Genes Within Populations

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Genes Within Populations
Chapter 20
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Genetic Variation and Evolution

• Darwin Evolution is descent with modification
• Evolution changes through time
• Species accumulate difference
• Descendants differ from their ancestors
• New species arise from existing ones

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Natural selection mechanism of evolutionary
change

• Natural selection proposed by Darwin as the
  mechanism of evolution
• individuals have specific inherited
  characteristics
• they produce more surviving offspring
• the population includes more individuals with
  these specific characteristics
• the population evolves and is better adapted to
  its present environment

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Darwins theory for how long necks evolved in
giraffes
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Natural selection mechanism of evolutionary
change

• Inheritance of acquired characteristics Proposed
  by Jean-Baptiste Lamarck
• Individuals passed on physical and behavioral
  changes to their offspring
• Variation by experiencenot genetic
• Darwins natural selection variation a result of
  preexisting genetic differences

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Lamarcks theory of how giraffes long necks
evolved
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Gene Variation in Nature

• Measuring levels of genetic variation
• blood groups
• enzymes
• Enzyme polymorphism
• A locus with more variation than can be
  explained by mutation is termed polymorphic.
• Natural populations tend to have more polymorphic
  loci than can be accounted for by mutation.
• DNA sequence polymorphism

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Godfrey H. Hardy English mathematicianWilhelm
Weinberg German physicianConcluded thatThe
original proportions of the genotypes in a
population will remain constant from generation
to generation as long as five assumptions are met
Hardy-Weinberg Principle
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Hardy-Weinberg Principle

• Five assumptions
• No mutation takes place
• No genes are transferred to or from other
  sources
• Random mating is occurring
• The population size is very large
• No selection occurs

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Hardy-Weinberg Principle

• Calculate genotype frequencies with a binomial
  expansion (pq)2 p2 2pq q2
• p individuals homozygous for first allele
• 2pq individuals heterozygous for both alleles
• q individuals homozygous for second allele
• because there are only two alleles p plus q
  must always equal 1

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Hardy-Weinberg Principle

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Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
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A population not in Hardy-Weinberg equilibrium
indicates that one or more of the five
evolutionary agents are operating in a population
Five agents of evolutionary change
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Agents of Evolutionary Change

• Mutation A change in a cells DNA
• Mutation rates are generally so low they have
  little effect on Hardy-Weinberg proportions of
  common alleles.
• Ultimate source of genetic variation
• Gene flow A movement of alleles from one
  population to another
• Powerful agent of change
• Tends to homogenize allele frequencies

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Agents of Evolutionary Change

• Nonrandom Mating mating with specific genotypes
• Shifts genotype frequencies
• Assortative Mating does not change frequency of
  individual alleles increases the proportion of
  homozygous individuals
• Disassortative Mating phenotypically different
  individuals mate produce excess of heterozygotes

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Genetic Drift

• Genetic drift Random fluctuation in allele
  frequencies over time by chance
• 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 A bottleneck effect
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Bottleneck effect case study
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Selection

• Artificial selection a breeder selects for
  desired characteristics

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Selection

• Natural selection environmental conditions
  determine which individuals in a population
  produce the most offspring
• 3 conditions for natural selection to occur
• Variation must exist among individuals in a
  population
• Variation among individuals must result in
  differences in the number of offspring surviving
• Variation must be genetically inherited

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Selection
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Selection
Pocket mice from the Tularosa Basin
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Selection to match climatic conditions

• Enzyme allele frequencies vary with latitude
• Lactate dehydrogenase in Fundulus heteroclitus
  (mummichog fish) varies with latitude
• Enzymes formed function differently at different
  temperatures
• North latitudes Lactate dehydrogenase is a
  better catalyst at low temperatures

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Selection for pesticide resistance
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Fitness and Its Measurement

• Fitness A phenotype with greater fitness
  usually increases in frequency
• Most fit is given a value of 1
• Fitness is a combination of
• Survival how long does an organism live
• Mating success how often it mates
• Number of offspring per mating that survive

