Abstract
The evolutionary basis for high species diversity in tropical regions of the world remains unresolved. Much research has focused on the biogeography of speciation in the Amazon Basin, which harbors the greatest diversity of terrestrial life. The leading hypotheses on allopatric diversification of Amazonian taxa are the Pleistocene refugia, marine incursion, and riverine barrier hypotheses. Recent advances in the fields of phylogeography and species-distribution modeling permit a modern re-evaluation of these hypotheses. Our approach combines comparative, molecular phylogeographic analyses using mitochondrial DNA sequence data with paleodistribution modeling of species ranges at the last glacial maximum (LGM) to test these hypotheses for three co-distributed species of leafcutter ants (Atta spp.). The cumulative results of all tests reject every prediction of the riverine barrier hypothesis, but are unable to reject several predictions of the Pleistocene refugia and marine incursion hypotheses. Coalescent dating analyses suggest that population structure formed recently (Pleistocene-Pliocene), but are unable to reject the possibility that Miocene events may be responsible for structuring populations in two of the three species examined. The available data therefore suggest that either marine incursions in the Miocene or climate changes during the Pleistocene—or both—have shaped the population structure of the three species examined. Our results also reconceptualize the traditional Pleistocene refugia hypothesis, and offer a novel framework for future research into the area.
Highlights
Tropical regions around the world are well known for their rich diversity of life
For A. cephalotes, the potential last glacial maximum (LGM) range spanned most of the Amazon Basin, with a contiguous population throughout the Guiana Shield (Figure 1D)
For A. laevigata, the model predicted the presence of a large area of unsuitable habitat spanning much of the Amazon Basin (Figure 1F)
Summary
Tropical regions around the world are well known for their rich diversity of life. The reasons why the tropics harbor more species than temperate and arctic regions remain unclear [1,2,3,4]. As is true for the study of speciation in general [9], much focus has been placed on the biogeography of processes generating diversity in the Amazon Basin, on how allopatry can be achieved in a landscape without obvious geographic barriers ( the presence of invisible barriers, such as ancient ‘‘arches’’ has been suggested [10,11,12,13]). A plethora of hypotheses have been suggested, three stand out as the most widely discussed. These are the Pleistocene refugia hypothesis, the marine incursion hypothesis, and the riverine barrier hypothesis
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