Modes of Speciation
Allopatric Speciation
When two populations of the same species become isolated from each other due to geographic barriers.
Sympatric Speciation
When two groups of the same species live in the same geographic location, but they evolve differently until they can no longer interbreed and are considered different species.
e.g. The divergence of “resident” and “transient” orca forms in the northeast Pacific. Resident and transient orcas inhabit the same waters, but avoid each other and do not interbreed.
Parapatric Speciation
Pretty rare! The populations that are diverging maintain a zone of contact and do not cease the exchange of genes completely. The species are spatially separated, but still exchange migrants. Nothing is stopping individuals from mixing and mating, but it doesn’t happen. The lower fitness of hybrids drives increased differentiation, eventually resulting in premating isolation.
e.g. Rock wallabies on the east coast of Australia; metal-tolerant and non-tolerant populations of the grass Anthoxanthum odoratum growing across a mine boundary.