Natural selection acts on the individual but evolution occurs at the population level. Changes in allele frequencies in a population over time are referred to as microevolution, which can occur quickly and is directly observable. On a grander scale, macroevolution is long-term change, characterized by speciation that mostly occurs over deep, or geologic, time. Accumulated microevolutionary change results in macroevolu-tionary change. Microevolution and macroevolution lie on a continuum of change from small- to large-scale, both resulting from the same genetic mechanisms and the same four forces: mutation, gene flow, genetic drift, and selection.
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