Parapatric Speciation Explained

Parapatric speciation occurs when populations of a species are geographically adjacent but experience different environmental conditions or selection pressures. Although there is potential for gene flow between these adjacent populations, reproductive isolation evolves, leading to the formation of distinct species.

Distribution Along Elevational Gradient

In this scenario, Species A occupies lower elevations (foothills to 1800m) and Species B occupies higher elevations (1800m to 3500m) in the Himalayas. Their distributions are continuous and adjacent, meeting at the 1800m elevation mark.

• Species A: Foothills to 1800m
• Species B: 1800m to 3500m

The 1800m level acts as a zone of contact where gene flow might still occur, but different selective pressures at lower versus higher altitudes favour the divergence of the two species. This pattern aligns perfectly with parapatric speciation.

Comparison with Other Speciation Modes

• Allopatric speciation requires a complete geographic barrier separating populations, which is absent in this continuous gradient.
• Sympatric speciation occurs without geographic separation, within the same area, which does not fit the distinct elevational ranges.
• Peripatric speciation involves the isolation of a small peripheral population, which is not described here.

Therefore, the adjacent distribution along a continuous environmental gradient strongly suggests parapatric speciation.