AP Biology · Lesson 3 of 8 · Unit 7

Hardy–Weinberg equilibrium

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Hardy–Weinberg equilibrium provides the mathematical baseline for measuring microevolution in a population. On the AP Biology exam (Topic 7.5), you will calculate allele and genotype frequencies from phenotype data, test whether a population is evolving (sometimes paired with a chi-square test in a free-response question), and predict how violating specific equilibrium conditions alters genetic makeup.

By the end you’ll be able to

Describe the conditions for Hardy–Weinberg equilibrium: a large population, no migration, no new mutations, random mating and no natural selection.
Explain why Hardy–Weinberg equilibrium is a valuable null hypothesis even though its conditions are never met.
Calculate allele and genotype frequencies using $p + q = 1$ and $p^2 + 2pq + q^2 = 1$.
Determine whether a population is evolving by comparing observed genotype frequencies with Hardy–Weinberg expectations.

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What the exam asks

Calculate allele frequencies ( $p$ and $q$ ), genotype frequencies ( $p^2$ , $2pq$ , $q^2$ ), or the expected number of individuals of a given phenotype/genotype in a population.
Describe the five conditions required for a population to remain in Hardy–Weinberg equilibrium.
Explain how Hardy–Weinberg serves as a null hypothesis ( $H_0$ ) for evolution.
Determine whether a population is evolving by comparing observed counts or frequencies to Hardy–Weinberg expectations.
Predict and justify how an environmental change, migration event, or mating shift alters allele or genotype frequencies.

Core ideas

The five conditions for equilibrium

A population is in Hardy–Weinberg equilibrium (meaning its allele and genotype frequencies remain constant from generation to generation) only if it meets five strict conditions. If a condition is violated, specific evolutionary changes occur:

Condition for Equilibrium	Effect of Violating the Condition
1. Extremely large population size	Genetic drift (random changes in allele frequencies, such as bottleneck or founder effects) occurs in small populations, often reducing genetic diversity and fixing or losing alleles.
2. No migration (no gene flow)	Gene flow (immigration or emigration of fertile individuals or gametes) adds or removes alleles, directly changing allele frequencies ( $p$ and $q$ ).
3. No net mutations	Mutations introduce brand-new alleles or convert one allele into another, directly changing allele frequencies.
4. Random mating	Nonrandom mating (such as inbreeding or assortative mating) changes genotype frequencies ( $p^2$ , , ) without directly changing allele frequencies ( and ). Inbreeding increases homozygosity ( and ) and decreases heterozygosity ().

Hardy–Weinberg as a null hypothesis

No natural population ever meets all five conditions simultaneously. However, Hardy–Weinberg equilibrium is a powerful null hypothesis ( $H_0$ ): it models what a population’s genetic structure would look like if no evolution were occurring at a given locus. By comparing observed data from a real population against Hardy–Weinberg expected values (often using a chi-square test), biologists can detect whether evolutionary agents are acting on that gene and estimate the rate of change.