How does parapatric speciation differ from other modes of speciation?

Parapatric speciation differs from other modes of speciation as it occurs when populations are partially, but not completely, geographically isolated from each other.

Parapatric speciation is one of the three primary modes of speciation, the others being allopatric and sympatric speciation. Unlike allopatric speciation, where populations are completely geographically isolated from each other, and sympatric speciation, where populations share the same geographic location, parapatric speciation occurs when populations are partially separated. This means that they are adjacent or overlapping in their geographic distribution, but there is limited gene flow between them due to the existence of a gradient or cline.

This gradient or cline could be a physical barrier, such as a mountain range, or an ecological gradient, such as a change in habitat type. However, it is not a complete barrier to gene flow, as in allopatric speciation. Instead, it creates a situation where different parts of the population experience different selective pressures. For example, one end of the gradient might be a wet environment, while the other end is dry. The organisms at each end of the gradient would then evolve to be better suited to their specific environment, leading to divergence between the two groups.

Over time, this divergence can lead to the formation of new species, even though there is still some gene flow between the populations. This is because the selective pressures at each end of the gradient are strong enough to overcome the homogenising effect of gene flow. The new species that form are then adapted to different parts of the gradient, and may no longer be able to interbreed with each other, even if they come into contact.

In contrast, in allopatric speciation, the complete geographical isolation prevents any gene flow between the populations, leading to divergence and the formation of new species. In sympatric speciation, new species form within the same geographic location, often due to disruptive selection or changes in mating preferences.

Therefore, parapatric speciation is a unique mode of speciation that combines elements of both allopatric and sympatric speciation. It involves partial geographical isolation, different selective pressures across a gradient, and the formation of new species despite some gene flow.