In parapatric speciation, populations that are in contact tend to diverge due to:

In parapatric speciation, populations that are in contact tend to diverge primarily due to environmental differences. This type of speciation occurs when populations are geographically separated but still have a zone of contact where they can interbreed. However, even though they are in proximity to each other, differences in their environments can create selective pressures that lead to distinct adaptations in each population.

For example, if one population is located in a region with different soil types, temperature, or food resources compared to the neighboring population, individuals that are better suited to their specific environment will have a reproductive advantage. Over time, these adaptations can lead to reproductive isolation, even in the presence of gene flow.

The divergence is not primarily driven by physical barriers or geographical boundaries, as parapatric speciation specifically involves populations that are adjacent to one another rather than completely isolated. Behavioral differences can play a role in mating preferences, but the essential factor driving the divergence in parapatric speciation is the difference in environmental conditions leading to natural selection for specific traits.