HISTORICAL DIVERSIFICATION OF BIRDS IN NORTHWESTERN SOUTH AMERICA: A MOLECULAR PERSPECTIVE ON THE ROLE OF VICARIANT EVENTS.

Studies of South American biodiversity have identified several areas of endemism that may have enhanced historical diversification of South American organisms. Hypotheses concerning the derivation of birds in the Chocó area of endemism in northwestern South America were evaluated using protein electrophoretic data from 14 taxonomically diverse species groups of birds. Nine of these groups demonstrated that the Chocó area of endemism has a closer historical relationship to Central America than to Amazonia, a result that is consistent with phytogeographic evidence. Within species groups, genetic distances between cis-Andean (east of the Andes) and trans-Andean (west of the Andes) taxa are, on average, roughly twice that between Chocó and Central American taxa. The genetic data are consistent with the hypotheses that the divergence of most cis-Andean and trans-Andean taxa was the result of either the Andean uplift fragmenting a once continuous Amazonian-Pacific population (Andean Uplift Hypothesis), the isolation of the two faunas in forest refugia on opposite sides of the Andes during arid climates (Forest Refugia Hypothesis), or dispersal of Amazonian forms directly across the Andes into the trans-Andean region (Across-Andes Dispersal Hypothesis). Disentangling these hypotheses is difficult due to the complexity of the Andean uplift and to the scant geologic and paleoclimatic information that elucidates diversification events in northwestern South America. Regarding the divergence of cis- and trans-Andean taxa, the genetic, geologic, and paleoclimatic data allow weak rejection of the Andean Uplift Hypothesis and weak support for the Forest Refugia and Andean Dispersal Hypotheses. The subsequent diversification of Chocó and Central American taxa was the result of Pleistocene forest refugia, marine transgressions, or parapatric speciation.