IP5: Hardy-Weinberg/Genetic
Drift/Gene Flow
EQ: Distinguish genetic drift from gene flow and genetic drift in terms
of (a) how they occur and (b) their implications for future genetic
variation in a population.
EK1A1: Natural Selection is a major
mechanisms of natural selection
EK1A3: Evolutionary change is also
driven by random processes
Important Vocabulary
• Locus – (plural = loci) – location of a specific gene on a
chromosome– you can kind of think of it as a trait (but
remember some traits…ie eye color…are controlled by
multiple genes)
• Gene Pool – all the different alleles (letter used to
represent a trait/gene) at all loci in all individuals in a
population.
– Human Blood – there are three different alleles (A, B, and
O). Most traits have only two alleles.
• Population – group of organisms living in the same area
that are able to reproduce futile offspring; they share
the same gene pool
The Smallest Unit of Evolution
• Natural Selection acts directly on individuals
 but individuals themselves do not evolve
 only populations change overtime
• Example from book: drought 1,200180
overtime population had larger beaks but
individual birds beaks did not grow
themselves
• Popular Example: Giraffe’s neck
Microevolution
• Microevolution: change in allele frequency in a
population over time
• Three main mechanisms cause allele frequency to
change:
– Natural Selection – aka Adaptive evolution
– Genetic Drift – chance events that alter allele
frequency change
– Genetic Flow – transfer of alleles between populations
• Only Natural Selection consistently improves
match of organisms to the environment (nonrandom)
Mutation and Sexual Reproduction
• Natural Selection can only happen if individuals
differ in their inherited characteristics
• Two processes that produce genetic differences:
1. Mutation – must lead to a phenotype change; must
be in gamete to be passed along
2. Sexual reproduction – recombines alleles
• Sexual Reproduction has a more significant
impact on creating variation in organisms
Sexual Reproduction
• Three mechanisms in sexual reproduction lead
to shuffling of genes:
1. Crossing Over
2. Independent Assortment of Homologous
Chromosomes
3. Fertilization
Mutation Rates
• Low in plants and animals
• Even lower in bacteria and viruses but
because generation span is so low (2 days for
HIV) we see quicker change
• Viruses w/RNA genome experience even more
mutations because they lack RNA repair
mechanisms in the lost
H-W used to determine if a population is
evolving at a particular locus
• Variation is needed for evolution, but just because
differences exists does not mean evolution
automatically occurs
• H-W Equation can be used to determine if evolution is
happening.
• One way to assess if evolution is occurring is to
determine what the genetic make up of a population
would be if it were not evolving at a particular locus.
– Compare scenarios from real data
• If there is NO difference in the data then evolution is NOT
occurring (H-W Equilibrium)
• If there ARE differences than evolution IS occurring
Hardy Weinberg Principle
• Frequencies of alleles and genotypes in a population
will remain constant from generation to generation if
only Mendelian Segregation (Independent
Assortment) and recombination of alleles
(fertilization) happen
– Such gene pool is said to be in equilibrium
• Deck of Cards Analogy – No matter how often you
reshuffle to deal new hands, the deck itself does not
change, more Kings do not appear
– The act of reshuffling itself does not change allele
frequencies
Hardy-Weinberg Conventions
(locus w/2 alleles and 3 genotypes)
•
•
•
•
•
•
•
p represents the frequency of the dominant allele
q represents the frequency of the recessive allele
Because there are only two alleles p + q = 1
p2 = homozygous dominant (HH) genotype freq.
2pq = heterozygous (Hh) genotype freq.
q2 = homozygous recessive (hh) genotype freq.
Because there are only three genotypes:
– p2 + 2pq + q2 = 1
What is the HW Eq and What does it
represent?
