The rapid evolution of a new species from a small population that is partially or totally isolated from the parent population; the rapid speciation occurs due to genetic drift and founder effect and usually involves a few mutations that have a big impact on the organisms' observable physical traits.

This is also called "saltational speciation".

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.