Allopatric speciation depends on geographic separation, while sympatric speciation separates gene pools within the same area through reproductive barriers and isolation.
Coverage 2013–2023 · Updated 15 Jul 2026
Allopatric speciation begins with a geographic barrier; sympatric speciation occurs while populations remain in the same geographic area. Both require reduced gene flow and eventual reproductive isolation.
In allopatry, physical separation stops or sharply reduces mating before differential selection and drift drive divergence. In sympatry, behavioural, temporal or ecological differences reduce mating within the shared area; chromosome changes can also isolate plant lineages.
Compare the routes by asking: Is there a geographic barrier? Which barrier reduces gene flow? What different selection pressures act? Is reproductive isolation established? Geographic, behavioural and temporal isolation can all contribute.
A river dividing a population is an allopatric starting point. Two populations breeding in different seasons in the same habitat can begin a sympatric route because temporal isolation reduces gene flow.
Sharing a location does not by itself demonstrate sympatric speciation, and physical separation does not by itself complete allopatric speciation; evidence of divergence and reproductive isolation is needed.
This objective is assessed through structured response, commonly using Outline / Compare / Distinguish.
Outline / Compare / Distinguish
Build the answer around this relationship: Allopatric speciation occurs when populations are geographically separated.
Defining both processes only as isolation without stating whether geography separates the populations.
Representative question
Compare allopatric speciation and sympatric speciation using the table below.
| Allopatric | Sympatric |
|---|---|
HL speciation questions usually ask you to choose the right layer. Route layer: allopatric means geographic isolation; sympatric means no spatial separation. Niche layer: adaptive radiation produces many related species from one ancestor as they occupy different niches, as in Darwin’s finches. Barrier layer: prezygotic barriers act before fertilization, postzygotic barriers act after hybrid formation. Chromosome layer: plant polyploidy can abruptly create reproductive isolation and fertile new lineages.
