Evolution
Hardy-Weinberg Equilibrium
Allele frequencies stay constant when no evolution — null hypothesis for population genetics
Hardy-Weinberg equilibrium describes a population where allele frequencies remain constant across generations — no evolution occurring. Predicted by G. H. Hardy and Wilhelm Weinberg (1908). Equation: p² + 2pq + q² = 1, where p and q are frequencies of two alleles; p² is homozygous dominant, 2pq heterozygous, q² homozygous recessive. Requirements: (1) Large population. (2) No mutation. (3) No migration. (4) No selection. (5) Random mating. If any violated: evolution occurs. Used as null hypothesis: if observed frequencies differ from H-W expected, evolution must be happening. Foundation of population genetics.
- Equationp² + 2pq + q² = 1
- pFrequency of dominant allele
- qFrequency of recessive allele
- DiscoveredHardy and Weinberg, 1908 (independent)
- Five conditionsLarge population, no mutation, no migration, no selection, random mating
- UseNull hypothesis for population genetics
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Why H-W matters
- Population genetics. Foundation principle.
- Disease estimation. Calculate carrier frequencies.
- Selection detection. Find genes under selection.
- Conservation biology. Population health indicators.
- Forensics. DNA profiling probabilities.
- Anthropology. Population history.
- Evolution. Test for evolutionary change.
Common misconceptions
- H-W is always true. Idealized; rarely exactly met.
- Most populations at H-W. Most show deviations.
- H-W means no evolution. Means no evolution at that locus, currently.
- p + q always = 1. Yes if only 2 alleles.
- Used only for diseases. Many applications.
- Deviations always selection. Could be drift, migration, etc.
Frequently asked questions
What's the Hardy-Weinberg equation?
For two alleles A and a with frequencies p and q (p + q = 1). Genotype frequencies: AA = p², Aa = 2pq, aa = q². Sum: p² + 2pq + q² = 1. Predicts genotype frequencies from allele frequencies. Constant across generations if no evolution. Used to: detect deviations indicating selection; calculate carrier frequency for diseases.
What are the five conditions?
For H-W to hold. (1) Large population: prevents drift. (2) No mutation: allele frequencies stable. (3) No migration: no gene flow. (4) No selection: all genotypes equally fit. (5) Random mating: no preference. Real populations rarely meet all; H-W is idealized. Deviations indicate which factor is operating — what evolutionary force is at work.
How is H-W used in genetics?
Common applications. (1) Estimate carrier frequency: cystic fibrosis (q² = 1/2500 in some populations); q = 1/50; carriers (2pq) ≈ 1/25. (2) Test for selection: deviations may indicate selection on certain genotypes. (3) Population size estimation: low heterozygosity may indicate recent bottleneck. (4) Random mating test: compare observed vs expected genotype frequencies.
How is H-W broken?
When any condition violated. (1) Small population: random fluctuations (genetic drift). (2) Mutation: introduces new alleles. (3) Migration (gene flow): adds/removes alleles. (4) Selection: differential reproduction. (5) Non-random mating: assortative (similar mates) or disassortative. Each cause type of evolution. Tracking deviations reveals what's happening.
What about more than two alleles?
Extended formula. For three alleles A1, A2, A3 with frequencies p, q, r (sum to 1): genotype frequencies (p+q+r)² = p² + q² + r² + 2pq + 2pr + 2qr. Each homozygote: f². Each heterozygote: 2 × f1 × f2. ABO blood: alleles I^A, I^B, I^O. Frequencies vary by population.
What is genetic drift?
Random changes in allele frequencies. Most important in small populations. Each generation: who reproduces is partly chance. Some alleles increase by chance; others decrease. Possible: alleles become fixed (frequency 1) or lost (frequency 0). Stronger effect in: small populations, neutral mutations. With selection: deterministic (large pop) or stochastic (small pop).
What's the founder effect?
Special case of genetic drift. Small group founds new population. Random sample of original gene pool. Allele frequencies different from source. Can lose rare alleles. Examples: Amish populations (high frequency of certain genetic disorders due to founders), Galapagos finches. Important in: speciation, population genetics, conservation.