Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


The high frequency with which rivers delimit phenotypically differentiated bird taxa is unique to Amazonia, where major rivers often form the boundaries between allospecies and subspecies pairs of understory terra firme forest birds. In contrast, many such forest species with life history traits similar to these differentiated forms show no variation in plumage across even the largest rivers. To determine whether such species are nonetheless genetically differentiated, I obtained tissue samples from populations of forest understory birds from opposite banks of the Napo and Amazon rivers of northeastern Peru. These included three species that are not phenotypically differentiated across the Amazon and three species that are not phenotypically differentiated across the Napo, as well as two species that are phenotypically differentiated across the Amazon. Protein electrophoretic analysis of allozymes revealed substantial genetic differences among river-separated birds that do and do not show plumage differences. The prevailing historical hypothesis to explain the high number of species of Amazonian birds states that isolation in Pleistocene forest fragments was the important vicariant event that permitted speciation. An alternative is that isolation on opposite banks of rivers after the formation of the Amazonian river system was the important vicariant event. The pattern of genetic variation reported in this study supports the latter hypothesis. Wright's coefficient ${\rm F\sb{ST}}$ was used as an indirect index of the extent of gene flow among populations in contiguous forest. For some Amazonian species, ${\rm F\sb{ST}}$ values are high compared to most temperate zone birds, especially considering the geographic proximity ($< 90$ km) among the compared Amazonian populations. Increased population subdivision due to reduced effective population size or reduced effective dispersal distance, coupled with an aversion to crossing habitat discontinuities exposed to full sunlight, could explain the effect of riverine barriers on genetic differentiation within such species. The increased population subdivision and response to riverine barriers in understory terra firme forest birds suggests that the genetic continuity of these birds will be disrupted severely by the fragmentation of formerly contiguous forest through the building of roads and associated agricultural clearing currently underway in the Amazon basin.