Allopatric vs Sympatric Speciation: What's the Difference?

Allopatric speciation happens when populations are separated by a geographical barrier. Sympatric speciation happens when populations become reproductively separated while living in the same area. In both cases, the populations eventually become different species if they can no longer interbreed to produce fertile offspring.

How allopatric speciation works

A barrier such as a mountain range or a river splits a population. Individuals on opposite sides can no longer breed with each other, so their gene pools are separated. A gene pool is all the alleles in a population; alleles are different versions of a gene.

Mutations introduce new alleles. Different environmental conditions can favour different alleles through natural selection. Over many generations, the populations' allele frequencies change and they become increasingly different. Eventually, even if the barrier disappears, they may no longer be able to produce fertile offspring together.

For example, if a river divides a population and the two sides face different predators, different inherited characteristics may improve survival on each side.

How sympatric speciation works

There is no geographical barrier. Instead, something prevents groups living in the same area from breeding with each other. For example, they might begin breeding at different times or develop different courtship behaviours. This is reproductive isolation: gene flow between the groups is reduced or stopped because they do not successfully interbreed.

Once their gene pools are separated, mutations and natural selection can cause the groups to diverge until they form different species.

Exam mistake to avoid: Do not say that sympatric speciation has no separation at all. It has no geographical separation, but reproductive separation is still essential. Equally, a barrier alone does not instantly create two species; divergence takes place over generations.