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Fitness and its Measurement
Body size and egg-laying in water striders
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Interactions Among Evolutionary Forces

• Mutation and genetic drift may counter selection
• The magnitude of drift is inversely related to
  population size

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Interactions Among Evolutionary Forces

• Gene flow may promote or constrain evolutionary
  change
• Spread a beneficial mutation
• Impede adaptation by continual flow of inferior
  alleles from other populations
• Extent to which gene flow can hinder the effects
  of natural selection depends on the relative
  strengths of gene flow
• High in birds wind-pollinated plants
• Low in sedentary species

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Interactions Among Evolutionary Forces
Degree of copper tolerance
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Maintenance of Variation

• Frequency-dependent selection depends on how
  frequently or infrequently a phenotype occurs in
  a population
• Negative frequency-dependent selection rare
  phenotypes are favored by selection
• Positive frequency-dependent selection common
  phenotypes are favored variation is eliminated
  from the population
• Strength of selection changes through time

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Maintenance of Variation
Negative frequency - dependent selection
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Maintenance of Variation
Positive frequency-dependent selection
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Maintenance of Variation

• Oscillating selection selection favors one
  phenotype at one time, and a different phenotype
  at another time
• Galápagos Islands ground finches
• Wet conditions favor big bills (abundant seeds)
• Dry conditions favor small bills

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Maintenance of Variation

• Fitness of a phenotype does not depend on its
  frequency
• Environmental changes lead to oscillation in
  selection

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Maintenance of Variation

• Heterozygotes may exhibit greater fitness than
  homozygotes
• Heterozygote advantage keep deleterious alleles
  in a population
• Example Sickle cell anemia
• Homozygous recessive phenotype exhibit severe
  anemia

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Maintenance of Variation

• Homozygous dominant phenotype no anemia
  susceptible to malaria
• Heterozygous phenotype no anemia less
  susceptible to malaria

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Maintenance of Variation
Frequency of sickle cell allele
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Maintenance of Variation

• Disruptive selection acts to eliminate
  intermediate types

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Maintenance of Variation
Disruptive selection for large and small beaks in
black-bellied seedcracker finch of west Africa
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Maintenance of Variation

• Directional selection acts to eliminate one
  extreme from an array of phenotypes

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Maintenance of Variation
Directional selection for negative phototropism
in Drosophila
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Maintenance of Variation

• Stabilizing selection acts to eliminate both
  extremes

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Maintenance of Variation
Stabilizing selection for birth weight in humans
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Experimental Studies of Natural Selection

• In some cases, evolutionary change can occur
  rapidly
• Evolutionary studies can be devised to test
  evolutionary hypotheses
• Guppy studies (Poecilia reticulata) in the lab
  and field
• Populations above the waterfalls low predation
• Populations below the waterfalls high predation

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Experimental Studies

• 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.

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Experimental Studies

• The evolution of protective coloration in guppies

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Experimental Studies

• The laboratory experiment
• 10 large pools
• 2000 guppies
• 4 pools with pike cichlids (predator)
• 4 pools with killifish (nonpredator)
• 2 pools as control (no other fish added)
• 10 generations

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Experimental Studies

• The field experiment
• Removed guppies from below the waterfalls (high
  predation)
• Placed guppies in pools above the falls
• 10 generations later, transplanted populations
  evolved the traits characteristic of
  low-predation guppies

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Experimental Studies
Evolutionary change in spot number
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The Limits of Selection

• Genes have multiple effects
• Pleiotropy sets limits on how much a phenotype
  can be altered
• Evolution requires genetic variation
• Thoroughbred horse speed
• Compound eyes of insects same genes affect both
  eyes
• Control of ommatidia number in left and right eye

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Experimental Studies
Selection for increased speed in racehorses is no
longer effective
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Experimental Studies
Phenotypic variation in insect ommatidia