It refers to evolution resulting from small specific genetic changes that can lead to formation of a new subspecies. When individuals with certain traits are more successful at reproduction, the ensuing generation will have more copies of that trait. Should the trend continue, eventually the traits will become so common in the population that the population profile will change.
Examples of microevolution : House sparrows have adapted to the climate of North America, mosquitoes have evolved in response to global warming, and insects have evolved resistance to our pesticides.
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