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BIO 416Evolution
Lecture #12
Population Genetics III
DestabilizingThe Equilbrium
February 9, 2009
Dr. Karen Schmeichel Oglethorpe University
Hardy
Weinberg
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Business:
•Lit Rev #1 Due 2/11•Darwin’s Birthday 2/12 –(10 pts)
•Exam 2/13 – Study Guide on Wednesday
•Exam – Through Ch 7
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Objectives:
•Reconsider H-W equilibrium using Card Simulations
•Discuss Factors that destabilize H-W•Cover the ways in which selection can alter changes in allelic and genotypic frequencies
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Hardy-Weinberg Equilibrium: a Null Model
1. Allele frequencies in a population will not change, generation after generation
2. If the allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq and q2
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Hardy-Weinberg-Castle Equilibrium:
Describes how allele and genotype frequencies do not change during the course of many generations, unless
destabilizing conditions exist
p2 +2pq + q2 = 1f(AA) f(Aa) f(aa)
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= 0.8 = 0.2
= 0.8
= 0.2
= 0.8 x 0.8 = 0.8 x 0.2
= 0.8 x 0.2 = 0.2 x 0.2
f(AA) = .64 f(Aa) = .32 f (aa) = .04
Another Way to conceptualize HW:
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In a given population with alleles A and a:
p + q = 1
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A Population in H-W Equilibrium will show same allelic frequency generation after generation (RARE, if ever!)
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A Population Genetics Simulation
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HW Equilibrium
is Predictive
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HW Equilibrium: a Null Model
1. Allele frequencies in a population will not change, generation after generation
2. If the allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq and q2
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How to “mind your
p’s and q’s”?
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Alloenzymes &
Orchid Exercise
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Heterozygosity as a Measure of
Genetic Diversity:The average individual is
heterozygous at 4-15%
of its genes
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Hardy Weinberg Conditions: Allele and genotype frequencies will not change from generation to generation if:
1.No selection2.No mutation3.No migration 4.No genetic drift5.Mating is random
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Can Selection Change allele frequencies
from one generation to the next?
(Game Rule: 75% of the heterozygotes survive
and 50% of the homozyg recessives survive)
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Violation ofConclusion #1
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Violation ofConclusion #2
Papua New Guinea’sFore Tribe and Kuru
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Dogma Following the Modern Synthesis:
Natural Selection should preserve the allele most conducive to
survival and reproduction and eliminate the rest.
The one best allele was called wild type and any other alleles
were considered mutants (extremely rare).
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Natural SelectionIs most potent
When a recessive
Allele is common,
Not rare(think: heterozygote)
FAST! SLOW!
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Selection Favoring Heterozygotes: “Overdominance”
(ex, Fig. 6.18 in Fruit Flies)A way of maintaining recessive alleles
Selection Favoring Homozygotes: “Underdominance”(ex, Fig 6.19 in Fruit Flies)
In this case, one allele tends to go to fixation, while the other allele is
lost (may be different in different populations)
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For 2/11: •Ch 7 & Prairie Chicken
Paper
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Mutation Alone
Causes SlowChange Over
Time – Generally
A weak Mechanism for
Evolution
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Mutation + Selection: The 1-2 Punch
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Mutation/Selection Balance(or frequency of deleterious allele
at equilibrium) is Described Mathematically:
q = s