Microevolution
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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.
Notes: Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the case of artificial selection, whereby various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors, or in the often-cited, but somewhat problematic case of systematic color change in the peppered moth, Biston betularia, which was observed over a 50-year period in England. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.
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