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 =