Source = Wikipedia

Microevolution is the occurrence of small-scale changes in allele 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, natural selection, gene flow, and genetic drift.

Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution. Ecological genetics concerns itself with observing microevolution in the wild. Typically, observable instances of evolution are examples of microevolution; for example, bacterial strains that have antibiotic resistance.

Microevolution can be contrasted with macroevolution; which is the occurrence of large-scale changes in gene frequencies, in a population, over a geological time period (i.e. consisting of lots of microevolution). The difference is largely one of approach. Microevolution is reductionist, but macroevolution is holistic. Each approach offers different insights into evolution.

Because microevolution can be observed directly, creationists agree that it occurs, though they tend to make a distinction between microevolution, macroevolution, and speciation.

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Macroevolution refers to evolution that occurs above the level of species, over long periods of time, that leads to speciation, in contrast to microevolution, which refers to smaller evolutionary changes (described as changes in allele frequencies) within a species or population. Within the Modern Synthesis school, microevolution is thought to be the only mode of evolution. The process of speciation (is isolated populations) can fall within the purview of either. Paleontology, evolutionary developmental biology, and comparative genomics contribute most of the evidence for the patterns and processes that can be classified as macroevolution.

Macroevolution is controversial in two ways:

It is disputed among biologists whether there are macroevolutionary processes that are not described by strictly gradual phenotypic change, of the type studied by classical population genetics. One of these two views is becoming less and less tenable as the role for genome-wide changes and developmental processes in evolution become clearer.
A misunderstanding about this biological controversy has allowed the concept of macroevolution to be coopted by creationists. They use this controversy as a supposed "hole" in the evidence for deep-time evolution.
However, microevolution and macroevolution both refer fundamentally to the same thing, changes in allele frequencies, and the scientific controversy is only about how those changes predominantly occur. Either way macroevolution uses the same mechanisms of change as those already observed in microevolution.