the evolution of populations
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The Evolution of Populations. Chapter 23 Biology – Campbell • Reece. What is a population? Species Gene pool. Population. Variations within a population AND Geographic variation How does variation occur? What is the ultimate source of new alleles? - PowerPoint PPT PresentationTRANSCRIPT
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The Evolution of The Evolution of PopulationsPopulations
Chapter 23Biology – Campbell • Reece
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PopulationPopulation
What is a population?
Species
Gene pool
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Genetic Variation & Genetic Variation & EvolutionEvolution
Variations within a population ANDGeographic variation
How does variation occur?What is the ultimate source of new
alleles?Where must mutations occur in order
to be passed to the next generation?
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Alleles in a PopulationAlleles in a Population
Allele frequency example…◦Red flower (R) is dominant over white flower (r)
◦In a population of 500, 20 have white flowers (rr)
◦The other 480 have red flowers (RR or Rr) 320 are RR, 160 are Rr
◦The dominant allele (R) accounts for 800 or 80% of the total (1000) number of genes
◦The recessive allele (r) accounts for 200 or 20%
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Hardy-Weinberg TheoremHardy-Weinberg Theorem
Describes a nonevolving populationThe frequencies of alleles and
genotypes in a population’s gene pool remain constant over generations◦Chance of RR – 0.8 x 0.8 = .64◦Chance of Rr – 0.8 x 0.2 = .16 + .16 (for rR) = .32
◦Chance of rr – 0.2 x 0.2 = 0.04◦The allele frequency does not change
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Hardy-Weinberg TheoremHardy-Weinberg Theorem
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Hardy-Weinberg TheoremHardy-Weinberg Theorem
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Hardy-Weinberg Hardy-Weinberg EquilibriumEquilibrium
p = one allele, q = other allelep + q = 1Frequency of RR = p2
Frequency of Rr/rR = 2pqFrequency of rr = q2
Hardy-Weinberg Equation:◦p2 + 2pq + q2 = 1
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5 Conditions for H-W 5 Conditions for H-W EquilibriumEquilibrium
1. Very large population size.2. No migration.3. No net mutations.4. Random mating.5. No natural selection.
We do not really expect a natural population to be in H-W equilibrium
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Altering Allele FrequenciesAltering Allele Frequencies
What might cause the allele frequencies to change?
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Genetic DriftGenetic Drift
What is genetic drift?
◦What size population is most likely to be affected?
◦Founder effect
◦Bottleneck effect
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Genetic DriftGenetic Drift
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Bottleneck EffectBottleneck Effect
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Bottleneck EffectBottleneck Effect
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Genetic DriftGenetic Drift
4 key points: Genetic drift…1. is significant in small populations2. can cause allele frequencies to change
at random3. can lead to a loss of genetic variation
within populations4. can cause harmful alleles to become
fixed
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Gene FlowGene Flow
What is gene flow?
What results from gene flow?
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Directional SelectionDirectional Selection
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Disruptive SelectionDisruptive Selection
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Stabilizing SelectionStabilizing Selection
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No ‘Perfect’ OrganismsNo ‘Perfect’ Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromisesChance, natural selection, and the
environment interact