Q.95. In allopatric mode of specialion, a new species forms due to
(A) Geographic isolation
(B) Genetic drift
(C) Formation of a few fertile individuals thar can not mare with other members of the same species
living in the same geographical area
(D) The formation of allopolyploid condition
Allopatric Speciation: How Geographic Isolation Drives New Species Formation
Allopatric speciation occurs when populations of the same species become physically separated, leading to genetic divergence and new species. This MCQ tests the core mechanism behind this primary mode of evolution in biology exams.
Correct Answer
(A) Geographic isolation
In allopatric speciation, a physical barrier like mountains, rivers, or oceans splits a population into isolated groups, preventing gene flow. Over time, these groups evolve separately via mutation, natural selection, and genetic drift, accumulating differences until reproductive isolation develops—forming distinct species. Examples include Darwin’s finches on the Galápagos Islands, where oceanic separation drove divergence.
Option Explanations
(A) Geographic Isolation (Correct)
This is the defining feature: extrinsic barriers (e.g., vicariance like rising landmasses or peripatric dispersal to islands) halt interbreeding, allowing independent evolution. Without gene flow, populations diverge rapidly, as modeled by Ernst Mayr.
(B) Genetic Drift
Genetic drift causes random allele frequency changes, contributing to divergence in small isolated populations (e.g., founder effects), but it’s a secondary process. Allopatric speciation requires prior geographic separation to enable drift’s effects; drift alone doesn’t initiate it.
(C) Formation of a Few Fertile Individuals That Cannot Mate with Others in Same Area
This describes sympatric speciation via behavioral or habitat isolation within the same locale, not allopatric. Small non-mating groups imply intrinsic barriers without geographic split, like disruptive selection.
(D) The Formation of Allopolyploid Condition
Allopolyploidy is a sympatric mechanism in plants, where hybrid genome duplication instantly creates reproductive isolation (e.g., wheat). It doesn’t involve geographic separation and is irrelevant to allopatric animal speciation.
| Option | Requires Geographic Separation? | Speciation Mode | Primary Mechanism |
|---|---|---|---|
| (A) Geographic isolation | Yes | Allopatric | Barrier-induced divergence |
| (B) Genetic drift | No (enhances it) | Any | Random changes |
| (C) Few non-mating individuals | No | Sympatric | Behavioral isolation |
| (D) Allopolyploidy | No | Sympatric | Polyploid hybrid |
This breakdown clarifies allopatric vs. other speciation for evolutionary biology studies.
