Transcript Ch_23 Population Genetics

Chapter 23.
Evolution of Populations
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Populations evolve
 Natural selection acts on individuals

differential survival
 “survival of the fittest”

differential reproductive success
 who bears more offspring
 Populations evolve
genetic makeup of
population changes
over time
 favorable traits
(greater fitness)
become more common
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
Bent Grass on
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toxic mine
site
Individuals DON’T evolve!!!
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Mutation & Variation
 Mutation creates variation

new mutations are constantly appearing
 Mutation changes DNA sequence
changes amino acid sequence?
 changes protein?

 change structure?
 change function?

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changes in protein may
change phenotype &
therefore change fitness
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Sex & Variation
 Sex spreads variation
one ancestor can have many
descendants
 sex causes recombination
 offspring have new combinations
of traits = new phenotypes

 Sexual reproduction recombines alleles
into new arrangements in every
offspring
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Variation impacts natural selection
 Natural selection requires a source of
variation within the population
there have to be differences
 some individuals must be more fit than
others

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Changes in populations
 Evolution of populations is really
measuring changes in allele frequency

all the genes & alleles in a population =
gene pool
 Factors that alter allele frequencies
in a population
natural selection
 genetic drift

 founder effect
 bottleneck effect

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gene flow
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Natural selection
 Natural selection adapts a population to
its environment

a changing environment
 climate change
 food source availability
 new predators or diseases
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combinations of alleles
that provide “fitness”
increase in the population
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Genetic drift
 Effect of chance events

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces population to
small number & then
population recovers
& expands again
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Founder effect
 When a new population is started by
only a few individuals
some rare alleles may be at high
frequency; others may be missing
 skew the gene pool of
new population

 human populations that
started from small group
of colonists
 example: white people
colonizing New World
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Distribution of blood types
 Distribution of the O type blood allele in native
populations of the world reflects original settlement
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Distribution of blood types
 Distribution of the B type blood allele in native
populations of the world reflects original migration
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Out of Africa
Likely migration paths of humans out of Africa
Many patterns of human traits reflect this migration
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Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance

 alleles lost from gene pool
 narrows the gene pool
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Cheetahs
 All cheetahs share a small number of
alleles
less than 1% diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an
important concept in
conservation biology of
endangered species
loss of alleles from gene
pool
 reduces variation
 reduces ability to
adapt
 at risk populations

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Gene flow
 Population spread over large area
migrations = individuals move from one
area to another
 sub-populations may have different
allele frequencies

 Migrations cause genetic mixing across
regions = gene flow
new alleles are moving
into gene pool
 reduce differences
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
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Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
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Any Questions??
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Chapter 23.
Measuring
Evolution of Populations
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Populations & gene pools
 Concepts
a population is a localized group of
interbreeding individuals
 gene pool is collection of alleles in the
population

 remember difference between alleles & genes!

allele frequency is how common is that
allele in the population
 how many A vs. a in whole population
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Evolution of populations
 Evolution = change in allele frequencies
in a population


hypothetical: what would it be like if
allele frequencies didn’t change?
non-evolving population
1. very large population size (no genetic drift)
2. no migration (movement in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (no selection)
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Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population

preserves allele frequencies
 Serves as a model


natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on
a population
 measuring evolutionary change
G.H. Hardy
AP mathematician
Biology
W. Weinberg
physician
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Hardy-Weinberg theorem
 Alleles
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 100%, so:
p+q=1
BB
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Bb
bb
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Hardy-Weinberg theorem
 Individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 100%, so:
p2 + 2pq + q2 = 1
BB
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Bb
bb
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Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
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q2 (rr): 16/100 = .16
q (r): √.16 = 0.4
p (R): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.48
bb
Must assume in H-W equilibrium!
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Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.48
BB
Bb
bb
p2=.10
=.45
BB
2pq=.80
2pq=.10
Bb
q2=.10
=.45
bb
Null hypothesis
Sampled data
How do you
explain
the data?
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How do allele frequencies change?
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Real world application of H-W
 Frequency of allele in human population
Example:
What % of human population carries
allele for PKU (phenylketonuria )
 ~ 1 in 10,000 babies born in the US is
born with PKU, which results in mental
retardation & other problems if untreated
 disease is caused by a recessive allele

 PKU = homozygous recessive (aa)
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H-W & PKU disease
 frequency of homozygous recessive individuals




q2 (aa) = 1 in 10,000 = 0.0001
frequency of recessive allele (q):
q = √0.0001 = 0.01
frequency of dominant allele (p):
p (A) = 1 – 0.01 = 0.99
frequency of carriers, heterozygotes:
2pq = 2 x (0.99 x 0.01) = 0.0198 = ~2%
~2% of the US population carries the PKU allele
300,000,000 x .02 = 6,000,000
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Any Questions??
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