Variations in Populations — AP Biology
1. What Are Variations in Populations? ★★☆☆☆ ⏱ 2 min
Variation in populations refers to differences in heritable nucleotide sequences (alleles) and resulting phenotypes among individuals within a single interbreeding population, defined as a group of the same species living in the same geographic area that can interbreed.
This topic is core to Unit 7: Natural Selection, which makes up 13–25% of the total AP Biology exam score per the official CED. It appears on both multiple-choice (MCQ) and free-response (FRQ) sections.
Genetic variation is the raw material for natural selection: without differences in heritable traits, natural selection cannot change the prevalence of adaptive traits over time. Variation is quantified using allele frequencies and genotype frequencies. Within-population variation drives microevolution, small-scale changes in allele frequency over generations that lead to macroevolutionary change over longer time scales.
2. Sources of Genetic Variation ★★☆☆☆ ⏱ 3 min
Genetic variation originates and is maintained in populations through multiple mechanisms, with mutation as the ultimate source of all new variation. Only germline mutations (occurring in gamete-producing cells) are passed to offspring and contribute to population-level variation. Most mutations are neutral or harmful, but rare beneficial mutations create new adaptive traits for natural selection to act on.
- **Sexual reproduction**: Crossing over in meiosis I, independent assortment of homologous chromosomes, and random fertilization generate new combinations of existing alleles, creating phenotypic variation without new mutation.
- **Gene flow**: Movement of individuals and their alleles between populations, which can introduce new alleles into a population.
- **Standing cryptic variation**: Most populations carry rare, existing alleles that are not expressed under typical environmental conditions, which can become visible when the environment changes.
- **Balancing selection**: Natural selection that maintains multiple alleles in a population, such as heterozygote advantage in sickle cell anemia.
Exam tip: When asked for sources of variation on an FRQ, always explicitly distinguish between ultimate sources (mutation) and proximate sources that reshuffle existing variation (gene flow, sexual recombination) to earn full points.
3. Allele and Genotype Frequency Calculation ★★★☆☆ ⏱ 3 min
To quantify variation in populations, biologists calculate allele and genotype frequencies. By convention, $p$ is the frequency of the dominant allele, and $q$ is the frequency of the recessive allele for a biallelic (two-allele) locus in a diploid population. For any population, the sum of allele frequencies for a locus equals 1, so $p + q = 1$ always holds, regardless of whether the population is evolving.
p = \frac{(2 \times \text{number of AA individuals}) + (\text{number of Aa individuals})}{2N}
Genotype frequencies also sum to 1: $f(AA) + f(Aa) + f(aa) = 1$. This calculation works for any population, evolving or not, and is the foundation for detecting microevolution (any change in allele frequency over generations).
Exam tip: Always write 'total alleles = 2 × number of diploid individuals' at the start of every calculation to avoid the most common student mistake of using the number of individuals as the total allele count.
4. Hardy-Weinberg Equilibrium ★★★☆☆ ⏱ 3 min
Hardy-Weinberg equilibrium (HWE) is a null model that describes the expected allele and genotype frequencies for a population that is NOT evolving. If all five HWE assumptions are met, allele and genotype frequencies will remain constant from generation to generation. The five assumptions are: (1) no new mutation, (2) no gene flow, (3) random mating, (4) no genetic drift (very large population size), (5) no natural selection. HWE is used to test for evolution: if observed genotype frequencies differ significantly from HWE expectations, one or more assumptions are violated, meaning evolution is occurring.
p^2 + 2pq + q^2 = 1
Where $p^2$ is the expected frequency of homozygous dominant (AA), $2pq$ is the expected frequency of heterozygous (Aa), and $q^2$ is the expected frequency of homozygous recessive (aa).
Exam tip: You will almost always start HWE calculations with $q^2$, because only homozygous recessive phenotypes can be directly distinguished from other phenotypes; dominant phenotypes include both homozygotes and heterozygotes that cannot be separated by observation.
5. Effects of Evolutionary Forces on Variation ★★★☆☆ ⏱ 3 min
Different evolutionary forces alter the amount of genetic variation in a population in predictable ways. Mutation increases variation by adding new alleles. Genetic drift (random changes in allele frequency due to chance events) almost always reduces variation, because rare alleles are much more likely to be lost by chance, especially in small populations. Two common forms of drift that reduce variation are the bottleneck effect (when a large population crashes to a small size due to a disturbance) and the founder effect (when a small group founds a new population with only a subset of the original variation).
Gene flow typically increases variation within a population, because it introduces new alleles from other populations. Natural selection has variable effects: directional selection (favoring one extreme phenotype) reduces variation; disruptive selection (favoring both extreme phenotypes) increases variation; stabilizing selection (favoring the intermediate phenotype) reduces variation; and balancing selection maintains multiple alleles, preserving variation.
Exam tip: Always match the scenario to the effect: population isolation or size reduction = genetic drift, new individuals entering = gene flow, new trait = mutation.
Common Pitfalls
Why: Students confuse number of individuals with number of alleles, forgetting most organisms discussed in AP Bio are diploid.
Why: Students memorize the rule for $q$ and incorrectly apply it to $p$.
Why: Courses focus on HWE calculation, so students forget its core purpose.
Why: Students memorize that mutation is the ultimate source and forget other mechanisms.
Why: Students learn drift is strongest in small populations and incorrectly generalize that it does not occur in large populations.
Why: Students associate evolution with natural selection and forget neutral processes.