Transcript Ch 16 Evolution of populations

CH 16
EVOLUTION OF
POPULATIONS
Crash Course: Population Genetics
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https://www.youtube.com/watch?v=WhFKPaRnTdQ
16-1 Genetic equilibrium
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Population genetics: study of evolution from a
genetic point of view
Basically how populations of a species evolve
But what is a population?
Group of members of the same species living in the
same area
Sources of genetic variation
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1.
Three main sources
Mutations: any change in sequence of DNA
 Replication
mistakes
 Radiation/environmental causes
2.
3.
recombination: reshuffling of genes
Random pairing of gametes
Bell curve
# of individuals with that trait
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Many traits in nature show trends like this
Phenotype continuum
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Number of phenotypes produced depends on how
many genes control that trait
Single gene traits- have two alleles
Two distinct phenotypes
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Polygenic traits- controlled by two or more genes
Results in multiple phenotypes
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Gene pool- all genes, including all different alleles,
that are present in a population
frequency (of an allele)- number of times alleles
occur in a gene pool
 Percentage
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Genetic definition of evolution?
Change in relative frequency of alleles in a
population over time
Phenotype frequency
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How often a specific phenotype is observed in a
population
Can be written mathematically
Frequency = # indiv. w/a particular phenotype
total # of indiv. in population
Hardy-Weinberg equilibrium
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When evolution is not occurring
 Allele
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frequencies remain the same
In order for evolution to not occur,
certain conditions must be met.
Evolution Versus Genetic Equilibrium
Hardy-Weinberg principle = Genetic Equilibrium
• Random Mating – Equal opportunity to produce offspring
• Large Population – Genetic Drift does not effect Allele Frequency
• No Movement into or out of Population – The gene pool must be
kept together (no new alleles)
• No Mutations – Mutations cause new forms of alleles changing the
frequency
• No Natural Selection – All genotypes must have equal probability
of surviving.
Hardy-Weinberg equilibrium
Allele frequency equation
p+q=1
 p = frequency of dominant allele
 q = frequency of recessive allele
Together, they make 100% of alleles for a gene in
that population
 If p = 34%, what is q? 0.66
 If q = 19%, what is p? 0.81
Hardy-Weinberg equilibrium
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Genotypic frequency equation
p2 + 2pq + q2 = 1
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p2 = homozygous dominant frequency
2pq = heterozygous frequency
q2 = homozygous recessive frequency
If p = .46, what is p2? 0.2116
If p = .12, what is q2? 0.7744 = 77%
If q =.31, what is 2pq? 0.4278
16-2 Disruption of genetic equilibrium
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Mutation
 Occur
at a relatively constant rate over time
 Can be sped up when exposed to mutagens
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Gene flow: process of genes moving from one
population to another
Immigration: moving into a population
Emigration: moving out of a population
Genetic Drift
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Alleles can become rare by chance
Over time a series of chance occurrences can cause
an alleles to become common in a population
Effects of genetic drift are more dramatic with small
population size
Founder effect: change in allele frequencies as a
result of migration of a small subgroup of a
population
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic
Drift
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Nonrandom mating
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Sexual selection: tendency of individuals to choose
a mate with certain traits.
Common in birds
Peacock display
Tropical birds of paradise - Papua New Guinea
The amazing Lyrebird - Australia
Natural selection
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Natural selection on a single gene traits can lead to
changes in allele frequencies
Natural selection on polygenic traits
3
possible effects
Directional selection
2.
Stabilizing selection
3.
Disruptive selection
https://www.youtube.com/watch?v=vCHdT9MWIaA
1.
Directional selection
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When individuals at one end of curve have higher
fitness than individuals in the middle or the other
end
Stabilizing selection
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When individuals near the middle have higher
fitness than the individuals at either end
Disruptive selection
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When individuals at upper and lower ends have
higher fitness than individuals near the middle
16-3 Formation of Species
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As new species evolve, populations become
reproductively isolated from each other
Reproductive isolation: when two members of
populations cannot interbreed and produce fertile
offspring
Separate gene pools
Isolation Mechanisms
Geographic Isolation:
- separation of animals in a specific
region
- formation of river, canyon, mountain
Isolation Mechanisms
Behavioral Isolation:
- differences in courtship or reproductive
behaviors
-meadowlark songs
Temporal isolation:
-two or more species reproduce at
different times
-orchids
Formation of species
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Allopatric speciation:
when species arise from
geographic isolation
 Different
places
https://www.youtube.com/w
atch?v=cSgulsydsQU
Reproductive isolation
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Prezygotic isolation: premating
isolation
Species may live
 in
different places
 Reproduce at different times
 Have different mating behaviors
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Postzygotic isolation: postmating
isolation
 Hybrids
may be weak
 Hybrids may be sterile
Sympatric
speciation
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Sympatric
speciation: when
two subpopulations
become isolated
while living in the
same area
Rates of speciation
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Gradualism:
speciation at gradual
and regular rate
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Punctuated
equilibrium: periods
of sudden, rapid
change followed by
periods of
littelchange