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 - percent 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
dominant
• 2pq = frequency of heterozygous
individuals
• q2 = frequency of individuals homozygous
recessive
4 simple rules for calculating equations:
1. Find what q2 and q are first
• They can be determined with the information given to you in a problem. The
q2 population are the homozygous recessive individuals. Once you solve for
q2, take the square root to find q.
2. Once you find what q is, you subtract that number from 1
• p + q = 1. That will give you p, or the number of dominant alleles in a
population
3. Square the p number to solve for p2
• This will give you the number of homozygous dominant individuals in a
population.
4. Now that we have p2, p, q2, and q, plug those numbers into the 2pq
• Check your work by plugging into each equation and see if they equal 1
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 =
98
200
q=
p=
p2 =
2pq =
= 0.49
p2 + 2pq + q2 = 1