(of biological species or speciation) Describing formation of two or more different species or populations that inhabit contiguous geographical areas (not overlapping or marginally overlapping) with the potential for gene flow between populations.

In parapatric speciation, the new species evolve from contiguous populations, but nonetheless, the population does not mate randomly.
Suppose now 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 favoured a different form.
If the transition between the two environments was sudden, a stepped cline will evolve at the border, the border would be recognized as a hybrid zone.
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 two populations would have separated while they were geographically contiguous, along an environmental gradient.