Download Hardy-Weinberg Principle

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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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Charles Darwin
Collected
specimens
During journey on
the HMS Beagle
Wrote about
conclusions in the
Origin of the
Species
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Darwin’s Finches of the Galapagos
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Wallace’s Line
The Wallace Line or
Wallace's Line is a
faunal boundary line
drawn in 1859 by the
British naturalist
Alfred Russel Wallace
that separates the
ecozones of Asia and
Wallacea, a
transitional zone
between Asia and
Australia.
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Modern Synthesis Theory
• 1940s – comprehensive
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theory of evolution
(Modern Synthesis
Theory)
Introduced by Fisher &
Wright & Haldane
Until then, many did not
accept that Darwin’s
theory of natural
selection could drive
evolution
S. Wright
Haldane
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Sir Ronald A. Fisher
Statistician combined
Mendel’s work and
natural selection
--also created many
biostatistics used
today
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Sewall Wright
Geneticist used
included selection in
path analysis
Founder of population
genetics with Fisher
and Haldane
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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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HardyWeinberg
Principle
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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
- violation: small populations have
fluctuations in allele frequencies (e.g., fire,
storm).
2.No migration
- violation: immigrants can change the
frequency of an allele by bringing in new
alleles to a population.
3.No net mutations
- violation: 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
- Violation: 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
- Violation: 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 = non-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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Estimating allelic frequencies:
•If one or more alleles are recessive, can’t
distinguish between heterozygous and
homozygous dominant individuals.
•Use Hardy-Weinberg to calculate allele
frequencies based on the number of homozygous
recessive individuals.
If q2 = 0.0043, then q = 0.065; p = 1 - q =
0.935
p2 = 0.8742, 2pq = 0.1216
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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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Allele and Genotype Frequencies
Each diploid individual in the population has 2 copies of
each gene. The allele frequency is the proportion of
all the genes in the population that are a particular
allele.
The genotype frequency of the proportion of a
population that is a particular genotype.
For example: consider the MN blood group. In a
certain population there are 60 MM individuals, 120
MN individuals, and 20 NN individuals, a total of 200
people.
The genotype frequency of MM is 60/200 = 0.3.
The genotype frequency of MN is 120/200 = 0.6
The genotype frequency of NN is 20/200 = 0.1
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The genotype frequency of MM is 60/200 = 0.3.
The genotype frequency of MN is 120/200 = 0.6
The genotype frequency of NN is 20/200 = 0.1
The allele frequencies can be determined by adding the
frequency of the homozygote to 1/2 the frequency of
the heterozygote.
The allele frequency of M is 0.3 (freq of MM) + 1/2 *
0.6 (freq of MN) = 0.6
The allele frequency of N is 0.1 + 1/2 * 0.6 = 0.4
Note that since there are only 2 alleles here, the
frequency of N is 1 - freq(M).
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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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Genetic Drift
Genetic drift is the random changes in allele
frequencies. Genetic drift occurs in all
populations, but it has a major effect on small
populations.
For Darwin and the neo-Darwinians, selection
was the only force that had a significant
effect on evolution. More recently it has
been recognized that random changes, genetic
drift, can also significantly influence
evolutionary change. It is thought that most
major events occur in small isolated
populations.
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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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Bottlenecks
Example: Pingalop atoll is an island in the South
Pacific. A typhoon in 1780 killed all but 30
people. One of survivors was a man who was
heterozygous for the recessive genetic
disease achromatopsia. This condition caused
complete color blindness. Today the island
has about 2000 people on it, nearly all
descended from these 30 survivors. About
10% of the population is homozygous for
achromatopsia This implies an allele
frequency of about 0.26.
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Founder’s Effect
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Founder Effect Example
Founder effect example: the Amish are a
group descended from 30 Swiss founders
who renounced technological progress.
Most Amish mate within the group. One of
the founders had Ellis-van Crevald
syndrome, which causes short stature,
extra fingers and toes, and heart defects.
Today about 1 in 200 Amish are
homozygous for this syndrome, which is
very rare in the larger US population.
Note the effect inbreeding has here: the
problem comes from this recessive condition
becoming homozygous due to the mating of
closely related people.
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Modes of Natural
Selection
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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
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Directional
Selection
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• Disruptive selection
-Favors extremes over
intermediate phenotypes
-Occurs when
environmental change
favors an extreme
phenotype
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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 current
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
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Co-evolution
-- when changes in at least two species'
genetic compositions reciprocally affect each
other's evolution
-- a change in the genetic composition of one
species (or group) in response to a genetic
change in another.
-Often occurs between parasite & host and flowers &
their pollinators, and predator and prey
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Coevolution pollinators
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Parasitic relationships
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