Question

Two sister species, A and B, are distributed along a continuous elevational gradient in the Himalayas. Species A is distributed from the foothills to 1800 metres above sea level and species B is distributed from 1800 to 3500 metres above sea level. Which one of the following modes of speciation best explains their distribution?

• A. Sympatric speciation
• B. Allopatric speciation
• C. Parapatric speciation
• D. Peripatric speciation

Hint:

Sister species with non-overlapping ranges meeting at one boundary usually diverged while geographically isolated, then expanded back into contact.

Answer 1

The Correct Option is B

This question gives two sister species with ranges that just touch at one elevation (1800 m) and never overlap. The trap is to look at today's map and assume the split is happening right now, across that boundary. Let's check what each term in the options actually needs.

1. Sympatric speciation: This needs the two forms to diverge while sharing the same space, with no geographic separation at any point. Species A and B occupy separate elevational belts, so this does not fit.
2. Allopatric speciation: This needs a period where the ancestral population was cut off into two groups with no gene flow between them, usually by a physical barrier or by being pushed into separate refugia. Himalayan sister species that now replace each other along elevation are known, from long term field studies, to have diverged in exactly this way, in isolated valleys or refugia during past climate shifts, before expanding back to meet at a boundary.
3. Parapatric speciation: This needs the two forms to diverge while still in contact along a gradient, with reduced but not zero gene flow across the boundary during divergence itself. The boundary at 1800 m here is a secondary contact zone reached after the species had already formed elsewhere, not a divergence happening live across the slope.
4. Peripatric speciation: This is a special version of allopatric speciation where a small founder population splits off at the margin of the main range. Nothing in the question suggests one of the two species began as a tiny marginal population, so this is too specific a fit.

The pattern described, closely related species occupying adjacent, non-overlapping elevational bands with a shared boundary, is the textbook signature of past geographic isolation followed by range expansion and secondary contact. That is allopatric speciation, even though the present-day map looks parapatric at first glance.

Let's summarize:

• A clean elevational replacement between sister species usually reflects an old geographic split, not an active one.
• Parapatric speciation would need ongoing contact and gene flow during divergence, which is not what produced this boundary.

So the mode of speciation that best explains the distribution of species A and B is allopatric speciation.