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A change in gene frequency within a population. Evolution at this scale can be observed over short periods of time - for example, between one generation and the next, the frequency of a gene for pesticide resistance in a population of crop pests increases. Such a change might come about because natural selection favored the gene, because the population received new immigrants carrying the gene, because some nonresistant genes mutated to the resistant version, or because of random genetic drift from one generation to the next.

Global assessment (1st work programme) University of California Museum of Paleontology, 2018b