The historical origins of palaeotropical intercontinental disjunctions in the pantropical flowering plant family Annonaceae

Abstract

Geographic isolation of sister taxa in the African and Asian tropics (palaeotropical intercontinental disjunction; PID) is a major biogeographic pattern explained by four competing hypotheses: rafting on the Indian tectonic plate (‘Gondwanan vicariance hypothesis’); migration facilitated by a northern mid-latitude corridor of frost-free climates during the Eocene (‘boreotropical migration hypothesis’); overland dispersal across Arabia associated with the Miocene Climatic Optimum; and transoceanic dispersal. The explanatory challenges posed by PIDs are addressed here using the pantropical flowering plant family Annonaceae as a study system. Molecular dating and ancestral area reconstructions were undertaken using plastid DNA sequence data (ca. 6 kb) derived from an extensive taxon sampling, incorporating ca. 75% of all genera, with phylogenetically informed sampling of species within genera that are distributed across the African and Asian tropics. Statistical dispersal-vicariance analysis and likelihood reconstructions indicated 12 intercontinental dispersal events between Africa and Asia. All but one of these dispersals were from Africa to Asia. Between 10 and 12 vicariance events were inferred, ranging from the late Palaeocene to the late Miocene, with mean divergence times of seven events in the Miocene. Although migration through the Eocene boreotropics has previously been highlighted as the predominant process underlying intercontinental disjunctions in Annonaceae, our results indicate that post-boreotropical processes have also had a major impact on shaping PIDs. Palaeogeographic reconstructions and the fossil record from the Arabian Peninsula support the plausibility of a hypothesized window of overland dispersal opportunity for lowland tropical forest taxa prior to climate deterioration commencing in the late Middle Miocene, providing an alternative to transoceanic dispersal. The patterns observed underscore the hypothesis that intercontinental floristic exchange, facilitated by both the Eocene boreotropics and the erosion of oceanic and climatic biogeographic barriers between Africa and Asia in the Miocene, had a substantial impact on the assembly of palaeotropical forest floras.