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?
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?
Show Hint
Sister species with non-overlapping ranges meeting at one boundary usually diverged while geographically isolated, then expanded back into contact.
Updated On: Jul 20, 2026
- Sympatric speciation
- Allopatric speciation
- Parapatric speciation
- Peripatric speciation
Show Solution
Verified By Collegedunia
The Correct Option is B
Solution and Explanation
Step 1: Read the pattern in the question.
Species A and B are sister species (closest living relatives), and their current ranges do not overlap: A holds the lower slopes up to 1800 m, and B takes over from 1800 m to 3500 m. Together, the two species cover the full elevational gradient with a single shared boundary at 1800 m.
Step 2: Recall the modes of speciation.
Sympatric speciation happens without any spatial separation, inside the same geographic range. Allopatric speciation needs the ancestral population to be split by a geographic barrier (a river, a mountain ridge, a stretch of unsuitable habitat) into two isolated populations that then diverge. Parapatric speciation happens when populations stay in contact along a shared boundary or a cline and diverge because of a strong environmental gradient, without ever being fully isolated. Peripatric speciation is a special case of allopatric speciation where a small population becomes isolated at the edge of the parent range and diverges rapidly because of its small size.
Step 3: Work out why the present-day picture looks parapatric but is not.
On a map, A and B sit side by side with a clean line at 1800 m, and that can look like an ongoing parapatric split. But in the Himalaya, this kind of elevational replacement between sister species is well documented, from long-term field studies on Himalayan birds, to be a leftover of past geographic isolation rather than an active split happening along the slope today. During the repeated climatic swings of the Pleistocene, ancestral populations were pushed apart into separate valleys or refugia with no gene flow between them. While isolated, they built up enough genetic difference to become distinct species. Only afterward did their ranges expand back out and meet again, producing the clean elevational boundary seen now.
Step 4: Rule out the other options.
Sympatric speciation is ruled out because it needs divergence with no spatial separation at all, and here the two forms occupy different places. Parapatric speciation is ruled out for the reason above: the boundary is a secondary contact zone from populations that diverged while apart, not an active divergence happening across the gradient right now. Peripatric speciation would need one of the two populations to have started as a small, marginal, founder population at the edge of the other's range, which the question does not indicate.
Final Answer:
The elevational replacement of species A and B is best explained as the outcome of speciation that took place while their ancestors were geographically isolated.
\[ \boxed{\text{Allopatric speciation}} \]
Species A and B are sister species (closest living relatives), and their current ranges do not overlap: A holds the lower slopes up to 1800 m, and B takes over from 1800 m to 3500 m. Together, the two species cover the full elevational gradient with a single shared boundary at 1800 m.
Step 2: Recall the modes of speciation.
Sympatric speciation happens without any spatial separation, inside the same geographic range. Allopatric speciation needs the ancestral population to be split by a geographic barrier (a river, a mountain ridge, a stretch of unsuitable habitat) into two isolated populations that then diverge. Parapatric speciation happens when populations stay in contact along a shared boundary or a cline and diverge because of a strong environmental gradient, without ever being fully isolated. Peripatric speciation is a special case of allopatric speciation where a small population becomes isolated at the edge of the parent range and diverges rapidly because of its small size.
Step 3: Work out why the present-day picture looks parapatric but is not.
On a map, A and B sit side by side with a clean line at 1800 m, and that can look like an ongoing parapatric split. But in the Himalaya, this kind of elevational replacement between sister species is well documented, from long-term field studies on Himalayan birds, to be a leftover of past geographic isolation rather than an active split happening along the slope today. During the repeated climatic swings of the Pleistocene, ancestral populations were pushed apart into separate valleys or refugia with no gene flow between them. While isolated, they built up enough genetic difference to become distinct species. Only afterward did their ranges expand back out and meet again, producing the clean elevational boundary seen now.
Step 4: Rule out the other options.
Sympatric speciation is ruled out because it needs divergence with no spatial separation at all, and here the two forms occupy different places. Parapatric speciation is ruled out for the reason above: the boundary is a secondary contact zone from populations that diverged while apart, not an active divergence happening across the gradient right now. Peripatric speciation would need one of the two populations to have started as a small, marginal, founder population at the edge of the other's range, which the question does not indicate.
Final Answer:
The elevational replacement of species A and B is best explained as the outcome of speciation that took place while their ancestors were geographically isolated.
\[ \boxed{\text{Allopatric speciation}} \]
Was this answer helpful?
0
0
Top GATE EY Applied Ecology & Evolution Questions
- The intermediate disturbance hypothesis was proposed to explain patterns of:
- Individual lizards were repeatedly presented with a predator model. Over successive trials, they showed a reduction in the duration of their alarm response. Which one of the following is this an example of?
- Among the following vertebrate classes, biparental care is most common in:
- A classical metapopulation at equilibrium is made up of local populations with:
- The population size at which net recruitment is the highest is also when the greatest amount can be harvested, while ensuring the long-term survival of the population. The amount harvested at this population size is known as:
View More Questions
Top GATE EY Speciation Questions
- A butterfly species in a homogenous forest patch starts exhibiting assortative mating, such that spotted individuals always mate with each other and plain individuals always mate with each other. This trait is heritable, and spotted parents always produce spotted offspring and plain parents always produce plain offspring. Which one of the following forms of speciation could this prezygotic reproductive barrier most likely lead to?
- The Biological Species Concept was proposed by _________.
Top GATE EY Questions
- If '\(\rightarrow\)' denotes increasing order of intensity, then the meaning of the words [\textit{simmer} \(\rightarrow\) \textit{seethe} \(\rightarrow\) \textit{smolder}] is analogous to [\textit{break} \(\rightarrow\) \textit{raze} \(\rightarrow\) \_\_\_\_\_\_]. Which one of the given options is appropriate to fill the blank?
- In a locality, the houses are numbered in the following way:The house-numbers on one side of a road are consecutive odd integers starting from \(301\), while the house-numbers on the other side of the road are consecutive even numbers starting from \(302\). The total number of houses is the same on both sides of the road. If the difference of the sum of the house-numbers between the two sides of the road is \(27\), then the number of houses on each side of the road is:
- For positive integers \( p \) and \( q \), with \( \frac{p}{q} \neq 1 \), \( \left( \frac{p}{q} \right)^q = p^{q-1 \). Then, which of the following is true?}
- Which one of the given options is a possible value of \(x\) in the following sequence? 3, 7, 15, x, 63, 127, 255
- On a given day, how many times will the second-hand and the minute-hand of a clock cross each other during the clock time 12:05:00 hours to 12:55:00 hours?
View More Questions