Chapter 18: Evolutionary Change in Populations

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Chapter 18 Evolutionary Change in Populations
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Remember

• Basis of evolution is GENETIC
• We study POPULATIONS (not in an individual
  lifetime)
• Variations are due to
• ENVIRONMENT
• HEREDITY

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Populations genetics

• Study of genetic variability within a population
  and of the forces that act on it

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5 factors responsible for evolutionary change

• Nonrandom mating
• Mutation
• Genetic drift
• Gene flow
• Natural selection

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Gene pool

• All the alleles for all genes present in the
  population
• Genotype frequency
• Phenotype frequency
• Allele frequency

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Hardy-Weinberg

• Genetic equilibrium allele and genotypes
  frequencies do not change between generations
• Ideal situation rare
• Provides model
• Equation

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• The occurrence of PKU is 1 per 10,000 births
• q2 0.0001
• q 0.01
• The frequency of normal alleles is
• p 1 q 1 0.01 0.99
• The frequency of carriers is
• 2pq 2 x 0.99 x 0.01 0.0198
• or approximately 2 of the U.S. population

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Conditions for Hardy-Weinberg

• 1.) Random Matings
• 2.) No net mutations
• 3.) Large population size
• 4. ) No migration
• 5.) No natural selection

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Hardy-Weinberg Problems

• Page 405 of textbook
• Review Questions
• ( 1-10)
• Check answers in Appendix
• Bring questions to class

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Microevolution

• Generation-to-generation changes in allele or
  genotype frequencies within a population
• 5 processes at work
• Nonrandom mating
• Inbreeding
• Assortative breeding
• Mutation
• Genetic drift
• Bottleneck effect
• Founder effect
• Gene flow
• Natural selection

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Fig. 23-8-3
CW CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CR
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
CR CR
Generation 1
Generation 2
Generation 3
p (frequency of CR) 0.7
p 0.5
p 1.0
q (frequency of CW ) 0.3
q 0.5
q 0.0
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Fig. 23-9
Original population
Bottlenecking event
Surviving population
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Fig. 23-11
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Natural Selection 3 types

• Stabilizing selection intermediate phenotypes
• Directional selection 1 extreme phenotype
• Disruptive selection favors phenotypic extremes

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Fig. 23-13
Original population
Frequency of individuals
Phenotypes (fur color)
Original population
Evolved population
(c) Stabilizing selection
(b) Disruptive selection
(a) Directional selection
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Genetic Variation is Necessary for Natural
Selection

• Sources of Variation
• Mutation
• Sexual reproduction
• Meiosis
• Crossing over
• Independent assortment
• Random union of gametes

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Genetic Polymorphism

• Presence in a population of 2 alleles for a give
  locus
• Variation
• Low frequency of alleles

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Balanced polymorphism

• Type of genetic polymorphism
• 2 alleles persist in a population over many
  generations because of natural selection
• 2 types
• Heterozygote advantage
• Sickle cell
• Frequency-dependent selection
• More value when rare

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Fig. 23-17
Frequencies of the sickle-cell allele
02.5
2.55.0
5.07.5
Distribution of malaria caused by Plasmodium
falciparum (a parasitic unicellular eukaryote)
7.510.0
10.012.5
gt12.5
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Fig. 23-18
Right-mouthed
1.0
Left-mouthed
Frequency of left-mouthed individuals
0.5
0
1981
82
83
84
85
86
87
88
89
90
Sample year
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Neutral Variation

• Variation that does not alter the ability of an
  individual to survive and reproduce so is not
  adaptive