The phylogenetic relationships of Costaceae, a tropical monocotyledonous family sister to the gingers (Zingiberaceae), were investigated with a combination of two chloroplast loci (trnLF and trnK) and one nuclear locus (ITS1 - 5.8s - ITS2). The resulting parsimony analysis of selected taxa which demonstrate the range of floral morphological variation in the family shows that the Caldavena-type floral morphology is ancestral to the group and that both Tapeinochilus species and a Monocostus + Dimerocostus clade represent recent divergences. The genus Costus is broadly paraphyletic but Costus subgenus Costus represents a large monophyletic radiation with low robustness at the species level. Within this clade, secondary analyses suggest that pollination syndrome, traditionally used for taxonomic and classification purposes within the genus Costus, is a relatively plastic trait of limited phylogenetic utility. This represents the first detailed investigation into intra-generic and intra-specific evolutionary relationships within the family Costaceae and presents some novel evolutionary trends with respect to floral morphology and biogeography.

Key words: Costaceae, floral morphology, phylogenetics