Figure 2.

Three biogeographic processes that can be used to provide calibrations for molecular clocks. (a) Vicariance involves the formation of a barrier that causes a reduction in gene flow. Here, the fragmentation of a landmass causes a population of palms to split into two. (b) Geodispersal involves the removal of a barrier to gene flow, allowing movement into a new area. Here, the emergence of an isthmus allows a shrew to colonize a new land mass. (c) Biological dispersal involves the movement of an organism across a barrier. In this example, a passerine on the mainland disperses to an island and establishes a new population. (Online version in colour.)