Hardy-Weinberg Equilibrium Conditions

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.

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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

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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

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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

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Quick Reference Table

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Self-Check Questions

1. Which two conditions both address the effects of random chance on allele frequencies, and how do they differ in what they control?

2. 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?

3. Compare and contrast how natural selection and mutation affect allele frequencies—which introduces new alleles, and which changes frequencies of existing alleles?

4. 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?

5. 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?