Evolution is best described as the change in what over time?

• A. Allele frequencies in a population
• B. Gene expression in individuals
• C. Protein structures within a single organism
• D. Chromosome counts within a population

Answer: (A)

Evolution is the change in the genetic makeup of a population over generations. This is best seen through allele frequencies—the proportions of different gene variants in the population. When those frequencies shift due to natural selection, genetic drift, mutation, or migration, the population evolves. For example, alleles that confer antibiotic resistance may become more common in a bacterial population under pressure, illustrating how evolution unfolds over time.

Gene expression in individuals describes how genes are turned on or off within a single organism and isn’t about genetic change across generations. Protein structures within a single organism reflect its current biology rather than population-level genetic shifts. Chromosome counts within a population can vary in some cases, but evolution is defined by changes in allele frequencies across generations, not by a single measure like chromosome number.