Hardy-Weinberg only holds when its conditions do
- The idea
- Predict
- See it
- Why
- Check
The idea
“Hardy-Weinberg says allele frequencies stay the same, so I can apply it to any population.”
Hardy-Weinberg arrives as equations, and equations feel true everywhere. The math is fine. The prediction that nothing changes only fits a population where nothing is acting.
The short version
The mistake: Hardy-Weinberg says allele frequencies stay the same, so I can apply it to any population.
What's actually true: Hardy-Weinberg predictions hold only with no selection, migration or mutation, random mating and a large population.
- Hardy-Weinberg describes a population that isn't evolving. It needs no selection, migration or mutation, random mating and a very large size.
- Break a condition and the prediction breaks. Selection and gene flow push allele frequencies. In a small population, genetic drift, change by chance, moves them.
- Nonrandom mating works differently. On its own, selfing or picking similar mates cuts heterozygotes, while allele frequencies stay put.
- So treat the model as a baseline. If allele frequencies change, or genotype frequencies clearly differ from p^2, 2pq and q^2, find the condition that is failing.
On the exam: Hardy-Weinberg is what happens when nothing acts on a population. Check every condition before you trust it.