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D4.1.14 (HL)—Hardy-Weinberg conditions

Hardy-Weinberg equilibrium requires no selection, mutation, migration, genetic drift or non-random mating in a large population in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.14
Level
HL

Exam analysis

Chance of appearing6%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–2

Common command terms

  • State
  • Identify

Recent exam appearances

May 2025Paper1A ["HL"] · TZ339[ 1 ]D4.1.14 (HL)—Hardy-Weinberg conditions
May 2015Paper3 ["HL"] · TZ12(c)(i)[ 2 ]D4.1.14 (HL)—Hardy-Weinberg conditions
May 2014Paper3 ["HL"] · TZ12(b)(ii)[ 1 ]D4.1.14 (HL)—Hardy-Weinberg conditions
November 2013Paper3 ["HL"] · TZ02(d)[ 1 ]D4.1.14 (HL)—Hardy-Weinberg conditions
May 2013Paper3 ["HL"] · TZ1D2(a)[ 2 ]D4.1.14 (HL)—Hardy-Weinberg conditions

Coverage 2012–2025 · Updated 16 Jul 2026

Hardy–Weinberg Equilibrium Requires Forces of Change to Be Absent

HL only
Condition Change prevented in the model
very large population random genetic drift is negligible
random mating genotype pairing is not biased
no selection genotypes contribute equally
no mutation alleles do not convert into new states
no migration no alleles enter or leave through gene flow

Under these conditions, random mating produces p²:2pq:q² genotype frequencies and allele frequencies remain stable across generations.

The model is an ideal baseline, not a claim that real populations satisfy every condition perfectly.

Departure from Hardy–Weinberg Identifies a Question, Not a Single Cause

HL only
  1. Estimate allele frequencies from the population sample.
  2. Use p², 2pq and q² to calculate expected genotype frequencies and counts.
  3. Compare expected with observed genotype counts using an appropriate statistical test.
  4. If the departure exceeds sampling expectation, investigate which assumptions are violated.
Possible violation Additional evidence to seek
selection survival or reproduction differs by genotype
non-random mating mate pairs differ from random expectation
migration immigrants carry different allele frequencies
drift small population and random change
mutation documented new allele conversion

A deviation does not by itself prove natural selection. Several forces, population structure, genotyping error or sampling can produce the pattern.

Population Genetics Measures How Selection and Other Forces Change Gene Pools

HL only

count allele copies in the gene pool → establish p and q → use Hardy–Weinberg as a no-change expectation → compare observed genotype or allele frequencies across generations → use phenotype, fitness and population evidence to identify selection or another evolutionary force

Pattern or action First model to examine
mean shifts toward one extreme directional selection
variance narrows around intermediate values stabilizing selection
both extremes rise and intermediates fall disruptive selection
observed genotypes differ from p²:2pq:q² one or more equilibrium assumptions may be violated
humans repeatedly choose parents artificial selection

Keep description, calculation and cause separate. A changed distribution describes an outcome; allele counts quantify evolution; causal claims require evidence about inheritance, survival, mating, migration, drift or deliberate breeding.

Hardy-Weinberg conditions

HL only

[ 2 ]

State two assumptions made when using the Hardy-Weinberg equation.
1.

Concept essentials

  • Hardy-Weinberg equilibrium is a null model for no evolution.
  • Random mating and a large population help maintain expected genotype frequencies.
  • Selection, mutation, migration and drift can disrupt equilibrium.