How do we know if a
population is evolving?
Hardy Weinberg Equilibrium
When is a population not evolving?
• How do we know if a gene pool has changed?
• The Hardy-Weinberg Principle can help answer these
questions.
How can a population’s genes
change over time?
• All of the alleles of a population’s genes together make up a gene
pool.
• Allele frequency - % of any specific allele in the gene pool.
• Genetic equilibrium – a population in which the frequency of
alleles remains the same over generations.
The Hardy-Weinberg Principle states:
• Genetic equilibrium will be reached if the frequency
of alleles remains stable generation after generation.
• Genetic equilibrium = no evolution occurring.
Hardy-Weinberg Principle
• Conditions necessary for genetic equilibrium are:
1. No mutation occurs
2. Immigration and emigration do not occur
(population is isolated from other populations) no gene flow.
3. Population is very large
4. Mating is totally random
5. All individuals survive and reproduce equally (no
natural selection)
Hardy-Weinberg Principle
• It is virtually impossible to meet these conditions.
• Allelic frequencies do change in populations, therefore
evolution occurs.
• The main application of this principle is calculating allele and
genotype frequencies in a population.
• In a population, the sum frequency of alleles will equal 1.
• This can be expressed as:
•p+q=1
• Where:
• p = frequency of the dominant allele
• q = frequency of recessive allele
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
• Where:
• p2 = frequency of individuals homozygous
for the dominant allele
• 2pq = frequency of heterozygous individuals
• q2 = frequency of individuals homozygous for
the recessive allele
Question
• If 98 out of 200 individuals in a population express the recessive
phenotype, what percent of the population are homozygous
dominant? (Recessive phenotype = homozygous genotype)
P+Q=1
Q2 =
Q=
P=
P2 =
2PQ =
P2 + 2PQ + Q2 = 1