evolution

Meaning: Evolution is a change in a population's allele frequencies across generations. Variation comes from mutation and recombination. Natural selection raises the frequency of heritable alleles that improve fitness, meaning reproductive success. Other mechanisms also shift frequencies: genetic drift (random change, stronger in small populations, as in bottlenecks and the founder effect), gene flow (migration between populations) and nonrandom mating such as sexual selection.

Equation: p² + 2pq + q² = 1, with p + q = 1. Here p and q are the frequencies of two alleles (unitless), and p², 2pq and q² are the expected genotype frequencies (unitless). Frequencies stay constant only when no evolution is occurring: a large population, random mating, no mutation, no migration and no selection.

On the MCAT: Passages give allele frequencies over generations or ask for Hardy-Weinberg calculations, often starting from q² as the frequency of the recessive phenotype. You predict the effect of a bottleneck, antibiotic exposure, or directional, stabilizing or disruptive selection. Phylogenetic trees and homologous versus analogous structures (convergent evolution) also appear.

Common confusion: Individuals do not evolve; populations do. Fitness means reproductive success, not strength. Mutations arise randomly with respect to need, and selection then acts on the variation already present.