🧬AP Biology

Hardy-Weinberg Equilibrium Conditions

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Why This Matters

Hardy-Weinberg Equilibrium is the null hypothesis of population genetics—it describes what happens when a population isn't evolving. You're being tested on your ability to recognize that evolution occurs when any one of these conditions is violated. The five conditions (large population, random mating, no selection, no migration, no mutation) aren't just a list to memorize; they represent the mechanisms of evolutionary change working in reverse. When you understand why each condition matters, you can predict how populations will change when real-world pressures kick in.

This topic connects directly to Unit 7's big ideas about natural selection, genetic drift, and gene flow. On the AP exam, you'll use Hardy-Weinberg equations ( $p + q = 1$ and $p^2 + 2pq + q^2 = 1$ ) to calculate expected genotype frequencies, then compare them to observed data to determine if evolution is occurring. Don't just memorize the conditions—know what evolutionary force each one prevents and what happens to allele frequencies when that condition fails.

Conditions That Prevent Random Chance Effects

Genetic drift occurs when random sampling error changes allele frequencies, and these conditions eliminate that randomness.

Large Population Size

Minimizes genetic drift—in large populations, random events affecting a few individuals won't significantly shift overall allele frequencies
Maintains heterozygosity over generations because rare alleles are less likely to be lost by chance alone
Threshold concept: effective population size ( $N_e$ ) matters more than census size; a population of 10,000 with only 100 breeding individuals behaves like a small population

No Genetic Drift

Eliminates stochastic changes in allele frequencies that occur due to random sampling of gametes each generation
Most critical in small populations—the smaller the population, the larger the potential swing in allele frequencies from generation to generation
Connects to bottlenecks and founder effects—both are special cases of drift that violate this condition and can rapidly change allele frequencies

Compare: Large population size vs. No genetic drift—both address random fluctuations, but large population size is the practical condition that minimizes drift, while "no drift" is the theoretical ideal. On FRQs, explain that drift still occurs in large populations, just with negligible effect.

Conditions That Prevent Directed Change

Natural selection and mutation actively push allele frequencies in particular directions; these conditions remove those directional forces.

No Natural Selection

All genotypes have equal fitness—no phenotype provides a survival or reproductive advantage in the current environment
Prevents directional, stabilizing, or disruptive selection from shifting allele frequencies toward favored variants
Real-world violation: when selection coefficients differ from zero, alleles conferring advantages increase in frequency across generations

No Mutations

Prevents introduction of new alleles into the gene pool, keeping the set of alleles constant
Mutation rates are typically low ( $10^{-5}$ to $10^{-9}$ per gene per generation), so this condition is approximately met in short-term studies
Long-term significance: over evolutionary time, mutation is the ultimate source of all genetic variation—without it, there's no raw material for selection

Compare: No natural selection vs. No mutations—selection sorts existing variation while mutation creates new variation. An FRQ might ask which force introduces novel alleles (mutation) versus which changes frequencies of existing alleles (selection).

Conditions That Prevent Gene Pool Mixing

These conditions keep the population genetically isolated and ensure alleles are distributed predictably.

No Migration (Gene Flow)

Prevents allele exchange between populations, so the focal population's allele frequencies aren't altered by immigrants or emigrants
Gene flow homogenizes populations—when it occurs, distinct populations become more genetically similar over time
Violation example: if individuals carrying allele $A$ immigrate into a population where $A$ is rare, the frequency of $A$ increases regardless of selection

Random Mating

All individuals equally likely to mate with any other individual, regardless of genotype or phenotype
Prevents assortative mating (like with like) or inbreeding, both of which alter genotype frequencies without changing allele frequencies
Key distinction: non-random mating changes genotype ratios (increasing homozygosity) but doesn't directly change allele frequencies—this is a common exam trap

Compare: No migration vs. Random mating—migration changes allele frequencies by adding or removing alleles, while non-random mating changes genotype frequencies while keeping allele frequencies constant. If you see excess homozygotes but unchanged allele frequencies, suspect non-random mating, not gene flow.

Quick Reference Table

Concept	Conditions That Address It
Preventing random fluctuations	Large population size, No genetic drift
Preventing directional allele change	No natural selection, No mutations
Preventing external gene pool mixing	No migration (gene flow)
Ensuring predictable genotype ratios	Random mating
Maintaining constant allele frequencies	All five conditions together
Most commonly violated in nature	Large population size (bottlenecks), No selection
Changes genotype but not allele frequency	Random mating (when violated)
Source of all new genetic variation	No mutations (when violated)

Self-Check Questions

Which two conditions both address the effects of random chance on allele frequencies, and how do they differ in what they control?
A population shows more homozygotes than Hardy-Weinberg predicts, but allele frequencies match expected values. Which condition is most likely violated, and why doesn't this change allele frequencies?
Compare and contrast how natural selection and mutation affect allele frequencies—which introduces new alleles, and which changes frequencies of existing alleles?
If an FRQ describes a small island population founded by 10 individuals from a mainland population, which Hardy-Weinberg conditions are violated, and what evolutionary mechanism does this represent?
A researcher calculates expected genotype frequencies using $p^2 + 2pq + q^2 = 1$ and finds they differ significantly from observed frequencies. What can she conclude about the population, and what should her next step be to identify which condition is violated?