Allele frequency is how common a specific allele is in a population, calculated by dividing the number of copies of that allele by the total number of all alleles for that gene. On AP Bio, a change in allele frequency over time is the working definition of evolution.
Allele frequency tells you how common one version of a gene is in a whole population. You find it by counting how many copies of that allele exist and dividing by the total number of copies of every allele for that gene. If a gene has two alleles and 70 out of 100 copies are the dominant one, that allele's frequency is 0.70.
Here's the part that matters for AP Bio: allele frequency is the dial that turns when a population evolves. Natural selection, genetic drift, mutation, and migration all do the same job in the end, they push these frequencies up or down. So when the CED talks about populations evolving in response to selective pressures (EK 7.2.A.2), it's really describing allele frequencies changing across generations. Phenotypic variation is what selection sees, but allele frequency is what actually gets passed on.
This term lives at the heart of Unit 7: Natural Selection, especially topic 7.2. Learning objective AP Bio 7.2.A asks you to describe why phenotypic variation matters, and 7.2.B connects molecular variation to fitness. Allele frequency is the bridge between those ideas. Variation gives selection something to act on, and the result shows up as shifting allele frequencies. It also ties back to Unit 5 (topic 5.5), where the same genotype can produce different phenotypes depending on the environment (EK 5.5.A.1), reminding you that genes and the environment together decide which alleles get favored. Evolution and Natural Selection is one of the big AP Bio themes, and tracking allele frequency is how you measure it.
Keep studying AP Biology Unit 7
Natural Selection (Unit 7)
Natural selection is the main engine that changes allele frequency. When an allele boosts fitness in a given environment, organisms carrying it reproduce more, so that allele's frequency climbs over generations. Selection acts on phenotypes, but the lasting effect is measured in allele frequency.
Genetic Drift (Unit 7)
Drift also changes allele frequency, but by random chance rather than fitness. In a small population, an allele can become more or less common just from luck of who reproduces, which is why drift and selection are easy to mix up but mechanistically different.
Gene Pool (Unit 7)
The gene pool is the total collection of all alleles in a population. Allele frequency is basically a snapshot of that pool, telling you what fraction of it each allele makes up. Evolution is a change in the gene pool's allele frequencies.
Mutation (Unit 7)
Mutation is the original source of new alleles, so it sets the raw frequencies that selection and drift then adjust. A brand-new beneficial mutation starts at an extremely low frequency and can rise fast if it raises fitness, like DDT resistance in mosquitoes.
Allele frequency shows up most often in multiple-choice questions about how populations change over time. A classic stem gives you the dark-form peppered moth and asks how its allele frequency shifts from before industrialization, through heavy pollution, to after cleanup. You'd say the dark allele rises when pollution favors camouflage, then falls once the environment is clean again. Another common stem describes the HbS (sickle cell) allele varying from 0.01 to 0.20 across regions of Africa and asks why, where the answer points to malaria as a selective pressure favoring heterozygotes. With DDT-resistance mosquitoes, you'd identify natural selection as the mechanism increasing the resistance allele's frequency. Your job is usually to connect a selective pressure to the direction the frequency moves, not to crunch a number. No released FRQ has used this term verbatim, but the concept underlies any evolution data question where you explain a change in a population.
Allele frequency counts individual alleles (the versions of a gene), while genotype frequency counts combinations of two alleles in individuals (like AA, Aa, aa). One population can have the same allele frequency split into very different genotype frequencies, which is exactly what Hardy-Weinberg math compares.
Allele frequency is the count of one allele divided by the total count of all alleles for that gene in the population.
A change in allele frequency over generations is the operational definition of evolution in AP Bio.
Natural selection, genetic drift, mutation, and migration all act by changing allele frequencies.
Selection acts on phenotypes, but the result you measure and report is the shift in allele frequency.
On the exam, connect a named selective pressure to the direction an allele's frequency moves, like the dark moth allele rising with pollution.
Allele frequency counts alleles, while genotype frequency counts pairs of alleles in individuals.
It's how common a specific allele is in a population, found by dividing the number of copies of that allele by the total number of alleles for that gene. On the exam, a change in allele frequency over time is what evolution actually means.
Yes. In AP Bio, evolution is defined as a change in a population's allele frequencies across generations, so if frequencies are shifting, the population is evolving.
Allele frequency counts single alleles like A or a, while genotype frequency counts the two-allele combinations individuals carry like AA, Aa, or aa. The same allele frequency can produce different genotype frequencies, which is the whole point of Hardy-Weinberg comparisons.
Natural selection, genetic drift, mutation, and gene flow (migration) all change allele frequency. Selection changes it based on fitness, drift changes it by chance, and mutation introduces brand-new alleles at low frequency.
Because malaria acts as a selective pressure. In regions with heavy malaria, carrying one HbS allele protects against the disease, so the allele's frequency stays higher (up to about 0.20) than in regions without that pressure.