Lecture 2 – Evolution of Populations

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Lecture 2 Evolution of Populations
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Key Concepts

• The Modern Synthesis
• Populations and the Gene Pool
• The Hardy-Weinberg Equilibrium
• Micro-evolution
• Sources of Genetic Variation
• Natural Selection
• Preservation of Genetic Variation

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Some preliminary definitions

• Species individual organisms capable of mating
  and producing fertile offspring
• Population a group of individuals of a single
  species
• Community a group of individuals of different
  species

Images species, population, community
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The Modern Synthesisintegrates our knowledge
about evolution

• Darwins natural selection
• Mendels hereditary patterns
• Particulate transfer (chromosomes)
• Structure of the DNA molecule

All explain how the genetic structure of
populations changes over time
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KEY POINT Environmental factors act on the
individual to control the genetic future of the
population Individuals dont evolve..populations
do
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Population a /- localized group of individuals
of one species
Image population of iris
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Critical Thinking

• How do we determine the boundaries of a
  population???

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Critical Thinking

• How do we determine the boundaries of a
  population???

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Recall basic genetic principles

• In a diploid species (most are), every individual
  has two copies of every gene
• One copy came from each parent
• Most genes have different versions alleles
• Diploid individuals are either heterozygous or
  homozygous for each gene
• Heterozygous Aa
• Homozygous AA or aa

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Recall basic genetic principles

• The total number of alleles for any gene in a
  population is the number of individuals in the
  population x 2
• If the population has 10 individuals, there are
  20 copies of the A gene some A alleles and
  some a alleles
• All these alleles comprise the gene pool

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Hardy-Weinberg Theorem

• Gene pool all alleles in a population
• All alleles have a frequency in the population
• There is a percentage of A and a percentage of
  a that adds up to 100
• Hardy-Weinberg Theorem demonstrates that allele
  frequencies dont change through meiosis and
  fertilization alone

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Hardy-Weinberg Theorem

• A simple, mathematical model
• Shows that repeated random meiosis and
  fertilization events alone will not change the
  distribution of alleles in a population
• Even over many generations

p2 2pq q2 1 we will not focus on the math
youll work on this in lab
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Hardy-Weinberg Theorem

• Meiosis and fertilization randomly shuffle
  alleles, but they don't change proportions
• Like repeatedly shuffling a deck of cards
• The laws of probability determine that the
  proportion of alleles will not change from
  generation to generation
• This stable distribution of alleles is the
  Hardy-Weinberg equilibrium

Doesnt happen in nature!!!
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Conditions for H-W Equilibrium

• No natural selection
• Large population size
• Isolated population
• Random mating
• No mutation

Doesnt happen in nature!!! The violation of each
assumption acts as an agent of microevolution
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The value of H-W???

• It provides a null hypothesis to compare to what
  actually happens in nature
• Allele frequencies DO change in nature
• BUT, they change only under the conditions of
  microevolution
• In nature, all the H-W assumptions are violated
• Result populations DO evolve

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Critical Thinking

• What are the limitations of the Hardy-Weinberg
  theorem???

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Critical Thinking

• What are the limitations of the Hardy-Weinberg
  theorem???
• Recall your basic genetics is this realistic???

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Critical Thinking
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Individuals Do Not Evolve

• Individuals vary, but populations evolve
• Natural selection pressures make an individual
  more or less likely to survive and reproduce
• But, it is the cumulative effects of selection on
  the genetic makeup of the whole population that
  results in changes to the species

The environment is a wall natural selection is a
gate
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The environment is the wall natural selection is
the gate
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Micro-evolutionpopulation-scale changes in
allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

Image natural variation in flower color same
image for all these summary slides
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Natural Selection the essence of Darwins theory
Differential reproductive success is the only way
to account for the accumulation of favorable
traits in a population
Cartoon beaver with chainsaw paws ? natural
selection does not grant organisms what they
need
More on this later.
More on this later.
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Micro-evolutionpopulation-scale changes in
allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

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Genetic Drift random changes in allele
frequency from generation to generation

• Reproductive events are samples of the parent
  population

• Larger samples are more representative than
  smaller samples (probability theory)

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Genetic Drift random changes in allele
frequency from generation to generation

• More pronounced in smaller and/or more segregated
  populations
• Bottleneck effect
• Founder effect

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Bottlenecking extreme genetic drift
Diagram bottlenecking
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Critical Thinking

• What events could cause a bottleneck???

