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D4.1.9 (HL)—Gene pool concept

A gene pool is the total set of alleles present in all individuals of a population in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.9
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–3

Common command terms

  • State
  • Discuss

Recent exam appearances

November 2025Paper1B ["HL"] · TZ34(d)[ 3 ]D4.1.9 (HL)—Gene pool concept
May 2016Paper2 ["HL"] · TZ04(b)[ 3 ]D4.1.9 (HL)—Gene pool concept
May 2015Paper3 ["HL"] · TZ12(c)(ii)[ 1 ]D4.1.9 (HL)—Gene pool concept
November 2013Paper3 ["HL"] · TZ02(c)[ 2 ]D4.1.9 (HL)—Gene pool concept
May 2013Paper3 ["HL"] · TZ22(a)[ 1 ]D4.1.9 (HL)—Gene pool concept

Coverage 2012–2025 · Updated 16 Jul 2026

A Gene Pool Contains Every Allele Available to an Interbreeding Population

HL only

A gene pool is the complete set of genes and their alleles in an interbreeding population. Individuals carry samples of that pool; gametes sample alleles again for the next generation.

Level What is counted?
diploid individual two allele copies at one autosomal locus
population all allele copies carried by all breeding individuals
allele frequency proportion of all locus copies that are a specified allele

When allele frequencies change from one generation to another, the gene pool is changing—this is population-level evolutionary change even before a new species appears.

Isolation Lets Population Gene Pools Diverge

HL only
  1. A barrier reduces or stops gene flow between populations.
  2. Mutation, selection and genetic drift act separately in each gene pool.
  3. Different environments can favor different inherited variants.
  4. Allele frequencies diverge across generations.
  5. Accumulated genetic and reproductive differences may eventually contribute to speciation.
Force Directional expectation
gene flow tends to make populations more similar
different local selection can make populations diverge predictably
drift in small populations can make frequencies diverge by chance
mutation introduces new alleles independently

Geographic allele-frequency databases can reveal divergence, but the pattern alone does not identify which force caused it; sampling and population definition must also be checked.

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.

Gene pool concept

HL only

[ 3 ]

Discuss how the isolation of populations due to the fragmentation of forests could lead to changes in gene pools.

Concept essentials

  • A gene pool includes all alleles in a population.
  • Evolution can be described as change in a gene pool over time.
  • Mutation, migration, drift and selection can alter the gene pool.