Allele frequency is how common a particular version of a gene (an allele) is in a population, calculated by counting that allele and dividing by the total number of alleles for that gene. In AP Bio, tracking whether allele frequencies stay constant or change is how you tell if a population is evolving.
Allele frequency tells you how common one version of a gene is across a whole population. To get it, you count how many times that allele shows up and divide by the total number of alleles for that gene in the population. Frequencies are written as decimals that add up to 1 (for a gene with two alleles, p + q = 1).
The big idea in AP Bio is what happens to those frequencies over time. If they stay constant generation after generation, the population is not evolving. If they shift, something is acting on the population. Per EK 7.5.A.2, you can calculate allele frequencies straight from genotype frequencies, which is the math at the heart of the Hardy-Weinberg model. So allele frequency isn't just a number, it's the thing you watch to answer the question "is this population evolving or not?"
This term lives in Unit 7: Natural Selection and ties three topics together. In topic 7.5, allele frequency is the variable the Hardy-Weinberg model predicts (LO AP Bio 7.5.A). In topic 7.1, natural selection is one of the forces that changes those frequencies (LO AP Bio 7.1.A and 7.1.B). In topic 7.6, measured shifts in allele frequencies count as mathematical evidence for evolution (LO AP Bio 7.6.A). Hardy-Weinberg gives you a null hypothesis, a 'no evolution' baseline, and allele frequency is what you compare against it. That makes this one of the most testable quantitative concepts in the whole course.
Keep studying AP Biology Unit 7
Hardy-Weinberg Equilibrium (Unit 7)
Hardy-Weinberg is the model that predicts what allele frequencies should look like if nothing is acting on a population. The five conditions (large population, no migration, no new mutations, random mating, no selection) are never fully met, so it works as a null hypothesis. When your observed frequencies don't match the prediction, you know evolution is happening.
Evolutionary Fitness (Unit 7)
Fitness is measured by reproductive success, and that success is exactly what shifts allele frequencies. Individuals with favorable phenotypes leave more offspring, so their alleles become more common over generations. Fitness is the cause; the change in allele frequency is the measurable result.
DNA Sequences (Units 6-7)
Comparing DNA nucleotide and protein amino acid sequences (EK 7.6.B.2) is another way to see allele-level differences across organisms. Allele frequency tracks change within one population over time, while sequence comparison reveals change between species and points to common ancestry.
Abiotic Factors (Units 4 & 7)
Environmental conditions like a drought or temperature swing act as selective pressures. When the environment changes, certain phenotypes survive better, and that fluctuation drives the rate and direction of allele frequency change (LO AP Bio 7.1.B).
Expect both calculation and reasoning questions. Multiple-choice stems often hand you genotype frequencies (like AA = 0.16, Aa = 0.48, aa = 0.36) and ask you to find allele frequencies or decide whether the population is in Hardy-Weinberg equilibrium. Other stems describe a change, such as a wildflower population whose petal-color allele frequencies shift over three generations or a beetle population after a drought, and ask what most likely caused it. Your job: read whether frequencies are stable (no evolution) or changing (a force is acting), and name the force. On FRQs, you may need to calculate p and q, then interpret what a shift means as evidence of natural selection or another mechanism. Watch for the trick where allele frequencies stay stable but genotype frequencies (like heterozygotes) don't match the prediction, which signals non-random mating, not changing allele frequencies.
Allele frequency counts individual alleles (versions of a gene); genotype frequency counts combinations within individuals (AA, Aa, aa). They're linked but not the same. You can have stable allele frequencies while genotype frequencies drift away from Hardy-Weinberg predictions, which is exactly the kind of curveball MCQs use to test whether you really understand the difference.
Allele frequency is how common one version of a gene is, found by counting that allele and dividing by the total alleles for that gene in the population.
If allele frequencies stay constant across generations the population is not evolving; if they change, a force like natural selection, migration, or genetic drift is acting on it.
You can calculate allele frequencies directly from genotype frequencies, which is the core math of the Hardy-Weinberg model (EK 7.5.A.2).
The five Hardy-Weinberg conditions are never truly met in nature, so the model serves as a null hypothesis you compare real data against.
Don't confuse allele frequency with genotype frequency; allele frequencies can stay stable even while genotype proportions shift.
It's how common a particular allele is in a population, calculated by counting that allele and dividing by the total number of alleles for that gene. In Unit 7, you use it to tell whether a population is evolving (frequencies change) or not (frequencies stay constant).
Yes. A measurable shift in allele frequencies over generations is the definition of evolution at the population level. If the frequencies stay stable, the population is in Hardy-Weinberg equilibrium and is not evolving.
Allele frequency counts the individual gene versions across the whole population, while genotype frequency counts the combinations inside individuals (AA, Aa, aa). They're connected, but allele frequencies can stay the same even when genotype frequencies (like heterozygotes) don't match Hardy-Weinberg predictions, often signaling non-random mating.
Add up all copies of an allele and divide by the total. For a gene with alleles A and a, the frequency of A (p) equals the AA frequency plus half the Aa frequency. So if AA = 0.16 and Aa = 0.48, then p = 0.16 + 0.24 = 0.40, and q = 0.60.
Because it gives you a 'no evolution' baseline to test against. The five conditions (large population, no migration, no new mutations, random mating, no selection) almost never all hold, so when real allele frequencies don't match the prediction, you know a real evolutionary force is at work.