Ridges and rivers: a test of competing hypotheses of Amazonian diversification using a dart-poison frog (Epipedobates femoralis)

Mitochondrial:DNA cytochrome b sequence data from a dart-poison frog, Epipe dobates femoralis, were used to test two hypotheses of Amazonian diversific ation: the riverine barrier and the ridge hypotheses. Samples were derived from sites located on both banks of the Rio Jurua and on both sides of the Iquitos Arch in western Amazonia. The phylogeographic structure was inconsi stent with predictions of the riverine barrier hypothesis. Haplotypes from opposite river banks did not form monophyletic clades in any of our phyloge netic analyses, nor was the topology within major clades consistent with th e riverine hypothesis. Further, the greatest differentiation between paired sites on opposite banks was not at the river mouth where the strongest bar rier to gene flow was predicted to occur. The results instead were consiste nt with the hypothesis that ancient ridges (arches), no longer evident on t he landscape, have shaped the phylogeographic relationships of Amazonian ta xa. Two robustly supported clades map onto opposite sides of the Iquitos Ar ch. The mean haplotypic divergence between the two clades, in excess of 12% , suggests that this cladogenic event dates to between five and 15 million years ago. These estimates span a period of major orogenesis in western Sou th America and presumably the formation of these ancient ridges.