Post on 12-Jan-2016
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 Equationp2 + 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 p2 + 2pq + q2 = 1
q2 = = 0.49
q =
p =
p2 =
2pq =