What is Hardy-WeinbergEquilibrium?
Hardy-Weinberg Equilibrium is a principle in population genetics
that states:
‘‘ Allele and genotype frequencies in a population remain constant
from generation to generation, provided no evolutionary forces
are acting’’.
It serves as a baseline (null model) to compare real populations
and detect changes.
4.
Hardy-Weinberg equilibrium
Hypothetical,non-evolving population
preserves allele frequencies in each generation (including recessives)
Serves as a model (null hypothesis)
natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on a population
measuring evolutionary change
G.H. Hardy
mathematician
W. Weinberg
physician
5.
Hardy-Weinberg Equilibrium Principle
The Hardy-Weinberg equilibrium (HWE) is a fundamental
concept in population genetics that describes a state where the
genetic variation in a population remains constant from one
generation to the next, provided certain conditions are met.
It provides a mathematical framework to study allele and genotype
frequencies in a population.
6.
Key Conditions forHardy-Weinberg Equilibrium
For a population to be in HWE, the following assumptions must hold:
Large population size – minimizes random genetic drift.
Random mating – individuals pair by chance, not by genotype or
phenotype.
No mutations – alleles are not altered by mutation.
No migration – no gene flow from other populations.
No natural selection – all genotypes have equal reproductive
success.
7.
When HWE Fails(Deviation from
Equilibrium)
1. Selection (Natural Selection)
Some genotypes have better survival or reproduction than
others.
Certain genotypes are more common or less common than
expected
Example:
In malaria-endemic areas:
AS genotype (carrier) survives better than AA or SS
Leads to higher frequency of heterozygotes
8.
When HWE Fails(Deviation from
Equilibrium)
2. Mutation
New alleles are created when DNA changes.
Gradual increase of a rare allele over time
Slight shift in allele frequencies (p and q)
Example:
A new recessive disease allele appears in a population
9.
When HWE Fails(Deviation from
Equilibrium)
3. Migration (Gene Flow)
People move in or out of a population, bringing new alleles.
Sudden change in allele frequencies
Population becomes more genetically mixed
Example:
A group with high frequency of allele “a” joins another population
Overall q increases
10.
When HWE Fails(Deviation from
Equilibrium)
4. Genetic Drift
Random changes in allele frequencies, especially in small
populations.
Allele frequencies change by chance, not because of advantage
Some alleles may disappear completely
Examples:
Bottleneck effect: disaster reduces population size
Founder effect: small group starts a new population
11.
When HWE Fails(Deviation from
Equilibrium)
5. Non-Random Mating (Inbreeding)
Individuals choose mates based on relatedness or traits (not random).
More homozygotes (AA, aa)
Fewer heterozygotes (Aa)
Example:
Relatives mating higher chance of same alleles pairing
→
12.
Importance Hardy-Weinberg Equilibrium
Provides a baseline to detect evolutionary forces such as
selection, mutation, migration, or genetic drift.
Helps in predicting genetic disease frequencies in
human populations.
Forms the basis for population genetics studies and
conservation biology.
13.
Applications of Hardy-WeinbergEquilibrium
1. Estimating & understanding Allele and Genotype
Frequencies
HWE provides a way to calculate allele frequencies (p and q) in a
population when genotype frequencies are known, and vice versa.
This is essential in studying genetic traits and disorders in
populations.
Sickle cell anemia is caused by a recessive allele (s) of the
hemoglobin gene (normal allele is A).
Using H-W, if you know the frequency of the disease (homozygous
recessive genotype, ss), you can estimate the frequency of the sickle
cell allele (s) in the population.
14.
Applications of Hardy-WeinbergEquilibrium
2 Predicting Carrier Frequencies for Genetic Diseases
Particularly useful for autosomal recessive diseases like sickle cell
anemia, cystic fibrosis, and Tay-Sachs disease.
Enables estimation of how many individuals are carriers
(heterozygotes) without symptoms, important for genetic counseling.
3. Estimating Carrier Frequency
Heterozygous carriers (AS) do not show disease symptoms but can
pass the allele on.
15.
