Abstract
Zoogeographical studies on distribution centers and population structure of Amazonian birds, lizards, and butterflies suggest that many subspecies and species originated from small isolated populations during several periods of ecologic-geographical separation. Repeated climatic fluctuations during
the Quaternary leading to vast changes in the vegetation cover of tropical South America have been postulated by earth scientists and support the above biological interpretation. Numerous zones of secondary contact of Neotropical birds indicate the former existence of ecological barriers in Amazonia that have
since disappeared. These contact zones are clustered in several well defined areas indicating that here entire faunas overlapped and partly fused, e.g. in northcentral Amazonia, in south-central Amazonia, in
upper Amazonia south of the Rio Marañón, as well as in portions of the forest region west of the Andes.
The ecological instability of the South American tropics during the Quaternary determined important aspects of the evolution of the Amazonian vertebrate faunas.
Forests survived arid climatic periods probably in restricted areas which served as refugia for the Amazonian forest faunas. Using indirect evidence derived from inequalities of rainfall and from avian distribution patterns I reconstructed the probable geographic location of fifteen tentatively postulated
forest refugia or groups of refugia in the lowlands of Middle and South America. Differentiation of isolated populations took place in these forest refugia due to varying selection pressure, chance and "plasticity" of systematic units following the model of geographic speciation.
Large Amazonian rivers modified or occasionally limited the dispersal of forest birds from the forest refugia in reexpanding forests, especially in species inhabiting the dark forest interior. The range limit of several ecologically incompatible allies stabilized along broad river courses which, in these cases,
constitute partial barriers to dispersal. In this way ecological competition is avoided. However, numerous component species of Amazonian superspecies are in direct contact and exclude one another geographically in uniform forests as a result of ecological competition.
Nonforest faunas advanced into Amazonia during periods of forest reduction, repeatedly establishing contact between the savanna faunas north and south of the Amazonian forest during the pleistocene
and post-Pleistocene. About 2400 years ago, readvancing forests led to the isolation of nonforest animal populations on savanna enclaves in the dry transverse zone of lower Amazonia.
The present complexity of the Amazonian bird fauna is fairly "recent" in origin. This suggestion probably applies generally to the faunas of the Neotropical Region.
