Molecular evolution, adaptive radiation, and geographic diversification inthe amphiatlantic family Rapateaceae: Evidence from ndhF sequences and morphology

Rapateaceae (16 genera, similar to 100 species) is largely restricted to th e tepuis and sandplains of the Guayana Shield in northern South America, wi th Maschalocephalus endemic to West Africa. The family has undergone extens ive radiation in flower form, leaf shape, habit, and habitat. To analyze th e evolution of these distributions and traits, we derived a molecular phylo geny for representatives of 14 genera, based on sequence variation in the c hloroplast-encoded ndhF gene. The lowland subfamily Rapateoideae is paraphy letic and includes the largely montane subfamily Saxofridericioideae as a m onophyletic subset. Overall, the morphological/anatomical data differ signi ficantly from ndhF sequences in phylogenetic structure, but show a high deg ree of concordance with the molecular tree in three of four tribes. Branch lengths are consistent with the operation of a molecular clock. Maschalocep halus diverges only slightly from other Monotremae: it is the product of re latively recent, long-distance dispersal, not continental drift-only its ha bitat atop rifted, nutrient-poor sandstones is vicariant. The family appear s to have originated approximately 65 Mya in inundated lowlands of the Guay ana Shield, followed by: (1) wide geographic spread of lowland taxa along r iverine corridors; (2) colonization of Amazonian white-sand savannas in the western Shield; (3) invasion of tepui habitats with frequent speciation, e volution of narrow endemism, and origin of hummingbird pollination in the w estern Shield; and (4) reinvasion of lowland white-sand savannas. The appar ent timing of speciation in the Stegolepis alliance about 6-12 Mya occurred long after the tepuis began to be dissected from each other as the Atlanti c rifted approximately 90 Mya. Given the narrow distributions of most monta ne taxa, this suggests that infrequent long-distance dispersal combined wit h vicariance accounts for speciation atop tepuis in the Stegolepis alliance .