When does parapatric speciation occur?

Parapatric speciation occurs when populations of a species become geographically adjacent but not completely separated, typically due to environmental differences across their range that lead to varying selective pressures. This form of speciation allows for the possibility of limited interbreeding between the populations, but also encourages divergence due to these selective pressures and the development of reproductive barriers.

The key element that facilitates parapatric speciation is the reduced gene flow caused by varying environments at the borders of populations. As individuals adapt to their local environments, reproductive isolation can emerge even without complete geographic isolation. Thus, while reproductive barriers might not arise due to geographical separation, they can develop when populations adapt to differing selective pressures, leading to a scenario where reproductive capability diminishes over time.

Different from geographic isolation, which characterizes allopatric speciation, the essence of parapatric speciation lies in the adjacent distribution of populations and how local adaptations influence their reproductive strategies. The dynamics of this process involve ecological factors and selection, making reproductive cessation in the absence of geographic barriers a relevant aspect of how divergence can happen in parapatric conditions.