AP® Biology · Unit 7 · Topic 7.5
The Hardy-Weinberg Model and Its Conditions
The Hardy-Weinberg model predicts what allele and genotype frequencies would look like if a population were not evolving, giving biologists a baseline to compare against real populations to detect evolutionary change.
No real population sits still genetically. So why build a model of one that never evolves? Because a model of non-evolution is a null hypothesis: a precise prediction of what allele and genotype frequencies would look like if nothing were acting on them. Calculate that prediction, compare it to what you actually measure in a real population, and any mismatch tells you something is going on, selection, drift, migration, mutation, or nonrandom mating. You cannot detect a force by looking at a population alone; you need a still baseline to measure the shift against.
The baseline holds only if five conditions are met: no mutation (no new alleles entering the gene pool), random mating (no mate preference skewing genotype combinations), no gene flow (no migration bringing in or removing alleles), infinite population size (no random drift from chance sampling in small groups), and no natural selection (no allele gives a survival or reproductive advantage). Break any one, and frequencies can shift from one generation to the next. Under these conditions, one generation of random mating is enough: starting from any set of genotype frequencies, the next generation reaches p2, 2pq, q2 for the genotypes and stays there as long as the conditions hold.
This connects directly to the rest of the unit: once you can calculate the equilibrium expectation, you can ask which force is pulling a population away from it, and that force is exactly what drives allele frequency change, the raw material for divergence and eventually speciation.
Key terms
- Allele frequency
- The proportion of all copies of a gene in a population that are a particular allele.
- Null hypothesis
- A baseline prediction assuming no effect, used for comparison to detect whether something is actually happening.
- Genetic drift
- Random change in allele frequencies due to chance, with larger effects in smaller populations.
- Gene flow
- The movement of alleles between populations through migration of individuals or gametes.
- p2 + 2pq + q2 = 1
- The Hardy-Weinberg equation giving expected genotype frequencies from allele frequencies p and q.
Watch it explained
- How it worksWalk me through the five conditions and what breaks when each one fails.
- How it worksHow does a null hypothesis let you detect that something is acting on a population?
- Going furtherWhat happens to genotype frequencies after one generation of random mating from any start?
- Common mistakeWhy isn't a population in Hardy-Weinberg equilibrium simply one that is not changing at all?
Check yourself
End of topic.