Genetic drift: chance events change the allele frequencies in a population, e.g. forest fire kills half the heath hens, but it kills all of the ones that have black feathers. A special case of genetic drift is "founder effect" in which a limited number of individuals start a new population that is reproductively isolated from the "parent population." This new population will have different allele frequencies from the parent population and may not even include its full gene pool.

Both of these mechanisms change allele frequencies in population, and that's what evolution is. "Founder effect" is an especially important mechanism in the development of new species.