In the Hardy–Weinberg model, two key conditions—no mutation and random mating—help keep allele frequencies stable and make genotype frequencies predictable. Understanding what each assumption means clarifies when the model can (and cannot) describe real populations.

No mutation (why it matters)

What the assumption means

The Hardy–Weinberg model assumes no new mutations arise in the gene pool and that existing alleles do not change into other alleles. If mutation occurred at meaningful rates, it would create new alleles or convert one allele into another, shifting allele frequencies across generations.

How mutation would disrupt equilibrium

Even though many mutations are rare, their effects accumulate over time, especially in large populations. Mutation violates Hardy–Weinberg because it:

• Introduces new alleles (e.g., $A \rightarrow a$ creates additional $a$ alleles)

• Changes existing allele frequencies directly, before any selection occurs

• Can interact with other forces (e.g., drift), accelerating divergence between populations

Important nuance for AP Biology

Hardy–Weinberg’s “no mutation” condition is best treated as “mutation is negligible over the time frame studied.” Over short intervals, mutation may be too rare to detect via allele-frequency measurements, so the model can still function as an approximate null model.

Random mating (why it matters)

What the assumption means

The model also assumes random mating among individuals in the population, meaning mates are chosen without regard to genotype or phenotype for the gene in question.

Link to predictable genotype frequencies

Random mating allows allele frequencies ($p$ and $q$) to combine like probabilities, producing stable, predictable genotype proportions after one generation of random mating.

This graph shows Hardy–Weinberg genotype frequencies as allele frequency changes: homozygotes occur at $p^2$ and $q^2$, while heterozygotes occur at $2pq$. The heterozygote curve peaks when $p=q=0.5$, illustrating why heterozygosity is maximized at intermediate allele frequencies. It visually reinforces that predictable genotype proportions come from probability-based random union of gametes. Source

This relationship depends on the assumption that gametes unite at random. If mating is not random, the genotype frequencies may deviate from $p^2$, $2pq$, and $q^2$.

What violates random mating (and what changes)

Common violations include:

• Assortative mating (choosing similar phenotypes)

• Disassortative mating (choosing different phenotypes)

• Inbreeding (mating among relatives)

• Sexual selection where mating success differs by phenotype

A key conceptual point: non-random mating primarily changes genotype frequencies (often increasing homozygosity under inbreeding), and may not immediately change allele frequencies by itself.

This figure tracks percent homozygosity across generations under different mating systems, showing that self-fertilization (an extreme form of inbreeding) drives homozygosity upward most rapidly. The slower curves for full-sib and half-sib mating illustrate that the strength of inbreeding determines how quickly heterozygosity is lost. It provides a concrete visual link between non-random mating and increased homozygote frequencies. Source

However, once genotype frequencies shift, other factors (like differential reproduction) can more easily lead to allele-frequency change.

Why the assumption is still useful

Random mating is rarely perfectly true, but it provides a clean baseline. If observed genotype frequencies differ from Hardy–Weinberg expectations, one possible explanation is non-random mating at that locus (as opposed to mutation, selection, migration, or drift).