Microevolution is the occurrence of small-scale changes in gene frequencies in a population over a few generations, also known as change at or below the species level. These changes may be due to several processes: mutation, gene flow, genetic drift, as well as natural selection. Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution. Biologists distinguish between microevolution and macroevolution, which is the occurrence of large-scale changes in gene frequencies in a population over a long period of time (and may culminate in the evolution of new species).
Typically, observable instances of evolution are examples of microevolution; for example, bacterial strains that have become resistant to antibiotics. Because microevolution can be observed directly, both pro-evolution and some anti-evolution groups agree that it is a fact of life.