Allopatric speciation

Definition

A mode of speciation in which populations diverge into separate species while geographically isolated by a physical barrier that prevents or substantially reduces gene flow. The isolation may arise through vicariance (splitting of a once-continuous range by a new barrier such as a mountain range, river, or glacier) or through dispersal (colonization of a distant area beyond the original range). Over time, genetic drift, natural selection in different environments, and the accumulation of reproductive isolating mechanisms can render the separated populations incapable of interbreeding even if secondary contact occurs.

Etymology

From Greek allos (other) + patra (native land, fatherland), referring to populations in different geographic areas; contrasted with sympatric (same place).

Example

The divergence of Hawaiian Drosophila fruit flies into hundreds of endemic species exemplifies allopatric speciation: founder populations colonized separate islands, became isolated by ocean barriers, and evolved distinct morphologies and mating behaviors; laboratory crosses between species from different islands often produce sterile or inviable offspring, confirming reproductive isolation established during geographic separation.

Synonyms

• geographic speciation
• vicariant speciation

Related Terms

• Sympatric speciation
• parapatric speciation
• reproductive isolation
• vicariance
• Gene flow
• Founder effect
• peripatric speciation

Usage Notes

Specialists distinguish allopatric speciation by vicariance (range split by a barrier) from peripatric speciation (small peripheral isolate diverges). The term is sometimes used loosely for any geographic isolation, but strict usage reserves it for cases where isolation is complete enough to drive divergence. Secondary contact after allopatric divergence can produce hybrid zones or reinforcement of isolating mechanisms. Contrast with sympatric speciation, where divergence occurs without geographic separation.