Speciation dynamics and biogeography of Neotropical spiral gingers (Costaceae)

• We generated a robust phylogeny representing the majority of known Costaceae species.
• Speciation rate varies greatly among clades of Costaceae.
• Costus radiation in the Neotropics arose at varied geographic contexts.

Species can arise via the divisive effects of allopatry as well as due to ecological and/or reproductive character displacement within sympatric populations. Two separate lineages of Costaceae are native to the Neotropics; an early-diverging clade endemic to South America (consisting of ca. 16 species in the genera Monocostus, Dimerocostus and Chamaecostus); and the Neotropical Costus clade (ca. 50 species), a diverse assemblage of understory herbs comprising nearly half of total familial species richness. We use a robust dated molecular phylogeny containing most of currently known species to inform macroevolutionary reconstructions, enabling us to examine the context of speciation in Neotropical lineages. Analyses of speciation rate revealed a significant variation among clades, with a rate shift at the most recent common ancestor of the Neotropical Costus clade. There is an overall predominance of allopatric speciation in the South American clade, as most species display little range overlap. In contrast, sympatry is much higher within the Neotropical Costus clade, independent of node age. Our results show that speciation dynamics during the history of Costaceae is strongly heterogeneous, and we suggest that the Costus radiation in the Neotropics arose at varied geographic contexts.

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