Two mechanisms are often used to explain current geographic distributions of organisms: vicariance (emergence of a barrier between areas that used to be continuous) and dispersal (migration of taxa across a geographic barrier into a new area). Much importance has been given to vicariant explanations, while the dispersal component has often been dismissed. Progress in phylogenetic methods using DNA sequences permit the rigorous testing of vicariant and dispersalist explanations for any given biota. In particular, the inclusion of a temporal component in biogeographic analyses, by means of the molecular clock, permits the discrimination between one and the other. Because vicariant events have a general effect over entire biotas, such events should lead to agreement in the estimated ages of splits between taxa from the affected areas, and can be used as a null hypothesis to be tested. Failure to find concordance in the age of the splits would suggest dispersal mechanisms were involved. Here I propose the use of Mesoamerican birds to further investigate these ideas. I select ten bird species which have highly congruent distribution in Mexico and Northern Central America in order to see (1) if geographic congruence is mirrored in phylogenetic relationships; (2) if there is a agreement in the age of the splits in the different species¿ trees, and (3) if there is concordance in the age of splits across a prominent geographic barrier. The results should lead to a better understanding of the speciation process in general, and the speciation dynamics of this biodiversity hotspot.
