What processes change allele frequencies in a population?
Topic 7.4 Population Genetics: explain how natural selection, mutation, gene flow, genetic drift and non-random mating change allele frequencies.
A focused answer to AP Biology Topic 7.4, covering the gene pool, allele frequencies, and the five mechanisms of microevolution (selection, mutation, gene flow, genetic drift, non-random mating), including bottleneck and founder effects, with a worked allele-frequency calculation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The College Board (Topic 7.4) wants you to explain how allele frequencies change in a gene pool through the mechanisms of microevolution: natural selection, mutation, gene flow, genetic drift (including bottleneck and founder effects), and non-random mating.
The gene pool and allele frequencies
The mechanisms of change
Genetic drift: bottleneck and founder effects
Both effects reduce genetic diversity, which can leave a population more vulnerable to disease and environmental change, connecting drift to the extinction-risk theme of this unit. Unlike selection, drift can even fix harmful or remove beneficial alleles purely by chance, because it does not depend on fitness.
Try this
Q1. State the difference between genetic drift and natural selection. [2 points]
- Cue. Drift changes allele frequencies by random chance, independent of fitness; selection changes them non-randomly, favoring alleles that improve fitness.
Q2. Explain how gene flow affects two neighboring populations. [2 points]
- Cue. Migration and breeding transfer alleles between them, so it tends to make their allele frequencies more similar and can introduce new alleles to a population.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (long FRQ excerpt). (a) Distinguish between genetic drift and natural selection as mechanisms of evolution. (b) Explain why genetic drift has a larger effect on small populations, and describe the founder effect as an example.Show worked answer →
A 4-point distinguish-and-explain FRQ on microevolution.
(a) Distinguish (2 points): (1 point) natural selection changes allele frequencies non-randomly, favoring alleles that improve fitness; (1 point) genetic drift changes allele frequencies by random chance, independent of fitness.
(b) Explain (2 points): (1 point) in a small population, chance events (such as which few individuals reproduce) have a proportionally larger effect on allele frequencies, so drift is stronger; (1 point) the founder effect occurs when a few individuals start a new population, carrying only a small, non-representative sample of the original gene pool, so allele frequencies differ by chance.
Markers reward the random-versus-non-random distinction and explaining why small populations are more affected by drift.
AP 2018 (style)1 marksSection I (multiple choice). The movement of alleles between populations when individuals migrate and breed is called: (A) genetic drift. (B) gene flow. (C) mutation. (D) the bottleneck effect.Show worked answer →
The correct answer is (B).
Gene flow is the transfer of alleles between populations by the movement and breeding of individuals (or gametes, such as pollen). It tends to make populations more similar. Genetic drift (A) and the bottleneck effect (D) are random changes; mutation (C) creates new alleles.
Related dot points
- Topic 7.5 Hardy-Weinberg Equilibrium: use the Hardy-Weinberg equations to calculate allele and genotype frequencies and test whether a population is evolving.
A focused answer to AP Biology Topic 7.5, covering the Hardy-Weinberg conditions, the equations p + q = 1 and p squared plus 2pq plus q squared = 1, and how to calculate and interpret allele and genotype frequencies, with worked calculations.
- Topic 7.1 Introduction to Natural Selection: explain the conditions required for natural selection and how it leads to changes in a population.
A focused answer to AP Biology Topic 7.1, covering Darwin's reasoning, the conditions for natural selection (variation, heritability, overproduction, differential reproduction), fitness, and how selection changes allele frequencies, with a worked example.
- Topic 7.12 Variations in Populations: explain why genetic variation within a population is important for survival and the response to environmental change.
A focused answer to AP Biology Topic 7.12, covering the sources and importance of genetic diversity, how variation buffers populations against change, the risks of low diversity, and the role of variation in evolution, with a worked example.
- Topic 7.10 Speciation: explain how reproductive isolation leads to speciation, including allopatric and sympatric speciation.
A focused answer to AP Biology Topic 7.10, covering the biological species concept, reproductive isolation (prezygotic and postzygotic barriers), allopatric and sympatric speciation, and rates of speciation, with a worked example.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)