AP Biology Population Genetics — AP Biology
1. Core Concepts and Standard Notation ★★☆☆☆ ⏱ 3 min
Population genetics is the quantitative study of the distribution and change in allele and genotype frequencies within biological populations, forming the core mathematical foundation for the study of evolution in AP Biology. It frames evolution as a measurable change in allele frequency over generations, rather than just a qualitative change in trait distribution, allowing researchers to test hypotheses about evolutionary forces. This topic accounts for ~15% of Unit 7 exam weight, appearing regularly on both multiple-choice and free-response sections.
- $p$ = frequency of the dominant allele in the population
- $q$ = frequency of the recessive allele in the population
- $p^2$ = frequency of the homozygous dominant genotype
- $2pq$ = frequency of the heterozygous genotype
- $q^2$ = frequency of the homozygous recessive genotype
2. Hardy-Weinberg Equilibrium (HWE) ★★★☆☆ ⏱ 5 min
The Hardy-Weinberg Equilibrium is a null model that describes a population that is *not evolving*, meaning allele and genotype frequencies remain constant from generation to generation. For a biallelic autosomal locus, the model is derived from probability rules for independent allele sampling from a gene pool, giving two core equations:
p + q = 1
p^2 + 2pq + q^2 = 1
Exam tip: Always count alleles correctly for diploid organisms: multiply the number of homozygotes by 2 before adding heterozygotes to get the total allele count. Never use individual counts directly for $p$ and $q$.
3. Mechanisms of Evolution (HWE Violations) ★★★★☆ ⏱ 4 min
HWE acts as a null model: when we observe deviations from HWE predictions, we can infer that one or more evolutionary mechanisms are acting on the population. Each deviation corresponds to a violation of one of the five HWE assumptions, and each mechanism alters allele and genotype frequencies in predictable ways:
- **Mutation**: Creates new alleles, changes frequency very slowly over time; it is the ultimate source of genetic variation.
- **Non-random mating (e.g., inbreeding)**: Increases homozygosity by genotype, but does not change overall allele frequency on its own.
- **Natural selection**: Differential survival and reproduction based on phenotype causes consistent, adaptive changes in allele frequency, modeled using relative fitness.
- **Genetic drift**: Random change in allele frequency due to sampling error in small populations; changes are non-adaptive.
- **Gene flow**: Movement of alleles between populations reduces genetic differentiation between groups.
Exam tip: When calculating relative fitness, always divide by the highest survival rate, not the sum of survival rates. This scales all fitness values between 0 and 1 for easy comparison.
4. Genetic Drift: Founder and Bottleneck Effects ★★★☆☆ ⏱ 3 min
Genetic drift is random fluctuation in allele frequency caused by sampling error in small populations, and it is a frequently tested HWE violation on the AP exam. Unlike natural selection, genetic drift causes non-adaptive changes: allele frequency shifts are random and unrelated to the fitness of the allele. Two commonly tested scenarios are:
- **Founder effect**: Occurs when a small subset of a larger population founds a new, isolated population. The founder group is not genetically representative of the original population, so allele frequencies change randomly.
- **Bottleneck effect**: Occurs when a large population is drastically reduced in size by a random event (e.g., natural disaster, habitat destruction). The surviving population has reduced genetic diversity and altered allele frequencies due to random sampling.
Exam tip: The most common AP exam question distinction: founder effect = new population, bottleneck effect = existing population reduced. Both are types of genetic drift.
Common Pitfalls
Why: Students confuse genotype frequency (count of individuals) with allele frequency (count of alleles) for diploid organisms
Why: Students mix up notation for allele frequency ($q$) and homozygous recessive genotype frequency ($q^2$), leading to algebra errors
Why: Students confuse genotype frequency change with allele frequency change
Why: Students mix up adaptive (selection) and non-adaptive (drift) evolutionary change
Why: Students memorize the assumptions but forget all must hold for no evolution