Question

Once upon a time there lived a population of mammals \(X\). Due to continental drift, the population got separated into two groups and continued to live independently for a long period of time. After approximately \(1\) lakh years, researchers attempted to interbreed the two populations, but they were unable to produce fertile offspring. This type of speciation has probably occurred due to:

• A. Temporal isolation
• B. Mechanical isolation
• C. Geographical isolation
• D. Both (B) and (C)

Hint:

Whenever a question mentions physical separation by mountains, rivers, oceans, islands, or continental drift, think of geographical isolation and allopatric speciation. This is one of the most frequently asked concepts in evolution.

Answer 1

The Correct Option is C

Concept: Speciation is the evolutionary process through which new species arise from existing populations. One of the most common mechanisms of speciation is

allopatric speciation, which occurs when populations become physically separated by geographical barriers such as mountains, rivers, oceans, or continental drift. Over long periods of time, the separated populations accumulate genetic differences due to mutation, natural selection, genetic drift, and adaptation to different environments. Eventually, these differences become so significant that individuals from the two populations can no longer produce fertile offspring, even if they come into contact again.

Step 1: Analyze the given situation. The question states that:

• Initially there was a single population of mammals.

• Due to continental drift, the population became separated into two groups.

• The two groups remained isolated for approximately \(1\) lakh years.

• Later, attempts to interbreed them failed to produce fertile offspring.
The most important clue is the separation caused by

continental drift.

Step 2: Identify the type of isolation. Continental drift creates a physical barrier between populations. Such separation prevents gene flow between the groups and is known as: \[ \boxed{\text{Geographical Isolation}} \] This geographical separation allows independent evolution of the two populations.

Step 3: Why not temporal isolation? Temporal isolation occurs when organisms reproduce at different times, such as:

• Different breeding seasons

• Different flowering periods

• Different mating times
The question does not mention any difference in breeding time. Therefore, temporal isolation is not the reason.

Step 4: Why not mechanical isolation? Mechanical isolation occurs when reproductive structures become incompatible, preventing mating. Examples include:

• Structural differences in reproductive organs

• Incompatibility between floral structures
The question does not provide evidence that mechanical incompatibility caused the separation. Instead, the primary reason given is physical separation by continental drift.

Step 5: Conclude the mechanism of speciation. Because the populations were separated by a geographical barrier for a very long period and evolved independently, the speciation occurred through: \[ \boxed{\text{Geographical Isolation}} \] which is the basis of allopatric speciation. Hence, the correct answer is \[ \boxed{\text{(C) Geographical Isolation}} \]