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Hardy Weinberg or
An Introduction to How
Microevolution Can Act on Alleles
in a Population
Questions for Discussion
What do Dominant and Recessive Mean?
Six Fingers is dominant…why do most of us
have 5?
Why do many Northern Europeans carry the
lethal recessive CF allele?
Are these alleles stable or is selection
Gene Pool
Gene pool is the total aggregate of genes
in a population at any one time.
Population is a localized group of
individuals belonging to the same species.
Species is a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring.*
Hardy-Weinberg Theorem
The frequencies of alleles and genotypes
in a population’s gene pool remain
constant unless acted on by a force other
than segregation and recombination. (eg.
Meiosis and random fertilization do not
affect a gene pool)
Equation: p2 + 2pq + q2 = 1
p2 + 2pq + q2 = 1
Represents: AA, A, aa
p + q =1 –
How many genotypes?
How many phenotypes?
This is known as the allele frequency.
Is the allele frequency the same at the
genotype frequency? Why or why not?
First step—always figure out WHAT you
know for certain!
Simple Problem:
Suppose Red is Dominant and White is
recessive in flowers.
If 96% of the flowers are red, what are the
frequencies of the individual genotypes in
the population.
p2 + 2pq + q2=1
To Start: What do we know for certain?
Allele frequencies remain in
equilibrium if
(Must be true to be IN
Very large population.
No migration.
No net mutations.
Random Mating.
No Natural Selection.
In Small Populations
Alleles can be selected for/against by
Genetic Drift:
Bottleneck Effect
Founder effect
Migration=Gene Flow
How does migration effect allele
Sexual Selection=Nonrandom mating
What does the data suggest about who
Drosophila females find “sexy”
Let’s use the equation to see if a
population is in equilibrium:
You study a population of frogs in a river
system in Colorado.
In the parent generation the allele frequencies
are p=.67 and q=.33;
You analyze the phenotypes tadpoles
produced and you note that: 11/100 are
homozygous recessive.
Is this gene at Hardy-Weinberg Equilibrium.
What can you infer about population size and
mating habits of the frogs in this population?
Long term data
You continue to look at the allele
frequencies in this population of frogs for
every year for 10 years.
At the end of 5 years, the allele
frequencies are p=.67 and q=.33
At the end of 10 years p=.5 and q=.5
What may have happened to this
Microevolution is…
Changing allele frequencies.
Our Model:
Goldfish eating Sharks!
How does natural selection alter allele
Over time…what can
microevolution do?
Which might lead to speciation
Which might lead reduced variability?