Natural populations evolve, but the Hardy–Weinberg model provides a baseline for what genetic stability would look like without evolution. AP Biology uses it to predict expected genotype patterns and test for evolutionary change.

The Hardy–Weinberg model (what it is for)

The model treats evolution operationally as a change in allele frequencies over time. If a population meets the model’s assumptions, its gene pool is stable, and the genotype distribution follows predictable proportions.

Hardy–Weinberg is especially valuable as a null model: it sets the “no evolutionary forces acting” expectation so that real data can be compared to it.

What the model tracks

For a single gene with two alleles (often written A and a), the model uses allele frequencies to predict genotype frequencies (AA, Aa, aa) under equilibrium conditions.

What the model predicts

Under Hardy–Weinberg equilibrium, the two allele frequencies fully determine the expected genotype frequencies in zygotes (and, if assumptions hold, in the whole population).

These relationships come from random union of gametes: if $p$ is the chance a gamete carries $A$ and $q$ is the chance it carries $a$, then offspring genotype probabilities follow basic probability multiplication.

A worked Hardy–Weinberg example that connects allele frequencies ($p$ and $q$) to expected genotype frequencies ($p^2$, $2pq$, $q^2$) in a population. The diagram shows how predicted genotype proportions match the parent generation when the population is at equilibrium, reinforcing that the model is a baseline for “no evolutionary forces acting.” Source

How predictions are used (conceptually)

• Measure allele or genotype data from a population sample.

• Use Hardy–Weinberg to generate expected genotype frequencies from allele frequencies (or infer allele frequencies from genotype data).

• Compare observed vs expected patterns to evaluate whether the population is consistent with “non-evolving” expectations at that locus.

Core assumptions (the “ideal, non-evolving population” conditions)

Hardy–Weinberg equilibrium only holds when no processes act to change allele frequencies, and when mating patterns do not distort genotype frequencies. The standard assumptions are:

• Very large population size (minimises chance fluctuations)

A probability distribution showing how allele frequency can change from one generation to the next purely by chance (genetic drift) in a small population. The wide spread of possible outcomes illustrates why small populations experience stronger random fluctuations, making Hardy–Weinberg expectations less reliable even without selection, mutation, or migration. Source

• Random mating with respect to the gene

• No mutation creating new alleles

• No migration (gene flow) adding or removing alleles

• No natural selection favouring any genotype

In AP Biology, these assumptions define the boundary conditions for applying the model as a baseline. If one or more assumptions are violated, the model may not fit the data.

Interpreting departures from Hardy–Weinberg

A mismatch between expected and observed genotype frequencies is evidence that at least one assumption is not met. Importantly:

• Deviations diagnose that “something is happening,” but they do not by themselves identify which specific evolutionary process is responsible.

• Genotype frequencies can shift even when allele frequencies do not (for example, non-random mating can alter genotype proportions without immediately changing $p$ and $q$).

Common pitfalls and precision points

• Equilibrium is locus-specific: a population might match Hardy–Weinberg at one gene but not another.

• Equations predict expected proportions, not exact counts: real samples include sampling noise; interpretation depends on sample size and study design.

• Alleles vs genotypes: selection and other forces act through phenotypes and genotypes, but evolution is defined by changes in allele frequencies.

• Two-allele simplification: the classic $p/q$ form assumes two alleles; the general logic extends to more alleles, but the equation expands accordingly.