1 Hardy-Weinberg Principle • Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation – Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.
Hardy-Weinberg Principle. Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions. Hardy-Weinberg Equilibrium. - PowerPoint PPT Presentation
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1
Hardy-Weinberg Principle
• Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation
– Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.
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Hardy-Weinberg Equilibrium
Population of cats n=10016 white and 84 blackbb = whiteB_ = black
Can we figure out the allelic frequencies of individuals BB and Bb?
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Hardy-Weinberg Principle
• Necessary assumptions
Allelic frequencies would remain constant if…
– population size is very large– random mating– no mutation– no gene input from external sources– no selection occurring
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Hardy-Weinberg Principle
• Calculate genotype frequencies with a binomial expansion
(p+q)2 = p2 + 2pq + q2
• p2 = individuals homozygous for first allele• 2pq = individuals heterozygous for alleles• q2 = individuals homozygous for second allele
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p2 + 2pq + q2
and
p+q = 1 (always two alleles)
• 16 cats white = 16bb then (q2 = 0.16)• This we know we can see and count!!!!!• If p + q = 1 then we can calculate p from q2
• Q = square root of q2 = q √.16 q=0.4• p + q = 1 then p = .6 (.6 +.4 = 1)• P2 = .36• All we need now are those that are heterozygous
(2pq) (2 x .6 x .4)=0.48
• .36 + .48 + .16
Hardy-Weinberg Principle
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Hardy-Weinberg Equilibrium
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Five Agents of Evolutionary Change
• Mutation– Mutation rates are generally so low they
have little effect on Hardy-Weinberg proportions of common alleles.
ultimate source of genetic variation• Gene flow
– movement of alleles from one population to another
tend to homogenize allele frequencies
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Five Agents of Evolutionary Change
• Nonrandom mating– assortative mating - phenotypically similar
individuals mate Causes frequencies of particular
genotypes to differ from those predicted by Hardy-Weinberg.
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Five Agents of Evolutionary Change
• Genetic drift – statistical accidents.– Frequencies of particular alleles may
change by chance alone. important in small populations
founder effect - few individuals found new population (small allelic pool)
bottleneck effect - drastic reduction in population, and gene pool size
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Genetic Drift - Bottleneck Effect
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Five Agents of Evolutionary Change
• Selection – Only agent that produces adaptiveevolutionary change
• Directional selection– Selection eliminates one extreme from a
phenotypic array.• Stabilizing selection
– Selection acts to eliminate both extremes from an array of phenotypes.
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Kinds of Selection
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Selection on Color in Guppies
• Guppies are found in small northeastern streams in South America and in nearby mountainous streams in Trinidad.
– Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.
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Evolution of Coloration in Guppies
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Selection on Color in Guppies
• High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
• Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.
– In the absence of predators, larger, more colorful fish may produce more offspring.
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Evolutionary Change in Spot Number
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Limits to Selection
• Genes have multiple effects– pleiotropy
• Evolution requires genetic variation– Intense selection may remove variation
from a population at a rate greater than mutation can replenish.