Hardy-Weinberg Equilibrium
-And how it affects evolution.
Hardy-Weinberg Principle
• Predicts how gene frequencies
(number of dominant and
recessive alleles) will be
transmitted from generation to
generation.
• The genes of a population are
known as the gene pool.
• The Hardy-Weinberg principle
states that there will be no
change in allele frequency of a
population over time. This is
called genetic equilibrium.
Hardy-Weinberg Equilibrium
• Genetic equilibrium will not
change over time• Frequencies in each
successive generation will
be the same as the original
parent generation.
Hardy-Weinberg Equilibrium
• There are five conditions that must be
met for Hardy Weinberg Equilibrium to
occur in a population (no changes in
allele frequencies):
–
–
–
–
–
large population
random mating
no mutations
no gene flow
no selection
Large Population
• A large population is less likely
to have random fluctuations
change allele frequencies than
smaller populations.
• Small populations are more
likely to experience genetic
drift- change in allele
frequency by chance
– Think of a coin toss. The more you toss
the coin, the greater your chances of
flipping 50% heads and 50% tails.
Random Mating
• Individuals must randomly
mate with other individuals
within a population.
• Non-random mating will
result in changes in allele
frequency.
Non-Random Mating
• Types of non-random
mating:
– Assortative mating- choosing
mates with similar phenotypes
– Competition- some males
compete for mates
Mutations
• Mutations- Spontaneous
changes in an allele.
• Increase variation in a
population when old
alleles change into new
alleles.
Gene Flow
• Gene flow- caused by
migration.
• New alleles are introduced
into a population when
new individuals move in.
• Alleles are removed from
the population when
individuals move out of a
population.
Selection
• Natural
selection
tends to
reduce
genetic
variability.
• -Three types:
– Stabilizing
– Disruptive
– Directional
Selection
•
•
•
Stabilizing selection
eliminates those
phenotypes most
different from the
norm, thus reducing
the frequency of
phenotypic extremes.
(AA, aa)
Directional selection
eliminates one extreme
and moves the
population toward the
other. (AA or aa)
Disruptive selection
eliminates average
phenotypes and
encourages the
extremes. (Aa) This
tends to result in
distinct phenotypes in
the same population.
Hardy-Weinberg Equilibrium
• For all populations to maintain
equilibrium (no changes in
allele frequencies over many
generations) all five conditions
must be met.
–
–
–
–
–
large population (no genetic drift)
random mating
no mutations
no gene flow
no selection
Hardy-Weinberg Equilibrium
• When one condition
changes, the gene pool or
allele frequency of the
population will change.
• These changes result in
different genotypes and
phenotypes (increased
variation) in the
population.
Hardy-Weinberg Equilibrium
• A population that is not in
Hardy-Weinberg equilibrium will
undergo natural selection
because of genetic variation.
• Natural selection will ultimately
favor the most fit individual(s) in
the population.
• This will result in changes in a
population over timeEVOLUTION OCCURS!
Hardy-Weinberg
Equilibrium
• How would a
mutation cause
changes in
allele frequency
in a population?
• How would this
allow evolution
to occur?
Hardy-Weinberg Equilibrium
Cheetah
• How would a
small
population
affect allele
frequencies
over several
generations?
• How would
this allow
evolution to
occur?
Hardy-Weinberg Equilibrium
• How would gene flow affect allele
frequencies in a population?
• How would this allow evolution to occur?
Hardy-Weinberg Equilibrium
• How would natural selection
change allele frequencies in a
population?
• How does this allow evolution to
occur?
Hardy-Weinberg Equilibrium
• How would
choosing a
mate
change
allele
frequencies
in a
population?
• How would
this allow
evolution to
occur?
Mechanisms for Change
• When populations are not
in Hardy-Weinberg
Equilibrium, evolution
occurs.
• The conditions that upset
Hardy-Weinberg
equilibrium are known as
the mechanisms of
change.
• Cause changes in alleles in
a population.
Mechanisms of Evolution
• Genetic Variation
• Descent and the genetic
differences that are heritable
and passed on to the next
generation;
• Mutation, migration (gene
flow), genetic drift, non-random
mating, and natural selection
as mechanisms for change;
• Result in changes over timeEvolution!