Dispersal versus vicariance

Article By:

Sanmartín, Isabel Department of Biodiversity and Conservation, Real Jardin Botanico, Madrid, Spain.

Last reviewed:2009

DOI:https://doi.org/10.1036/1097-8542.YB090078

Scientists have long been fascinated by the existence of disjunct (geographically discontinuous) distribution patterns such as the one shown in Fig. 1a, in which the members of a group of organisms are distributed across the southern continents, now separated by thousands of miles of ocean. How did this type of widely scattered distribution originate? Traditionally, two alternative explanations have been proposed: dispersal across a preexisting geographical barrier (for example, a mountain chain); or vicariance, the fragmentation of a widespread ancestral distribution by the appearance of a new barrier. Both biogeographical processes result in the isolation of a population by a geographic barrier, followed by differentiation of a new taxon by allopatric (geographically separated) speciation. However, the barrier in the dispersal explanation is older than the geographic disjunction, whereas the appearance of the geographic barrier is responsible for the geographic disjunction in the vicariance explanation, so it cannot be older than the resulting speciation event. Although vicariance and dispersal are not mutually exclusive processes—the opening of the Gibraltar Strait between North Africa and Iberia in the Pliocene was simultaneously a vicariance event for terrestrial organisms and a dispersal event for marine organisms—the history of biogeography as an evolutionary science could be considered until recently as the history of a debate between dispersal and vicariance explanations.

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