Which mode of speciation occurs via geographic isolation?

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

Answer: (A)

Allopatric speciation hinges on geographic isolation. When a physical barrier—like a mountain range, river, or distance—splits a population, the two groups no longer exchange genes. Over many generations, they experience different environmental pressures and random genetic drift, so they accumulate distinct genetic differences. Eventually, they diverge enough that, if they meet again, they can no longer interbreed, resulting in two separate species. This geographic separation is the key feature that defines this mode of speciation.

Parapatric speciation involves adjacent populations with a limited contact zone and reduced gene flow due to environmental gradients, not complete geographic separation. Sympatric speciation occurs within the same geographic area without a barrier, often through ecological niche differentiation or genetic changes like polyploidy. Gradualism describes the tempo of evolutionary change over time, not a mode of speciation tied to geographic context.