Applications of Hardy-WeinbergEquilibrium
4. Detecting Evolutionary Forces
Deviations from HWE indicate that factors like natural selection,
mutation, migration, non-random mating, or genetic drift
are acting on the population.
Useful for studying how populations evolve and adapt over time.
5. Forensic Science
HWE helps calculate the probability of a genetic profile in a
population, which is used in DNA fingerprinting and paternity
testing.
16.
Applications of Hardy-WeinbergEquilibrium
6. Conservation Biology
Helps assess the genetic diversity of endangered species populations
and their risk of inbreeding, guiding conservation efforts.
7. Population Genetics Research
Serves as a null model or baseline to study gene flow, population
structure, and the effects of mating patterns.
8. Public Health and Epidemiology
Assists in understanding the spread and prevalence of genetic
diseases, informing screening programs and healthcare policies
17.
Clinical Applications inMedical Laboratory
Science
a) Genetic Disease Screening
Estimate carrier frequency for diseases like:
Sickle cell disease
Cystic fibrosis
Tay-Sachs disease
b) Genetic Counseling
Predict risk of inheritance in families
Identify carrier parents
Guide reproductive decisions
18.
Clinical Applications inMedical Laboratory
Science
c) Laboratory Diagnostics
Compare observed vs expected genotype frequencies
Detect:
Selection
Inbreeding
Population stratification
d) Epidemiology
Monitor distribution of disease alleles in populations
Track changes over time
19.
Clinical Applications inMedical Laboratory
Science
e) Quality Control in Genetic Studies
In research labs, deviation from HWE may indicate:
Sampling error
Genotyping errors
Population bias
20.
Why Natural SelectionCan’t Make Perfect
Organisms
This means that nature’s process of “choosing” the best traits in
animals and plants doesn’t create flawless or ideal living things.
Instead, it works with what’s available and has limits, so organisms
are always a bit imperfect or “good enough” rather than perfect.
1. Selection Can Only Work on What Already Exists
Natural selection can’t invent new traits from scratch. It can only
pick from the traits that already exist in the population.
Think of it like choosing the best cards from the ones you’re dealt—
you can’t get new cards, only play with what you have.
21.
Why Natural SelectionCan’t Make Perfect
Organisms
2. Evolution Is Limited by History
Organisms inherit their bodies and genes from their ancestors, so
new traits must build on what came before.
It’s like remodeling an old house—you can’t redesign everything
completely; you work with the structure already there.
22.
Why Natural SelectionCan’t Make Perfect
Organisms
3. Adaptations Are Often Compromises
Some traits help in one way but cause problems in another.
For example, having a big brain is great for thinking, but it also
means needing more energy to survive.
So, nature balances these trade-offs rather than creating perfect
solutions.
23.
Why Natural SelectionCan’t Make Perfect
Organisms
4. Chance, Natural Selection, and the Environment Work
Together
Chance means random events (like natural disasters) can affect
which traits survive.
Natural selection favors traits that help organisms survive and
reproduce.
Environment influences what traits are useful—what’s good in one
place might not be in another.
24.
Why Natural SelectionCan’t Make Perfect
Organisms
This means that nature’s process of “choosing” the best traits in
animals and plants doesn’t create flawless or ideal living things.
Instead, it works with what’s available and has limits, so organisms
are always a bit imperfect or “good enough” rather than perfect.
1. Selection Can Only Work on What Already Exists
Natural selection can’t invent new traits from scratch. It can only
pick from the traits that already exist in the population.
Think of it like choosing the best cards from the ones you’re dealt—
you can’t get new cards, only play with what you have.
26.
Hardy-Weinberg theorem
CountingAlleles
assume 2 alleles = B, b
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
frequencies must add to 1 (100%), so:
p + q = 1
27.
Hardy-Weinberg theorem
CountingIndividuals
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 1 (100%),
so:
p2
+ 2pq + q2
= 1
28.
Application of H-Wprinciple
Sickle cell anemia
inherit a mutation in gene coding for hemoglobin
oxygen-carrying blood protein
recessive allele = Hs
Hs
normal allele = Hb
low oxygen levels causes
RBC to sickle
breakdown of RBC
clogging small blood vessels
damage to organs
often lethal
29.
