Achange in allele frequencies in a set of organisms over time is called

Microevolution

Microevolution is the change in the allele frequencies within a set of organisms (population) over time primarily due to four processes namely, mutation, natural and artificial selection, gene flow, and genetic drift. 

The term "microevolution" is coined to differentiate it from "macroevolution," which occurs over much longer periods of time and results in greater differences in the characteristics of the organisms in the population. 

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