Genes Within Populations

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Genes Within Populations

• Chapter 19

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Outline

• Gene Variation
• Hardy Weinberg Principle
• Agents of Evolutionary Change
• Measuring Fitness
• Interactions Among Evolutionary Forces
• Forms of Selection
• Selection on Color in Guppies
• Limits to Selection

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Gene Variation is Raw Material

• Evolution is descent with modification
• Darwin
• Through time, species accumulate differences such
  that ancestral and descendent species are not
  identical.

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Microevolution leads
to changes
within populations
Macroevolution leads to great
phenotypic changes
resulting in distinctive lineages
(e.g.
species)
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Gene Variation is Raw Material

• Natural selection and evolutionary change
• Some individuals in a population possess certain
  inherited characteristics that play a role in
  producing more surviving offspring than
  individuals without those characteristics.
• The population gradually includes more
  individuals with advantageous characteristics.

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Darwin versus Lamarck
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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.

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Hardy-Weinberg Principle

• Population genetics - study of properties of
  genes in populations
• blending inheritance (phenotypic inheritance) was
  widely accepted
• new genetic variants would quickly be diluted

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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
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Hardy-Weinberg Principle

• Necessary assumptions
• 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
• (pq)2 p2 2pq q2
• p individuals homozygous for first allele
• 2pq individuals heterozygous for alleles
• q individuals homozygous for second allele

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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
• 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
• artificial - breeders exert selection
• natural - nature exerts selection
• variation must exist among individuals
• variation must result in differences in numbers
  of viable offspring produced
• variation must be genetically inherited
• natural selection is a process, and evolution is
  an outcome

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Five Agents of Evolutionary Change

• Selection pressures
• avoiding predators
• matching climatic condition
• pesticide resistance

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Measuring Fitness

• Fitness is defined by evolutionary biologists as
  the number of surviving offspring left in the
  next generation.
• relative measure
• Selection favors phenotypes with the greatest
  fitness.

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Interactions Among Evolutionary Forces

• Levels of variation retained in a population may
  be determined by the relative strength of
  different evolutionary processes.
• Gene flow versus natural selection
• Gene flow can be either a constructive or a
  constraining force.
• Allelic frequencies reflect a balance between
  gene flow and natural selection.

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Natural Selection Can Maintain Variation

• Frequency-dependent selection
• Phenotype fitness depends on its frequency within
  the population.
• Negative frequency-dependent selection favors
  rare phenotypes.
• Positive frequency-dependent selection eliminates
  variation.
• Oscillating selection
• Selection favors different phenotypes at
  different times.

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Heterozygote Advantage

• Heterozygote advantage will favor heterozygotes,
  and maintain both alleles instead of removing
  less successful alleles from a population.
• Sickle cell anemia
• Homozygotes exhibit severe anemia, have abnormal
  blood cells, and usually die before reproductive
  age.
• Heterozygotes are less susceptible to malaria.

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Sickle Cell and Malaria
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Forms of Selection

• Disruptive selection
• Selection eliminates intermediate types.
• 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.
• thoroughbred horses
• Gene interactions affect allelic fitness
• epistatic interactions

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Summary

• Gene Variation
• Hardy Weinberg Principle
• Agents of Evolutionary Change
• Measuring Fitness
• Interactions Among Evolutionary Forces
• Forms of Selection
• Selection on Color in Guppies
• Limits to Selection

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