A speciation in which the zones of two diverging populations are only partially separated that the species may still come in contact or cross the barrier from time to time, until selection for specific behaviours or certain isolating mechanisms will eventually prevent them from interbreeding.
This speciation is exemplified by the Ensatina salamanders, which form a ring round the Central Valley in California.
Word origin: Gk para-, comb. form repr. pará (prep.) beside, alongside of, by, beyond + Greek patrā, fatherland.