Parapatric Speciation - Background
In parapatric speciation, strong selection along a gradient can lead to the genetic divergence of a continuous population. In the simplest case, the population exists in two different (but adjacent) environments. Because the selection pressures are different in the two environments, two distinct forms will likely develop in the population. Mating between the two forms will lead to hybrids, which will likely be selected against in either of the two environments. The hybrids may, however, be able to survive at the point where the two environments come together - the hybrid zone.
If the hybrids have decreased fitness, selection will favor individuals that choose to mate with their own type (positive assortative mating), which is one type of prezygotic isolation mechanism. This reduces the number of hybrids produced and strengthens the divergence of the population (this process is termed reinforcement).