Fig. 1
Three hypotheses on the role of rivers affecting diversification in Amazonian taxa. Hypotheses arranged left to right in order of increasing complexity, as minimum number of assumed parameters (see "Introduction"). (a) Riverine Barrier Hypothesis (RBH). Large rivers as semipermeable barriers to dispersal and gene flow. Example from terra firme adapted anuran amphibians14 (b) Interfluvial Barrier Hypothesis (IBH). Terra firm regions (green) between large rivers (black lines) as semipermeable barriers to dispersal and gene Example from riverine-adapted apteronotid electric fishes. Basin abbreviations: A, Amazon; E, Essequibo; O, Orinoco; T, Tocantins; UM, Upper Madeira. (c) River Network Hypothesis. Amazonian rivers by Strahler stream order (SO). Biogeographic portals between Amazonian tributaries and adjacent basins as double-headed arrows. CP, Casiquiare Portal; RP, Rupununi Portal; IP, Izozog Portal. Data from HydroSHEDS. (d) Proportions of river segments by SO for length, area and volume. (e) River Capture Hypothesis. Major reticulations of large rivers in the Central Amazon. Solid blue lines are current thalwegs; dashed lines paleo-thalwegs. (f) Large river corridors (dark blue) of the Central Amazon during the middle Pleistocene15. Note how river capture produces a reticulated history of connections among riverine (R) and terra firme (T) areas as biogeographic barriers and corridors. Maps created in ArcGIS Pro v.3.2.2 (Esri, Redlands, CA, USA; http://www.esri.com). Panel (a) modified from14 https://creativecommons.org/licenses/by/4.0/.
