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D4.1.11 (HL)—Changes in allele frequency

Allele frequencies change when selection, mutation, migration, genetic drift or non-random mating alters reproductive contribution in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.11
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper2
Typical marks1–4

Common command terms

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Recent exam appearances

May 2023Paper1 ["HL"] · TZ116[ 1 ]D4.1.11 (HL)—Changes in allele frequency
May 2016Paper2 ["HL"] · TZ03(c)[ 2 ]D4.1.11 (HL)—Changes in allele frequency
May 2016Paper3 ["HL"] · TZ010(a)(i)[ 1 ]D4.1.11 (HL)—Changes in allele frequency
May 2015Paper2 ["HL"] · TZ24(c)[ 4 ]D4.1.11 (HL)—Changes in allele frequency

Coverage 2015–2023 · Updated 16 Jul 2026

Neo-Darwinism Connects Darwin’s Selection to Mendelian Alleles

HL only

mutation supplies new alleles → meiosis and fertilization create genotypes → genotypes influence heritable phenotypes → selection acts through individual survival and reproduction → allele frequencies change in the population gene pool

Darwinian description Genetic description
individuals vary individuals carry different alleles and genotypes
some leave more offspring alleles are transmitted unequally
population becomes adapted advantageous inherited variants rise in frequency

Selection is not the only force that changes allele frequency. Mutation, migration and genetic drift also alter gene pools, though not necessarily because one phenotype has higher fitness.

Antibiotics Cause Directional Selection of Resistant Bacteria

HL only
  1. Resistance variation exists before or arises independently of treatment.
  2. The antibiotic kills susceptible bacteria more effectively.
  3. Resistant survivors reproduce and contribute a larger fraction of the next population.
  4. Resistance alleles rise in frequency; plasmid transfer can spread them between cells.

The antibiotic does not teach individual bacteria to become resistant. It changes the survival difference between pre-existing or newly mutated variants.

A mixed bacterial population is exposed to antibiotic; susceptible cells die, resistant survivors reproduce, and resistance can also move by plasmid transfer.

Artificial and Natural Selection Differ in Selector, Not in Inheritance

HL only
Feature Artificial selection Natural selection
source of reproductive advantage human breeder chooses parents environmental survival and mating pressures
variation required heritable variation heritable variation
population result chosen alleles change frequency alleles linked to higher fitness change frequency
guaranteed organismal benefit no; target serves human goal no universal benefit; adaptation is environment-specific

Antibiotic resistance is usually natural selection caused by a human-created pressure, not artificial selection: humans do not deliberately choose resistant bacteria as breeding parents.

Both processes reveal the same inheritance principle—repeated unequal reproduction changes a population over generations.

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.

Changes in allele frequency

HL only

[ 4 ]

If a wild population of cats contained both curled ears and normal ears, explain how the proportion of these two phenotypes could change in the population.

Concept essentials

  • Selection changes allele frequencies through differential reproductive success.
  • Genetic drift is random and strongest in small populations.
  • Migration can add or remove alleles from a population.