Hardy-Weinberg Equilibrium

Tracking Microevolution in Populations

So, how do we know if a population is evolving?

The Hardy-Weinberg equation can be used to test whether a population is evolving◦The Hardy-Weinberg principle states that

allele and genotype frequencies within a sexually reproducing, diploid population will remain in equilibrium unless outside forces act to change those frequencies

◦This state of non-evolution is known as Hardy Weinberg Equilibrium

For Hardy-Weinberg Equilibrium to exist, five conditions must be met:◦Very large population (to prevent what?)◦No gene flow between populations◦No mutations◦Random mating◦No natural selection

Is this likely, or even possible?

◦However, it is convenient to use the Hardy-Weinberg equation to take a snapshot of the allele and genotype frequencies in a population

◦Doing this over multiple generations can show if a population is evolving

The Hardy-Weinberg Equation

◦ Imagine that there are two alleles in a blue-footed booby population: W and w

W is a dominant allele for a nonwebbed booby foot

w is a recessive allele for a webbed booby foot

The Hardy-Weinberg Equation

◦ Consider the gene pool of a population of 500 boobies

320 (64%) are homozygous dominant (WW) 160 (32%) are heterozygous (Ww) 20 (4%) are homozygous recessive (ww)

Equation for allele frequency in a Hardy-Weinberg population – (KEY: p = Dominant allele and q = recessive allele)

p + q = 1

Frequency of dominant allele (W) = 80% = p◦ 80% of alleles in the booby population are W

Frequency of recessive allele (w) = 20% = q◦ 20% of alleles in the booby population are w◦ Frequency of alleles must be 100% or 1.0

Equation for genotype frequency in a Hardy-Weinberg population – (KEY: p2 = homozygous dominant (WW), 2pq = heterozygous (Ww),

and q2 = homozygous recessive (ww)

p2 + 2pq + q2 = 100% = 1.0

Frequency of all three genotypes must be 100% or 1.0 homozygous dominant + heterozygous + homozygous recessive =

100%

If we use these genotype frequencies in a Punnett square, we can predict that the next generation will have the same frequencies as the one before it

The Hardy Weinberg equation is useful in public health science

◦It is often used to determine the frequency of alleles for certain inherited diseases, such as cystic fibrosis or phenylketonuria, or PKU

◦These can be determined by the occurrence per number of births