Application of H-Wprinciple
Sickle cell frequency
High frequency of heterozygotes
1 in 5 in Central Africans = Hb
Hs
unusual for allele with severe
detrimental effects in homozygotes
1 in 100 = Hs
Hs
usually die before reproductive ageWhy is the Hs
allele maintained at such
high levels in African populations?
Why is the Hs
allele maintained at such high
levels in African populations?
Suggests some selective advantage of being
heterozygous…
30.
Application of H-Wprinciple
Malaria Single-celled eukaryote parasite
(Plasmodium) spends part of its life
cycle in red blood cells
31.
Application of H-Wprinciple
Heterozygote Advantage
In tropical Africa, where malaria is common:
homozygous dominant (normal)
die or reduced reproduction from malaria: Hb
Hb
homozygous recessive
die or reduced reproduction from sickle cell anemia: Hs
Hs
heterozygote carriers are relatively free of both: Hb
Hs
survive & reproduce more, more common in population
Hypothesis:
In malaria-infected cells, the
O2 level is lowered enough to
cause sickling which kills the
cell & destroys the parasite.
Hardy-Weinberg equilibrium describes
populationsthat are not evolving.
Genotype frequencies stay the same if five conditions are met.
very large population: no genetic drift
no emigration or immigration: no gene flow
no mutations: no new alleles added to gene pool
random mating:
no sexual selection
no natural selection:
all traits aid equally
in survival
34.
Hardy-Weinberg equilibrium describes
populationsthat are not evolving.
Real populations rarely meet all five conditions.
Real population data is
compared to a model.
Models are used to
studying how populations
evolve.
35.
The Hardy-Weinberg equationis used to predict
genotype frequencies in a population.
Predicted genotype frequencies are compared with actual frequencies.
used for traits in simple dominant-recessive systems
"The Hardy-Weinberg equation
is based on Mendelian genetics.
It is derived from a simple
Punnett square in which p is the
frequency of the dominant allele
and q is the frequency of the
recessive allele."
– must know frequency of recessive homozygotes
– p2
+ 2pq + q2
= 1
36.
Genetic driftchanges allele frequencies due to chance alone.
37.
Gene flowmoves alleles from one population to another.
In nature,populations evolve.
– expected in all populations
most of the time
– respond to changing
environments
42.
SPECIATION
SPECIATION: is anevolutionary process by which a new species comes into
: is an evolutionary process by which a new species comes into
being. Usually new and distinct species is formed in the course of
being. Usually new and distinct species is formed in the course of
evolution
evolution
KEY CONCEPT
New species can arise when populations are isolated.
43.
The isolation ofpopulations can lead
to speciation.
Populations become isolated when there is no gene flow.
Isolated populations adapt to their own environments.
Genetic differences can add up over generations.
44.
Reproductive isolationcan occur between isolated populations.
– members of different
populations cannot
mate successfully
– final step to
becoming separate
species
• Speciation is the rise of two or more species from one
existing species.
45.
Populations can becomeisolated in
several ways.
1. Behavioral barriers can cause isolation.
called behavioral isolation
includes differences in courtship or mating behaviors
46.
2. Geographicbarriers can cause isolation.
– called geographic isolation
– physical barriers divide population
• 3. Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
Overview: The SmallestUnit of Evolution
One misconception is that organisms evolve during their
lifetimes
Natural selection acts on individuals, but only populations
evolve
Consider, for example, a population of medium ground finches
on Daphne Major Island
During a drought, large-beaked birds were more likely to
crack large seeds and survive
The finch population evolved by natural selection
49.
Microevolution isa change in allele frequencies in
a population over generations
Three mechanisms cause allele frequency change:
Natural selection
Genetic drift
Gene flow
Only natural selection causes adaptive evolution
50.
Variation inheritable traits is a prerequisite for evolution
Mendel’s work on pea plants provided evidence of discrete
heritable units (genes)
Genetic variation makes evolution
possible
51.
