How can we measure whether a population is evolving?
Use the Hardy-Weinberg principle to calculate allele and genotype frequencies and test for change
A focused answer to the WACE Year 12 Biology dot point on the Hardy-Weinberg principle. Covers the equations, the equilibrium conditions, worked allele and genotype frequency calculations, and how deviation from equilibrium indicates evolution.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
What this dot point is asking
SCSA wants you to apply the two Hardy-Weinberg equations to calculate frequencies, state the conditions for equilibrium, and interpret a deviation from equilibrium as evidence of evolution. A strong answer defines each symbol and shows the working.
The principle and its purpose
The Hardy-Weinberg principle provides a null model: it predicts what allele and genotype frequencies should be if no evolutionary forces are acting. It is useful precisely as a baseline, because real populations rarely meet all the conditions, so comparing observed frequencies with the predicted ones reveals whether and how a population is changing.
The two equations
For a gene with two alleles:
- Let p be the frequency of the dominant allele and q be the frequency of the recessive allele. Then p plus q equals 1.
- The genotype frequencies are given by p squared plus 2pq plus q squared equals 1, where p squared is the frequency of homozygous dominant individuals, 2pq is the frequency of heterozygotes, and q squared is the frequency of homozygous recessive individuals.
The conditions for equilibrium
A population stays at Hardy-Weinberg equilibrium only if five conditions hold:
- no mutation,
- no gene flow (no migration in or out),
- no natural selection,
- random mating,
- a very large population (no genetic drift).
If any condition is broken, allele frequencies can change and the population evolves.
Worked calculation
Interpreting deviation as evolution
The power of the principle is in the comparison. If the observed genotype frequencies match p squared, 2pq and q squared, the population is at equilibrium and is not evolving for that gene. If the observed frequencies differ significantly, then one of the five conditions has been broken, for example selection against the recessive phenotype or gene flow from another population, and the population is evolving.
A practical caution
The recessive allele frequency must be found from the homozygous recessive group (q squared), because that is the only genotype you can identify by phenotype alone under simple dominance. You cannot tell homozygous dominant from heterozygous individuals just by looking, which is exactly why the equation is needed to estimate carrier numbers.
Why this matters for continuity
Hardy-Weinberg turns evolution into something measurable. By calculating expected frequencies and comparing them with reality, biologists can detect selection, migration or drift in a real population, estimate the hidden number of carriers of a genetic condition, and quantify how fast a gene pool is changing. It connects the abstract idea of evolution to numbers that can be tested.