Transcript Hardy Weinberg Equilibrium - Salisbury Composite High School

Hardy Weinberg Equilibrium
p2 + 2pq + q2 = 1
Two scientists independently derived the basic principle of
population genetics called the Hardy – Weinberg Principle. This
principle states that:
If all factors remain constant, the gene pool in a
population will have exactly the same
composition generation after generation. This
condition is called genetic equilibrium.
If the genetic equilibrium of a population is upset, the
population is said to be evolving.
Evolution
 "the sum total of the
genetically inherited
changes in the individuals
who are the members of a
population's gene pool."
 Evolution is simply a
change in frequencies of
alleles in the gene pool of
a population.
Population
 A group of the same species living in the
same place at the same time
Gene pool
all of the genes / alleles
that occur in a
population.
Ex) human gene pool for
blood type are IA, IB,
and i.
Allele frequency
– % or proportion of
that allele in the
population
Conditions
 Evolution will NOT occur and Hardy-
Weinberg equilibrium will be met if the
following conditions are met:
Conditions
1. No Mutation
Conditions
2. The population is
infinitely large
- laws of probability
must apply
Conditions
3. All members of the
population breed
Conditions
4. All mating is totally
random
Conditions
5. Everyone produces
the same number of
offspring
Conditions
6. There is no
migration in or out of
the population
Equation
 Equation used to find genotype frequencies:
p² + 2pq + q² = 1
And
p + q = 1
p is the frequency of the dominant allele
q is the frequency of the recessive allele
p2 is the frequency of the homozygous
dominant genotypes
q2 is the frequency of the homozygous
recessive genotypes
Equation
2pq is the frequency of the heterozygotes
Example
 Albinism is only expressed in the
phenotype of homozygous recessive
individuals (aa).
 The average human frequency of
albinism in North America is only
about 1 in 20,000.
Question
 Calculate the frequencies of the alleles and
all three genotypes in this population.
Solution
 Synthetic Theory of Evolution: Sample Hardy-
Weinberg Problem
Examples:
1. In a population, 21% of the individuals are homozygous
dominant, 49% are heterozygous and 30% are
homozygous recessive. What percentage of the next
generation are predicted to be homozygous recessive?
2. 16% of a population is observed to have a continuous
hairline (recessive). What percentage of the
population possesses the dominant allele? If there are
500 members in the population, how many would be
heterozygous?
3. A recessive genetic disorder occurs in 9% of the
population. What percentage of the population will be
carriers for the disorder? What percentage will be
homozygous dominant?
Quiz – Theoretical Ideas
 http://w3.dwm.ks.edu.tw/bio/activelearner/18/
ch18summary.html
Disturbances to Equilibrium
There are some situations that may make H-W
equilibrium of alleles more likely to change:
1) Mutations
Whether a mutation is good or bad, often depends on the
environment. A harmful mutation can turn out to
have a selective advantage if the environment
changes over time.
2) Non-random Mating

Individuals are often attracted to one another because
they value specific traits. Ex. In humans, wolves, elk
3) Inbreeding
-will reduce genetic diversity, thus decrease
frequency of some alleles
4) Genetic Drift
- a reduction in the gene pool variation caused purely by
chance. Usually in small populations. If a specific allele
doesn’t reproduce (by chance) it may be lost entirely.
Genetic Drift Example
5 ) Gene Flow
Migration – is the movement of genes into (immigration) /
out of (emigration) the population. Some genes may
migrate more readily than others.
6 ) Bottleneck Effect
 occurs when a part
of the population is
eliminated by
chance.
7) Founder Effect
- occurs when the founders of a new population have a
specific genotype. Ex. polydactyl hands in Amish in
Pennsylvania.
8) Natural Selection
Selective Advantage: the most important reason
for changes to H-W equilibrium
New mutations may arise that give the organism
an advantage over others of the same species
 These alleles become more common with
time
 Means that some alleles are helping
individuals to survive and reproduce
I. Stabilizing Selection:
 atypical phenotypes are eliminated, and an average is
favored. Ex. birth weight or color.
II. Directional Selection
– an atypical phenotype is selected for because of a
progression of change in the environment. Ex. horse
evolution, peppered moth.
III. Disruptive Selection
 two or more phenotypes are selected due to different
characteristics within a habitat. Ex. fish that feed on
bottom vs fish that feed on top.
Speciation
 Divergence producing new species,
two types:
1.Allopatric speciation: physical
separation of species drives the
splitting of one species into two (or
more)
Eg. Grand Canyon Squirrels
Darwin’s Finches
 May not be
immediately obvious
Eg. Anole lizards
in Cuba – not
physically
separated now,
but were 5 million
years ago
Allopatry Animation

2. Sympatry
 Division of one species into two or more in
absence of physical barriers
 Disputed by some
H-W Equilibrium - Summary
 Does not change unless a force is acting upon
it
 This force is often natural selection – leads to
evolution