Pool
•Members of a species
can interbreed &
produce fertile
offspring
Species have a shared
gene pool
Gene pool – all of the
alleles of all individuals
in a population
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The Gene Pool
•Different species
do NOT exchange
genes by
interbreeding
Different species
that interbreed
often produce
sterile or less viable
offspring e.g. Mule
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Populations
•A group of the
same species living
in an area
No two individuals
are exactly alike
(variations)
More Fit
individuals survive &
pass on their traits
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Speciation
•Formation of new
species
•One species may
split into 2 or more
species
A species may
evolve into a new
species
Requires very long
periods of time
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Modern
Evolutionary
Thought
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Modern Synthesis Theory
• Combines Darwinian
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selection and
Mendelian inheritance
Population genetics study of genetic
variation within a
population
Emphasis on
quantitative
characters (height,
size …)
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Modern Synthesis Theory
• 1940s – comprehensive
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theory of evolution
(Modern Synthesis
Theory)
Introduced by Fisher &
Wright
Until then, many did not
accept that Darwin’s
theory of natural
selection could drive
evolution
S. Wright
A. Fisher
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Modern Synthesis Theory
• TODAY’S theory on evolution
• Recognizes that GENES are responsible for
the inheritance of characteristics
• Recognizes that POPULATIONS, not
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individuals, evolve due to natural selection
& genetic drift
Recognizes that SPECIATION usually is due
to the gradual accumulation of small genetic
changes
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Microevolution
• Changes occur in gene pools due to
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mutation, natural selection, genetic
drift, etc.
Gene pool changes cause more
VARIATION in individuals in the
population
This process is called
MICROEVOLUTION
Example: Bacteria becoming unaffected
by antibiotics (resistant)
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The Hardy-Weinberg Principle
• Used to describe a non-evolving
population.
• Shuffling of alleles by meiosis and
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random fertilization have no effect
on the overall gene pool.
Natural populations are NOT
expected to actually be in HardyWeinberg equilibrium.
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The Hardy-Weinberg Principle
• Deviation from Hardy-Weinberg
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equilibrium usually results in
evolution
Understanding a non-evolving
population, helps us to understand
how evolution occurs
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5 Assumptions of the H-W Principle
1.Large population size
- small populations have fluctuations in allele
frequencies (e.g., fire, storm).
2.No migration
- immigrants can change the frequency of an
allele by bringing in new alleles to a
population.
3.No net mutations
- if alleles change from one to another, this
will change the frequency of those alleles
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5 Assumptions of the H-W Principle
3.Random mating
- if certain traits are more desirable,
then individuals with those traits will be
selected and this will not allow for random
mixing of alleles.
4.No natural selection
- if some individuals survive and reproduce
at a higher rate than others, then their
offspring will carry those genes and the
frequency will change for the next
generation.
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Traits Selected for Random Mating
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The Hardy-Weinberg Principle
The gene pool of a NON-EVOLVING
population remains CONSTANT over multiple
generations (allele frequency doesn’t change)
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
Where:
p2 = frequency of AA genotype
2pq = frequency of Aa
q2 = frequency of aa genotype
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The Hardy-Weinberg Principle
Determining the Allele Frequency using
Hardy-Weinberg:
1.0 = p + q
Where:
p = frequency of A allele
q = frequency of a allele
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Allele Frequencies Define Gene Pools
500 flowering plants
480 red flowers
320 RR
160 Rr
20 white flowers
20 rr
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x
(80%)
160 x
(20%)
2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
R
1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2
r
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Microevolution
of Species
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Causes of Microevolution
• Genetic Drift
- the change in the gene pool of a small
population due to chance
• Natural Selection
- success in reproduction based on heritable
traits results in selected alleles being passed to
relatively more offspring (Darwinian inheritance)
- Cause ADAPTATION of Populations
• Gene Flow
-is genetic exchange due to the migration of
fertile individuals or gametes between
populations
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Causes of Microevolution
• Mutation
- a change in an organism’s DNA
- Mutations can be transmitted in gametes to
offspring
• Non-random mating
- Mates are chosen on the basis of the best
traits
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Genetic Drift
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Factors that Cause Genetic Drift
• Bottleneck Effect
- a drastic reduction in population (volcanoes,
earthquakes, landslides …)
- Reduced genetic variation
- Smaller population may not be able to adapt to new
selection pressures
• Founder Effect
- occurs when a new colony is started by a few
members of the original population
- Reduced genetic variation
- May lead to speciation
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Loss of Genetic Variation
• Cheetahs have little genetic variation in
their gene pool
• This can probably be attributed to a
population bottleneck they experienced
around 10,000 years ago, barely
avoiding extinction at the end of the
last ice age
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Founder’s Effect
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Modes of Natural Selection
• Directional Selection
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Favors individuals at one end of the phenotypic
range
Most common during times of environmental change
or when moving to new habitats
• Disruptive selection
- Favors extreme over intermediate phenotypes
- Occurs when environmental change favors an
extreme phenotype
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Directional
Selection
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Disruptive Selection
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Modes of Natural Selection
• Stabilizing Selection
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Favors intermediate over extreme phenotypes
Reduces variation and maintains the cureent
average
Example: Human birth weight
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Variations in
Populations
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Geographic Variations
• Variation in a species
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due to climate or
another geographical
condition
Populations live in
different locations
Example: Finches of
Galapagos Islands &
South America
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Heterozygote Advantage
• Favors heterozygotes (Aa)
• Maintains both alleles (A,a) instead of
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removing less successful alleles from a
population
Sickle cell anemia
> Homozygotes exhibit severe anemia, have
abnormal blood cell shape, and usually die
before reproductive age.
> Heterozygotes are less susceptible to
malaria
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Sickle Cell and Malaria
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Other Sources of Variation
• Mutations
- In stable environments, mutations often result in
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little or no benefit to an organism, or are often
harmful
Mutations are more beneficial (rare) in changing
environments (Example: HIV resistance to
antiviral drugs)
• Genetic Recombination
- source of most genetic differences between
individuals in a population
• Co-evolution
-Often occurs between parasite & host and
flowers & their pollinators
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