Hardy-Weinberg
Equilibrium
Tracking Microevolution in Populations
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So, how do we know if a population is
evolving?
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The Hardy-Weinberg equation can be used
to test whether a population is evolving
◦ The Hardy-Weinberg principle states that
allele and genotype frequencies within a
sexually reproducing, diploid population will
remain in equilibrium unless outside forces act
to change those frequencies
◦ This state of non-evolution is known as Hardy
Weinberg Equilibrium

For Hardy-Weinberg Equilibrium to exist,
five conditions must be met:
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Very large population (to prevent what?)
No gene flow between populations
No mutations
Random mating
No natural selection
Is this likely, or even possible?
◦ However, it is convenient to use the HardyWeinberg equation to take a snapshot of the
allele and genotype frequencies in a population
◦ Doing this over multiple generations can show
if a population is evolving
The Hardy-Weinberg Equation
◦ Imagine that there
are two alleles in a
blue-footed booby
population: W and w
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W is a dominant
allele for a
nonwebbed booby
foot
w is a recessive allele
for a webbed booby
foot
◦ Consider the gene pool of a population of
500 boobies
 320 (64%) are homozygous dominant (WW)
 160 (32%) are heterozygous (Ww)
 20 (4%) are homozygous recessive (ww)
The Hardy-Weinberg Equation
Equation for allele frequency in a Hardy-Weinberg population
– (KEY: p = Dominant allele and q = recessive allele)
p+q=1

Frequency of dominant allele (W) = 80% = p

Frequency of recessive allele (w) = 20% = q
◦ 80% of alleles in the booby population are W
◦ 20% of alleles in the booby population are w
◦ Frequency of alleles must be 100% or 1.0
Equation for genotype frequency in a Hardy-Weinberg
population – (KEY: p2 = homozygous dominant (WW), 2pq =
heterozygous (Ww), and q2 = homozygous recessive (ww)
p2 + 2pq + q2 = 100% = 1.0

Frequency of all three genotypes must be 100% or 1.0
 homozygous dominant + heterozygous + homozygous recessive
= 100%

If we use these genotype frequencies in a Punnett
square, we can predict that the next generation will
have the same frequencies as the one before it

The Hardy Weinberg equation is useful in
public health science
◦ It is often used to determine the frequency of
alleles for certain inherited diseases, such as
cystic fibrosis or phenylketonuria, or PKU
◦ These can be determined by the occurrence per
number of births