In parapatric speciation, the new species evolve from contiguous populations.
Parapatric speciation occurs as follows:
Suppose that a population initially existed in an area to which it was well adapted, and that it then started to expand into a contiguous area in which the environment favored a different form. If the transition between the two environments was sudden, a stepped cline will evolve at the border.
As selection worked on the population in the new area, different genes would accumulate in it and the two populations would diverge to become adapted to their respective environments. If they diverged almost to be different species, the border would be recognized as a hybrid zone. The two populations would have separated while they were geographically contiguous, along an environmental gradient.
In contrast to the allopatric theory of speciation, the two populations on either side of the hybrid zone have diverged without any period of geographic separation.