If a population is not in Hardy-Weinberg equilibrium, at least one of five required conditions has been violated: no mutation, no migration, no natural selection, no genetic drift (meaning the population must be large), and random mating. On a biology exam, the correct answer is whichever option describes one of these evolutionary forces acting on the population. Here’s how to identify it and understand why each force disrupts equilibrium.
What Hardy-Weinberg Equilibrium Actually Requires
The Hardy-Weinberg principle predicts that allele frequencies in a population will stay the same from generation to generation, but only if five strict conditions are all met simultaneously. In equation form, the genotype frequencies follow p² + 2pq + q² = 1, where p is the frequency of one allele and q is the frequency of the other. If the observed genotype numbers in a population don’t match what this equation predicts, something is pushing allele or genotype frequencies around.
The five conditions are:
- No mutation: No new alleles are being created or changed.
- No migration (gene flow): No individuals are entering or leaving the population.
- No natural selection: All genotypes survive and reproduce equally well.
- Large population size (no genetic drift): The population is large enough that random chance doesn’t shift allele frequencies.
- Random mating: Individuals don’t choose mates based on genotype.
No real population meets all five conditions perfectly. Hardy-Weinberg equilibrium is a theoretical baseline, like a frictionless surface in physics. Its value is in showing you which forces are acting on a population by revealing where reality departs from the prediction.
Natural Selection Changes Who Survives
Natural selection is the most common correct answer on these questions because it’s the most intuitive disruption. When one allele gives organisms a survival or reproductive advantage, carriers of that allele leave more offspring. Over generations, the helpful allele becomes more common and the harmful one becomes rarer. This directly changes allele frequencies, which breaks the equilibrium prediction.
A classic real-world example is the sickle cell allele in regions where malaria is common. In parts of Africa, about 40% of the population carries one copy of the sickle cell allele (heterozygous), while roughly 4% carry two copies and develop sickle cell disease. The allele frequency sits around 24%. People with one copy gain significant resistance to malaria, so natural selection favors them. But people with two copies face serious health consequences, so selection also works against the allele. This push and pull, called balanced polymorphism, maintains the allele at a stable but non-random frequency that wouldn’t exist without selection pressures.
Genetic Drift Hits Small Populations Hard
Genetic drift is the random fluctuation of allele frequencies due purely to chance. In a small population, a few individuals might happen to reproduce more than others for no reason related to fitness. Over time, this randomness can cause alleles to disappear entirely or become far more common than expected. The smaller the population, the stronger the effect.
Two specific scenarios amplify drift. A population bottleneck occurs when a population shrinks dramatically, such as after a natural disaster, and the survivors carry only a subset of the original genetic variation. A founder effect happens when a small group splits off to colonize a new area. That small group may carry allele frequencies very different from the original population simply by chance. Both situations reduce genetic variation and push allele frequencies away from equilibrium predictions.
Gene Flow Introduces or Removes Alleles
When individuals move between populations, they carry alleles with them. If immigrants have different allele frequencies than the resident population, their arrival shifts the overall frequencies. The bigger the difference in allele frequencies between the two populations, the larger the effect. Even occasional migration can prevent a population from reaching or staying in equilibrium. Gene flow also occurs when members of one population mate with members of another without physically relocating, as long as their gametes mix across population boundaries.
Mutation Creates New Variation
Mutation introduces entirely new alleles into a population. A single base-pair change in DNA can create a new version of a gene that didn’t previously exist. While mutation rates for any individual gene are typically low per generation, they represent a constant, slow source of new genetic material. On its own, mutation shifts allele frequencies only slightly over short timescales, but it provides the raw material that other forces like natural selection then act on.
Non-Random Mating Skews Genotype Ratios
When organisms choose mates based on particular traits (or genotypes), the resulting offspring don’t appear in the ratios Hardy-Weinberg predicts. This is called assortative mating. If individuals with similar genotypes preferentially mate with each other, the population ends up with more homozygotes (two identical alleles) and fewer heterozygotes (two different alleles) than expected. Inbreeding is an extreme form of this: closely related individuals share alleles, so their offspring are more likely to be homozygous.
Non-random mating is a subtle disruptor because it can change genotype frequencies without necessarily changing allele frequencies right away. Over time, though, the shift in genotype ratios creates conditions where selection and drift act differently than they would in a randomly mating population.
How to Identify the Right Answer
When you see this question on an exam, you’re typically given observed genotype counts that don’t match the expected p², 2pq, and q² values, along with several possible explanations. To find the correct answer, look for whichever option describes one of the five evolutionary forces listed above. Common distractors include statements that actually support equilibrium, like “the population is very large” (which resists drift) or “mating is random” (which maintains expected ratios).
A few practical tips for working through these problems:
- Too few heterozygotes: Often points to non-random mating, inbreeding, or population subdivision.
- Too many heterozygotes: Can suggest heterozygote advantage (a form of natural selection).
- Allele frequencies shifting between generations: Points to selection, drift, migration, or mutation.
- Small population size mentioned: Genetic drift is likely the answer.
- Organisms moving between groups: Gene flow or migration is the cause.
You can verify a deviation statistically by comparing observed and expected genotype counts using a chi-square test with 1 degree of freedom. If the resulting p-value falls below 0.05, the difference is large enough to reject the assumption of equilibrium. But on most biology exams, you’re asked to reason through the cause rather than calculate it.