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 occurs2. Immigration and emigration do not occur

(population is isolated from other populations) - no gene flow.

3. Population is very large4. Mating is totally random5. 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 =