D4.1.10 (HL)—Allele frequencies

Allele frequencies describe how common each allele is in a population gene pool in evolving populations in evolving populations in evolving populations.

For allele A in a diploid population:

$f(A)=\frac{2N_{AA}+N_{Aa}}{2N_{total}}$

Genotype	Individuals	A copies contributed
AA	30	60
Aa	50	50
aa	20	0
total	100	110 of 200 copies

f(A) = 110/200 = 0.55; therefore f(a) = 0.45. Heterozygotes contribute one copy of each allele, not two copies of the dominant allele.

A barrier reduces or stops gene flow between populations.
Mutation, selection and genetic drift act separately in each gene pool.
Different environments can favor different inherited variants.
Allele frequencies diverge across generations.
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.

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.

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Define allele frequency.

Concept essentials

Allele frequency is a proportion within the population gene pool.
For two alleles, p plus q equals one.
A change in allele frequency is evidence of evolutionary change.

D4.1.10 (HL)—Allele frequencies

Allele Frequency Counts Allele Copies, Not Individuals

Isolation Lets Population Gene Pools Diverge

Population Genetics Measures How Selection and Other Forces Change Gene Pools

Concept essentials