Genetic Variation
Geneticvariation among individuals is caused by
differences in genes or other DNA segments
Phenotype is the product of inherited genotype and
environmental influences
Natural selection can only act on variation with a genetic
component
Variation Within aPopulation
Both discrete and quantitative characters contribute
to variation within a population
Discrete characters can be classified on an either-or
basis
Quantitative characters vary along a
continuum/continuous within a population
54.
Variation Within aPopulation
Discrete trait: a trait that has distinct values (presence or
absence), rather than a range of phenotypes, usually
encoded by one or a few genes. E.g. Sickle cell anameia,
ABO blood type, number of eggs in a bird clutch.
Quantitative (Continuous) trait: a trait that has a
continuum of phenotypes and is encoded by multiple genes.
E.g Body size, height, weight, intelligence (IQ), Running
speed, hair color, skin color, lifespan
55.
Genetic variationcan be measured as gene variability or
nucleotide variability
For gene variability, average heterozygosity measures
the average percent of loci that are heterozygous in a
population
Nucleotide variability is measured by comparing the
DNA sequences of pairs of individuals
56.
Variation Between Populations
Most species exhibit geographic variation, differences
between gene pools of separate populations
For example, Madeira is home to several isolated
populations of mice
Chromosomal variation among populations is due to
drift, not natural selection
Some examplesof geographic variation occur as a cline,
which is a graded change in a trait along a geographic axis
For example, mummichog fish vary in a cold-adaptive
allele along a temperature gradient
This variation results from natural selection
Sources of GeneticVariation
New genes and alleles can arise by mutation or gene
duplication
61.
Animation: Genetic Variationfrom Sexual Recombination
Genetic Variation from Sexual Recombination
Right-click slide / select “Play”
Right-click slide / select “Play”
62.
Formation of NewAlleles
A mutation is a change in nucleotide sequence of DNA
Only mutations in cells that produce gametes can be
passed to offspring
A point mutation is a change in one base in a gene
63.
The effectsof point mutations can vary:
Mutations in noncoding regions of DNA are often
harmless
Mutations to genes can be neutral because of
redundancy in the genetic code
Mutations that result in a change in protein
production are often harmful
Mutations that result in a change in protein production
can sometimes be beneficial
64.
Altering Gene Numberor Position
Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
Duplication of small pieces of DNA increases genome size
and is usually less harmful
Duplicated genes can take on new functions by further
mutation
An ancestral odor-detecting gene has been duplicated
many times: humans have 1,000 copies of the gene, mice
have 1,300
65.
Rapid Reproduction
Mutationrates are low in animals and plants
The average is about one mutation in every 100,000 genes
per generation
Mutation rates are often lower in prokaryotes and higher
in viruses
66.
Sexual Reproduction
Sexualreproduction can shuffle existing alleles into
new combinations
In organisms that reproduce sexually, recombination of
alleles is more important than mutation in producing
the genetic differences that make adaptation possible
67.
The Hardy-Weinberg equationcan be
used to test whether a population is
evolving
The first step in testing whether evolution is occurring in a
population is to clarify what we mean by a population
68.
Gene Pools andAllele Frequencies
A population is a localized group of individuals capable of
interbreeding and producing fertile offspring
A gene pool consists of all the alleles for all loci in a
population
A locus is fixed if all individuals in a population are
homozygous for the same allele
69.
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Beaufort Sea
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70.
The frequencyof an allele in a population can be
calculated
For diploid organisms, the total number of alleles at a
locus is the total number of individuals times 2
The total number of dominant alleles at a locus is 2
alleles for each homozygous dominant individual
plus 1 allele for each heterozygous individual; the
same logic applies for recessive alleles
71.
By convention,if there are 2 alleles at a locus, p and q
are used to represent their frequencies
The frequency of all alleles in a population will add up to
1
For example, p + q = 1
72.
For example,consider a population of wildflowers that is
incompletely dominant for color:
320 red flowers (CR
CR
)
160 pink flowers (CR
CW
)
20 white flowers (CW
CW
)
Calculate the number of copies of each allele:
CR
(320 2) 160 800
CW
(20 2) 160 200
73.