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Critical Thinking

• What events could cause a bottleneck???

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Conservation implications cheetahs are a
bottlenecked species
Image cheetah
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Extreme range reduction due to habitat
destruction and poaching Cheetahs were
naturally bottlenecked about 10,000 years ago by
the last major ice age (kinked tail) The species
is at risk of extinction
Maps historic and current range of cheetahs
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Australian Flame Robin, California Condor,
Mauritian Kestrel..and many more, all driven
nearly to extinction..
Images bottlenecked and now endangered species
Some colorful results of a quick web search on
bottlenecked species
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Founder Effect extreme genetic drift

• Occurs when a single individual, or small group
  of individuals, breaks off from a larger
  population to colonize a new habitat
• Islands
• Other side of mountain
• Other side of a river
• This small group may not represent the allele
  distribution of the parent population

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Founder Effect
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Long distance dispersal events can lead to the
founder effect
Image a founding population of seeds possibly
also the bird if its a gravid female
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Critical Thinking

• What do you think follows long distance dispersal
  to a new ecosystem???

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Critical Thinking

• What do you think follows long distance dispersal
  to a new ecosystem???

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Micro-evolutionpopulation-scale changes in
allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

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Gene Flow

• Mixes alleles between populations
• Immigration
• Emigration
• Most populations are NOT completely isolated

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Critical Thinking

• Will gene flow tend to increase or decrease
  speciation???

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Critical Thinking

• Will gene flow tend to increase or decrease
  speciation???

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Gene Flow
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Micro-evolutionpopulation-scale changes in
allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

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Selective Breeding
Image peacock with mating display
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Critical Thinking

• Animal behaviors are obvious examples
• Can you think of others???

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Critical Thinking

• Animal behaviors are obvious examples
• Can you think of others???

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Micro-evolutionpopulation-scale changes in
allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

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Mutations

• Random, rare, but regular events
• The only source of completely new traits

Diagram mutations
just for fun..
Cartoon - jackalope
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Evolution random eventsxthe gate
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Review Micro-evolutionpopulation-scale changes
in allele frequencies

• Natural Selection
• Genetic Drift
• Gene Flow
• Selective Mating
• Mutation

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Sources of Genetic Variation

• Natural selection acts on natural variation
• Where does this variation come from???
• Meiosis
• Mutation
• Additional mechanisms help preserve variation
  (later)

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Meiosis key source of variation
Diagram meiosis I
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Diagram meiosis II
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Random, Independent Assortment of Homologous
Chromosomes
n 2
Diagram results of meiosis with n2
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Probability theory reveals that for random,
independent events

• If each event has 2 possible outcomes
• In this case, one side of the plate or the other
• The possible number of distribution combinations
  2n, where n the number of events
• In this case, the distribution event is the
  distribution of chromosomes to the gametes
• n the haploid number of chromosomes
• If n is 2, then combinations are 22 4

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Random, Independent Assortment of Homologous
Chromosomes
n 2
Diagram results of meiosis with n2
Four possible distributions
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Probability theory reveals that for random,
independent events

• If each event has 2 possible outcomes
• In this case, one side of the plate or the other
• The possible number of distribution combinations
  2n, where n the number of events
• In this case, distribution refers to the
  distribution of chromosomes to the gametes
• n the haploid number of chromosomes
• If n is 23, then combinations are 223 8.4
  million!

