Population Change

Objective: Calculate simple frequencies and interpret population change.

For a diploid population with N individuals at an autosomal locus, there are 2N gene copies under the standard model. If genotypes are AA, Aa, and aa, frequency of A is p = (2nAA + nAa)/(2N), and frequency of a is q = 1 − p. Genotype frequencies count individuals and need not equal allele frequencies. Evolution at this scale can be described as change in allele frequencies between generations, while phenotypic change may also reflect environmental differences. Sampling, selection, drift, mutation, and migration can all affect measured values. Calculations require an explicit count of sampled individuals and assumptions about ploidy.

Objective: Distinguish random sampling from movement between populations.

Genetic drift is chance change in allele frequencies due to finite sampling of gametes and offspring. Its effects tend to be stronger in smaller populations, although it operates in populations of many sizes. A bottleneck sharply reduces population size, and a founder event establishes a population from a small subset, both potentially changing frequencies by sampling. Gene flow transfers alleles between populations through movement and reproduction, often reducing genetic differences between them, though outcomes depend on circumstances. A frequency shift attributed to drift should not be rephrased as if the environment selected that allele; mechanism matters.

Objective: Use equilibrium assumptions as a diagnostic model.

For two alleles with frequencies p and q = 1 − p, Hardy-Weinberg genotype proportions are p² for AA, 2pq for Aa, and q² for aa after random mating under simplifying assumptions including a large population and no selection, mutation, or migration at that locus. This is a null model for comparison, not a claim that real populations meet all assumptions. A departure from expected proportions can have several causes, such as nonrandom mating, population structure, selection, or sampling error; it does not uniquely identify one mechanism. One generation of genotype counts and repeated allele-frequency measurements answer related but distinct questions.