Allopatric speciation - the isolation of one portion of the population by some physical barrier that renders the new population incapable of reproducing offspring with the original population
Example: Tomato species
Parapatric speciation - a sub-population becoming established in a new ecological niche not previously occupied by the species; this has occurred for some annual plants
Sympatric speciation - a subpopulation that occupies the same niche as the remainder of the species develops a unique mutation that prevents it from mating with the original population; the new mutation provides the new species an ecological advantage which permits its establishment as the primary species in the same niche.
Example: Spartina townsendii