Allopatric speciation occurs when populations become geographically separated, whereas sympatric speciation occurs while populations remain in the same geographical area. Both require reproductive isolation and divergence until the populations form separate species.
The Mechanism
Speciation is the splitting of one species into two or more species. Under the biological species concept, separate species cannot interbreed to produce fertile offspring.
The two processes differ mainly in how reproductive isolation begins:
Feature | Allopatric speciation | Sympatric speciation |
|---|---|---|
Location | Populations occupy geographically separate areas | Populations occupy the same geographical area |
Initial barrier | A physical barrier, such as a river, mountain or ocean | A non-geographical barrier, such as different breeding behaviour, ecological niches or chromosome numbers |
Gene flow | Reduced or stopped by geographical separation | Reduced despite the populations living together |
Selection | Different environments create different selection pressures | Different niches or mating patterns create differential selection within the same area |
Example | Populations separated by the Congo River can undergo divergence | Polyploidy can immediately isolate a plant reproductively from its original population |
In allopatric speciation, a geographical barrier divides a population. Mutation and sexual reproduction generate variation, while different selection pressures and genetic drift cause the separated gene pools to diverge. If reproductive isolation remains after the barrier disappears, speciation has occurred.
In sympatric speciation, no physical barrier exists. Gene flow is reduced because groups use different resources, breed at different times, choose different mates or, especially in plants, develop different chromosome numbers through polyploidy. Divergence then produces reproductive isolation.
A common misconception is that individual organisms “adapt” and become a new species. Speciation instead results from changes in the heritable characteristics of populations over generations.
Exam Technique
For an AHL distinguish question on A4.1, state both sides explicitly: allopatric speciation involves geographical separation; sympatric speciation does not. For a longer comparison, add the shared outcomereproductive isolationand explain how reduced gene flow and differential selection cause divergence.
