Population
Genetics
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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
MODERN SYNTHESIS THEORY
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Combines Darwinian
selection and Mendelian
inheritance
Population genetics study of genetic variation
within a population
Emphasis on quantitative
characters
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MODERN SYNTHESIS THEORY
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Today’s theory on evolution
Recognizes that GENES are responsible for the
inheritance of characteristics
Recognizes that POPULATIONS, not 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
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Changes occur in gene pools due to 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 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
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 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
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GENE POOLS
• A population’s gene pool is the total of all
genes in the population at any one time.
• Each allele occurs with a certain
frequency (.01 – 1).
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THE HARDY-WEINBERG THEOREM
• Used to describe a non-evolving
population.
• Shuffling of alleles by meiosis and
random fertilization have no effect on
the overall gene pool.
• Natural populations are NOT expected
to actually be in Hardy-Weinberg
equilibrium.
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THE HARDY-WEINBERG THEOREM
Deviation from Hardy-Weinberg
equilibrium usually results in evolution
• Understanding a non-evolving population,
helps us to understand how evolution
occurs
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SOURCES OF GENETIC VARIATION
(DISRUPTION OF H-W LAW)
1. Mutations
- if alleles change from one to another, this will
change the frequency of those alleles
2. Genetic recombination
- crossing over; independent assortment
3. Migration
- immigrants can change the frequency of an
allele by bringing in new alleles to a population.
- emigrants can change allele frequencies by
taking alleles out of the population
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SOURCES OF GENETIC VARIATION
(DISRUPTION OF H-W LAW)
4. Genetic Drift
- small populations can have chance fluctuations
in allele frequencies (e.g., fire, storm).
- bottleneck; founder effect
5. 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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Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population remains
constant over multiple generations; i.e., the allele
frequency does not change over generations of time.
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
where p2 = frequency of AA genotype; 2pq = frequency
of Aa plus aA genotype; q2 = frequency of aa genotype
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