Parapatric Speciation

‘Para’ means ‘near,’ and ‘patria’ means ‘country. In parapatric speciation, two sister groups of a species develop reproductive isolation while they are still exchanging genes. Here, populations are geographically adjacent rather than being completely separated.

Unlike allopatric speciation, in which populations are isolated by physical barriers, or sympatric speciation, which occurs within a shared habitat, parapatrically separated groups have overlapping geographic areas.

Due to this partial separation, the gene flow is limited between the two groups, which allows other factors like natural selection, mutation, and genetic drift to cause divergence over time.

Factors Causing Parapatric Speciation

In parapatric speciation, mating is non-random, there is unequal gene flow and a physical barrier may or may not exist between the newly formed species and the parent population.

Here are the key factors that may lead to parapatric speciation:

Environmental Gradient: When a parent population gets spread across different habitats with different environmental conditions, populations at different points along the gradient experience different selective pressures, such as variations in temperature, soil type, or food availability. It leads to local adaptations, causing divergence.
Limited Gene Flow: Although the diverging and the original populations are adjacent, gene flow between them is reduced, especially between those at the extreme ends of the range. With time, this limited gene flow allows genetic differences to accumulate within the diverging group.
Natural Selection: Different environmental conditions favor different traits in each population. For example, organisms in a dry area may evolve drought resistance. This selection drives evolutionary change and divergence.
Hybrid Zones: In regions where populations overlap, hybrid zones may form. Hybrids in these areas often have lower fitness than individuals from either parent population due to mismatched traits, reinforcing reproductive isolation and inducing divergence.
Behavioral Isolation: Differences in behaviors, such as mating calls, feeding habits, or breeding times, can also contribute to reproductive isolation between neighboring populations, causing speciation.

How Does Parapatric Speciation Occur

Since, in parapatric speciation, a continuous population extends across a gradient of environmental conditions, this can result in individuals in different parts of the range experiencing different selective pressures. As these selections favor different traits, populations on either end of the gradient may evolve distinct characteristics, leading to reproductive isolation in the diverging group.

However, since the diverging populations are still partially connected to the parent group, there may be a hybrid zone where individuals from both populations interbreed. Over time, behavioral differences cause reproductive isolation due to reduced interbreeding between the two groups, thus leading to speciation.

Examples of Parapatric Speciation

Sweet Vernal Grass (Anthoxanthum odoratum)

This sweet vernal grass species grows near mines, where some populations are exposed to high levels of toxic metals like lead and zinc. Over time, the population near the contaminated soils has evolved to tolerate heavy metals, while adjacent populations in uncontaminated areas have not. The two populations remain geographically adjacent, but there is reduced gene flow between them due to differences in flowering time and ecological preferences, leading to parapatric speciation.

Greenish Warbler (Phylloscopus trochiloides)

The greenish warbler, the ring species, is found around the Tibetan Plateau. While the neighboring populations can interbreed, the populations at the two ends of the ring have diverged so significantly that they can no longer interbreed, even though they are geographically adjacent in certain regions. Differences in song patterns and ecological adaptations drive this divergence.

Little Greenbul (Andropadus virens)

The little greenbul is a bird species found in different habitats across West Africa. Populations in forested areas and nearby savannas experience different ecological pressures. These two populations are not completely isolated but have developed adaptations to their specific environments, proving genetic divergence has led to parapatric speciation.