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I.
Population Genetics
A.
Population
•
“Localized group of individuals…capable of
interbreeding and producing fertile offspring”
•
Isolated from other populations of the same species to
some extent
Fig. 23.6
I.
Population Genetics
A.
Population
•
Gene pool – All genes in a population at a
given time
•
•
•
Fixed allele – Only one allele at a particular locus in a
population
In a population, each allele has a frequency
Ex: Population of wildflowers with two alleles
(CR, CW) at locus for flower pigment
•
•
•
•
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Homozygous CR – Red flowers
Homozygous CW – White flowers
Heterozygous – Pink flowers
Population – 320 plants with red flowers, 160 with
pink flowers, 20 with white flowers
Question – Allele frequencies for CR and CW?
I.
Population Genetics
B.
Hardy-Weinberg Theorem
•
Describes distribution of alleles and traits in
populations that are not evolving
•
•
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Population in genetic equilibrium
Frequencies of alleles and genotypes in a gene
pool remain constant over generations
Mendelian inheritance preserves variation
I.
Population Genetics
B.
Hardy-Weinberg Theorem
•
Permits calculation of allele frequencies from
genotype frequencies and vice-versa
Frequency of any allele can be described as a
number between 0 and 1
Ex: Population of wildflowers (alleles CR, CW)
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Sum of allele frequencies = 1
Frequencies of alleles = p, q
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p+q=1
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(p + q)2 = p2 + 2pq + q2 = 1
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p2 – Freq. of CRCR individuals in population
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2pq – Freq. of CRCW individuals in population
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q2 – Freq. of CWCW individuals in population
Fig. 23.8
I.
Population Genetics
B.
Hardy-Weinberg Theorem
•
Proportion of alleles in population doesn’t
change over successive generations, provided
five conditions are met
1)
2)
No net mutations
Random mating
•
Mates not selected based on genotype
3) No natural selection
•
Favors certain geno- and phenotypes, leading to
changes in allele frequencies
4) Extremely large population size
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Random fluctuation (genetic drift) more prevalent
and influential in small populations
5) No gene flow
II.
Microevolution
•
Evolution occurs when populations
don’t meet all the H-W assumptions
Process by which a population’s
genetic structure changes =
microevolution
•
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Changes in allele frequencies result from five
evolutionary processes
1)
2)
3)
4)
5)
Mutation
Nonrandom mating
Natural selection
Genetic drift
Gene flow
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