Allopatric speciation

First geographic isolation...

• Unless populations are geographically isolated they will continue to interbreed

Allopatric speciation of squirrels in the Grand Canyon (see Campbell fig. 24.7, p. 451). The canyon is a barrier to dispersal by small mammals, and as a consequence the isolated populations can diverge.

...then genetic isolation

• Populations diverge to the point where they no longer interbreed
• This may be due to adaptation to different environments, or genetic drift

(see Campbell p. 453, fig. 24.9)

Founder event speciation ("peripheral isolates")

Allopatric speciation may affect many lineages at the same time

1. Geographic events affecting many lineages

The formation of the Panamanian Isthmus has affected many marine lineages, e.g. snapping shrimps Seven out of seven snapping shrimp lineages have split into species pairs after the Isthmus closed.

2. Cospeciation of host and parasite