• P2 + 2pq + q2 = 1
• Expected frequency of AA genotype +
expected frequency of Aa + expected
frequency of aa = 1
• AA + Aa + aa = 1
Conditions for H-W’s Equilibrium
In order for a population to be in HW Eq the following
conditions must be met:
1. No mutation
2. Random mating
3. No natural selection
4. Large Population (No Genetic Drift)
5. No gene flow
• These conditions are rarely met and therefore changes in
gene pools are usually taking place
• Common for one locus to be evolving while others are not
• Process can be so SLOW its not detectable
How to Use H-W EQ
• For a locus with two alleles (A and a) in a
population at risk from an infectious
neurodegenerative disease, 16 people had
genotype AA, 92 had genotype Aa, and 12 had
genotype aa. Use the H-W eq to determine
whether this population appears to be evolving.
• To answer this questions we will compare
expected frequencies of the alleles in the
population to actual frequencies
Genotypes AA
Actual
16
Expected
?
Aa
92
?
aa
12
?
How to use H-W eq continued
• Before we can use the HW eq: P2 +2pq+q2 = 1 we need
to determine p.
• p represents the total number of A alleles in the
population
• Because 16 individuals are AA and 92 individuals are Aa
there are a total of 124 A alleles in the population (16 +
16 + 92 = 124)
• Remember p is the frequencies of A so to get the
frequency we need to divide # of A/total # of A
possible. Because there are 120 individuals total
(16+92+12) and each has 2 alleles, there are 240 alleles
possible (2 X 120)
• Therefore p = 124/240 = .52 or 52% of the population
has at least 1 A allele
How to use H-W eq continued
• We can use the same method to determine
what % of the population would carry the a
allele. Or because there are only two types of
genotypes in the population we know:
– p + q = 1 (frequency of A + frequency of a = 100 of
the alleles in the population)
• Therefore if p = .52 then q = .48
How to use H-W eq continued
• Now that we know the value of p and q we
can plug them into the HW equation to
determine our expected frequencies of
genotypes:
• P2+2pq+q2 = 1
– P2 = (.52)2 = .27 or 27% of the pop. = AA
– 2pq = 2(.52)(.48) = .50 or 50% of the pop. = Aa
– q2 = (.48)2 = .23 or 23% of the pop. = aa
How to use H-W eq continued
• Expected % of Genotype
– P2 = (.52)2 = .27 or 27% of the pop. = AA
– 2pq = 2(.52)(.48) = .50 or 50% of the pop. = Aa
– q2 = (.48)2 = .23 or 23% of the pop. = aa
• To compare the expected data with the actual
data we need to determine out of a population of
120 how many would be expected to have each
genotype:
– 27% of 120 = about 32 AA
– 50% of 120 = about 60 Aa
– 23% of 120 = about 28 aa
How to use H-W eq continued
• Now that we have our expected number of
individuals who would have each genotype if
the population was at H-W equilibrium we can
compare it to the actual number of individuals
who have each genotype
Genotypes AA
Actual
16
Expected
32
Aa
92
60
aa
12
28
• What can we conclude? Is the population
evolving?
Conclusion/Next Steps
• Due to the differences between the actual
genotypes and expected genotypes we can
conclude evolution is taking place…
• …but WHY?
Genotypes AA
Actual
16
Expected
32
Aa
92
60
aa
12
28
Recall the Five Conditions of HW
1. No mutation
2. Random mating
3. No natural selection
4. Large Population (No Genetic Drift)
5. No gene flow
When a population is not in equilibrium we turn
to the 5 conditions to determine the reason for
the change in gene pool
1. Mutations
• Mutation: an altered gene (point mutations,
frame shift mutations, chromosomal mutations) –
modify the gene pool
• New mutation can alter allele frequencies, but
because they are rare, the change from one
generation to the next is small – mutation can
usually be ruled out as the main cause of the
genetic change to the gene pool
• Must be in a gamete to be past along
• However, as we learned last week if the mutation
provides a significant increase in fitness, due to
natural selection that mutant allele can quickly
increase in the population’s gene pool
2. Random Mating
• Nonrandom mating (for example – humans
mating dogs that have particular traits together
for a desired outcome) can affect the frequencies
of homozygous and heterozygous genotypes but
by itself usually has no effect on allele
frequencies in the gene pool
• What is the different between genotype
frequencies and allele frequencies
– Genotype = both alleles (AA, Aa, aa)
– Allele = only one letter (A or a)
3. Natural Selection - Nonrandom
• Adaptive evolution by acting on organisms phenotype;
creates a better match between the organism and the
environment
• Happens because a different proportion of genotypes
are being passed to future generations due to
differential fitness as a result of environmental change
• Yet another example:
– Fruit flies have an allele that provides resistance to
insecticides including DDT. Prior to the use of DDT in early
1930s the allele frequency of the resistant allele was 0%.
