vi·car·i·ance 
(vī-kârē-əns, vĭ-)
(vī-kârē-əns, vĭ-)n.
The division of a widespread group of organisms by a geographic barrier, such as a mountain range or a body of water, often resulting in the evolution of related species on either side of the barrier.
[Latin vicārius, substitute (since after a vicariant speciation event, one species can be thought of as "substituting for" a related species on the other side of the geographic barrier); see VICARIOUS + -ANCE.]
vi·cari·ant adj. & n.