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Lecture #2 – Evolution of
Populations
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Key Concepts:
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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 Synthesis
integrates our knowledge about
evolution
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Darwin’s natural selection
Mendel’s 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 don’t 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 don’t 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 – you’ll
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
Doesn’t happen in nature!!!
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Conditions for H-W Equilibrium:
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No natural selection
Large population size
Isolated population
Random mating
No mutation
Doesn’t 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 HardyWeinberg theorem???
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Critical Thinking
• What are the limitations of the HardyWeinberg 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-evolution:
population-scale changes in allele
frequencies
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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
Darwin’s theory
Cartoon – beaver with chainsaw paws 
“natural selection does not grant
organisms what they “need””
More on this later….
More on this later….
Differential reproductive success is the only
way to account for the accumulation of
favorable traits in a population
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Micro-evolution:
population-scale changes in allele
frequencies
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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)
Parent pop = 10% blue
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Larger pop = ~29% blue
Smaller pop = 100% blue
Genetic Drift – random changes in allele
frequency from generation to generation
• More pronounced in smaller and/or more
segregated populations
Bottleneck effect
Founder effect
Parent pop = 10% blue
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Segregated pop = ~29% blue
Segregated pop = 100% blue
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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Maps – historic and current range
of cheetahs
Extreme range
reduction due to
habitat destruction
and poaching
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Cheetahs were
naturally bottlenecked
about 10,000 years
ago by the last major
ice age (kinked tail)
The species is at risk
of extinction 30
Australian Flame Robin, California Condor,
Mauritian Kestrel
…..and many more, all driven nearly to extinction…..
Images – bottlenecked and now endangered species
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Some colorful results of a quick web search on “bottlenecked species”
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 it’s 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-evolution:
population-scale changes in allele
frequencies
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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-evolution:
population-scale changes in allele
frequencies
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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-evolution:
population-scale changes in allele
frequencies
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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 events
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“the gate”
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Review: Micro-evolution:
population-scale changes in allele
frequencies
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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 n=2
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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 n=2
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 > 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
Diagram – patterns of selection
diversifying
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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 Selection
rare clone is less infected
Graph – frequency dependent selection results
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Balanced Polymorphisms – Phenotypic Variation
multiple 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???
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The Modern Synthesis
Populations and the Gene Pool
The Hardy-Weinberg Equilibrium
Micro-evolution
Sources of Genetic Variation
Natural Selection
Preservation of Genetic Variation
85