In populations collected after the 1960s (following the use
of DDT) the allele frequency was 37%.
4. Genetic Drift - Random
• Chance events that cause an unpredictable change in a
populations gene pool (allele frequency) typically effects a
small population
– Tends to reduce genetic variation due to the loss of alleles
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CR
CW CW
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
Two main types of Genetic Drift
Founders Effect
Bottleneck Effect
•
•
•
Individuals become isolated from a
larger population  establish new
population  likely gene pool differs
from original population
Accounts for a number of relative
high frequency of certain inherited
disorders among isolated human
populations
Original
population
Bottlenecking
event
Surviving
population
•
Sudden reduction in population size
due to a change in environment;
resulting gene pop may no longer be
reflective of the original population
If population remains small further
genetic drift can occur
Effects of Genetic Drift
1. Genetic drift is significant in small
populations.
2. Genetic drift causes allele frequencies to
change at random.
3. Genetic drift can lead to a loss of genetic
variation within populations.
4. Genetic drift can cause harmful alleles to
become fixed.
5. Gene Flow (Immigration and
Emigration) - Random
• Consists of the movement of alleles among
population
– Insects bring pollen from a different population
– Increase of travel – more human diversity
• Tends to reduce differences between populations
over time.
• Can prevent populations from fully adapting to
environment
• More likely than mutation to alter allele
frequency directly
You should now be able to:
1. Define the terms population, locus, gene pool, and
relative fitness.
2. List the five conditions of Hardy-Weinberg
equilibrium.
3. Apply the Hardy-Weinberg equation to a population
genetics problem.
4. Explain why natural selection is the only mechanism
that consistently produces adaptive change.
5. Explain the role of population size in genetic drift.
6. Be able to explain the difference between natural
selection and genetic drift in the way they occur and
the impact they have on the gene pool.
Learning Objectives
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•
•
•
•
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Learning objective 1.1The student is able to convert a data set from a table of numbers that
reflect a change in the genetic makeup of a population over time and to apply mathematical
methods and conceptual understandings to investigate the cause(s) and effect(s) of this
change. [See SP 1.5, 2.2; Essential knowledge 1.A.1]
Learning objective 1.2The student is able to evaluate evidence provided by data to
qualitatively and/or quantitatively investigate the role of natural selection in evolution. [See
SP 2.2, 5.3; Essential knowledge 1.A.1]
Learning objective 1.3The student is able to apply mathematical methods to data from a
real or simulated population to predict what will happen to the population in the future.
[See SP 2.2; Essential knowledge 1.A.1]
Learning objective 1.6The student is able to use data from mathematical models based on
the Hardy- Weinberg equilibrium to analyze genetic drift and effects of selection in the
evolution of specific populations. [See SP 1.4, 2.1; Essential knowledge 1.A.3]
Learning objective 1.7The student is able to justify the selection of data from mathematical
models based on the Hardy-Weinberg equilibrium to analyze genetic drift and the effects of
selection in the evolution of specific populations. [See SP 2.1, 4.1; Essential knowledge
1.A.3]
Learning objective 1.8The student is able to make predictions about the effects of genetic
drift, migration and artificial selection on the genetic makeup of a population. [See SP 6.4;
Essential knowledge 1.A.3]