To calculatethe frequency of each allele:
p freq CR
800 / (800 200) 0.8
q freq CW
200 / (800 200) 0.2
The sum of alleles is always 1
0.8 0.2 1
74.
The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population
that is not evolving
If a population does not meet the criteria of the Hardy-
Weinberg principle, it can be concluded that the
population is evolving
75.
Hardy-Weinberg Equilibrium
TheHardy-Weinberg principle states that frequencies
of alleles and genotypes in a population remain constant
from generation to generation
In a given population where gametes contribute to the next
generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a
population
76.
Figure 23.7
Alleles inthe population
Gametes produced
Each egg: Each sperm:
80%
chance
20%
chance
80%
chance
20%
chance
Frequencies of alleles
p = frequency of
q = frequency of
CW
allele = 0.2
CR
allele = 0.8
77.
Hardy-Weinberg equilibriumdescribes the constant
frequency of alleles in such a gene pool
Consider, for example, the same population of 500
wildflowers and 1,000 alleles where
p freq CR
0.8
q freq CW
0.2
78.
The frequencyof genotypes can be calculated
CR
CR
p2
(0.8)2
0.64
CR
CW
2pq 2(0.8)(0.2) 0.32
CW
CW
q2
(0.2)2
0.04
The frequency of genotypes can be confirmed using a
Punnett square
79.
Punnett square
Auseful tool to do genetic crosses
For a monohybrid cross, you need a square divided by four….
Looks like
a window
pane…
We use the
Punnett square
to predict the
genotypes and phenotypes of
the offspring.
If pand q represent the relative frequencies of the only
two possible alleles in a population at a particular locus,
then
p2
2pq q2
1
where p2
and q2
represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
82.
Conditions for Hardy-Weinberg
Equilibrium
The Hardy-Weinberg theorem describes a hypothetical
population that is not evolving
In real populations, allele and genotype frequencies do change
over time
83.
The fiveconditions for nonevolving populations are
rarely met in nature:
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
84.
Natural populationscan evolve at some loci, while being
in Hardy-Weinberg equilibrium at other loci
85.
Applying the Hardy-WeinbergPrinciple
We can assume the locus that causes phenylketonuria (PKU) is in
Hardy-Weinberg equilibrium given that:
PKU is an inherited inability to metabolize phenylalanine which if
untreated causes brain and nerve damage
1. The PKU gene mutation rate is low
2. Mate selection is random with respect to whether
or not an individual is a carrier for the PKU allele
3. Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
4. The population is large
5. Migration has no effect as many other populations have
similar allele frequencies
86.
The occurrenceof PKU is 1 per 10,000 births
q2
0.0001
q 0.01
The frequency of normal alleles is
p 1 – q 1 – 0.01 0.99
The frequency of carriers is
2pq 2 0.99 0.01 0.0198
or approximately 2% of the U.S. population
87.
Three majorfactors alter allele frequencies and bring about
most evolutionary change:
Natural selection
Genetic drift
Gene flow
Natural selection, genetic drift, and gene flow
can alter allele frequencies in a population
88.
Natural Selection
Differentialsuccess in reproduction results in
certain alleles being passed to the next generation in
greater proportions
For example, an allele that confers resistance to DDT
increased in frequency after DDT was used widely in
agriculture
89.
Genetic Drift
Thesmaller a sample, the greater the chance of deviation
from a predicted result
Genetic drift describes how allele frequencies fluctuate
unpredictably from one generation to the next
Genetic drift tends to reduce genetic variation through
losses of alleles
The Founder Effect
The founder effect occurs when a few individuals
become isolated from a larger population
Allele frequencies in the small founder population can be
different from those in the larger parent population
95.
The Bottleneck Effect
The bottleneck effect is a sudden reduction in
population size due to a change in the environment
The resulting gene pool may no longer be reflective of
the original population’s gene pool
If the population remains small, it may be further affected
by genetic drift
Understanding thebottleneck effect can increase
understanding of how human activity affects other
species
100.
Case Study: Impactof Genetic Drift on
the Greater Prairie Chicken
Loss of prairie habitat caused a severe reduction in the
population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic variation,
and only 50% of their eggs hatched
