Geographic barriers between populations of a species can result in divergence of genes, morphology, or behaviors that can lead to speciation. The Yellow-backed Oriole (Icterus chrysater) is distributed from southern Mexico to Colombia, but with a major range disjunction of 600 km in Costa Rica. We examined molecular and morphological data for differences between northern and southern populations. We sequenced the mitochondrial control region and six nuclear introns. Genetic data show strong north–south population structure with evidence of gene flow. The evidence of gene flow between populations is surprising because of the large geographic break between populations. We also measured six morphological characters from specimens collected along the species’ distribution and found shallow north–south divergence.

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