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Probability is Multiplicative

• 8.4 million x 8.4 million gt 70 trillion!!!
• That is the number of possible combinations of
  maternal and paternal chromosomes in the
  offspring of a randomly mating pair of humans

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Recombination increases the potential variation
to infinity
Diagram recombination
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Critical Thinking

• Can meiosis produce totally new traits???

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Critical Thinking

• Can meiosis produce totally new traits???

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Natural Selection as a Mechanism of Evolutionary
Adaptation

• Natural selection acts on the variation produced
  by meiosis and mutation
• Selection increases the fitness of a population
  in a given environment
• Fitness ???

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Natural Selection as a Mechanism of Evolutionary
Adaptation

• Natural selection acts on the variation produced
  by meiosis and mutation
• Selection increases the fitness of a population
  in a given environment
• Fitness

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Natural selection has limits

• Individuals vary in fitness
• Natural selection promotes the most fit
• Selection acts on the phenotype the whole,
  complex organism
• Results from the combination of many different
  genes for any organism
• These genes are expressed in the whole, complex
  environment
• Selection is always constrained by the whole,
  complex evolutionary history of the species

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Critical Thinking

• Can evolution respond to needs???

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Critical Thinking

• Can evolution respond to needs???

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Patterns of Change by Natural Selection

• Directional Selection
• Diversifying Selection (AKA disruptive)
• Stabilizing Selection

Diagram patterns of natural selection
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Remember, all populations exhibit a range of
natural variation
Diagram patterns of natural selection
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Directional Selection

• Phenotypes at one extreme of the range are most
  successful
• Color
• Pattern
• Form
• Metabolic processes
• The population shifts to favor
  the successful phenotype

Diagram directional selection
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Diversifying Selection

• Multiple, but not all, phenotypes are successful
• Patchy environments
• Sub-populations migrate to new habitats
• The population begins to fragment and new species
  begin to diverge

Diagram diversifying selection
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Stabilizing Selection

• The intermediate phenotypes are most successful
• Homogenous environments
• Stable conditions
• The range of variation within the population is
  reduced

Diagram stabilizing selection
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Critical Thinking

• Which selection mode will most quickly lead to
  the development of diversity???

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Critical Thinking

• Which selection mode will most quickly lead to
  the development of diversity???

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directional
diversifying
Diagram patterns of selection
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Critical Thinking

• Can you think of a real-life example of an
  adaptive phenotype???

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Critical Thinking

• Can you think of a real-life example of an
  adaptive phenotype???

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Preservation of Natural Variation

• Diploidy
• Balanced Polymorphism
• Neutral Variation

Images natural variation in flower color
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Diploidy 2 alleles for every gene

• Recessive alleles retained in heterozygotes
• Not expressed
• Not eliminated, even if the recessive trait is
• aa may be eliminated, while Aa is preserved in
  the population
• Recessive alleles function as latent variation
  that may prove helpful if environment changes

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Balanced Polymorphism

• Heterozygote advantage
• Frequency dependent selection
• Phenotypic variation

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Balanced Polymorphism heterozygote advantage
a mutation in the gene that codes for hemoglobin
causes a single amino acid substitution in the
protein, RBC shape changes from round to sickle
shape
Sickle-cell Anemia
Map global distribution of sickle cell
allele Images normal and sickled red blood
cells
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Balanced Polymorphisms Frequency Dependent
Selectionrare clone is less infected
Graph frequency dependent selection results
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Balanced Polymorphisms Phenotypic
Variationmultiple morphotypes are favored by
heterogeneous (patchy) environment
Images balanced polymorphisms in asters and
snakes
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Neutral Variation

• Genetic variation that has no apparent effect on
  fitness
• Not affected by natural selection
• May provide an important base for future
  selection, if environmental conditions change

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Key Concepts QUESTIONS???

• The Modern Synthesis
• Populations and the Gene Pool
• The Hardy-Weinberg Equilibrium
• Micro-evolution
• Sources of Genetic Variation
• Natural Selection
• Preservation of Genetic Variation