• genetic diversity;
  • inbreeding;
  • phenology;
  • rarity;
  • reproduction;
  • speciation

There are many genera shared between Australian and Papua New Guinean rainforests. Species in the rare rainforest herbaceous genus Romnalda have a relictual and disjunct distribution within the major rainforest blocs of southern Queensland, north Queensland, and New Guinea. There are only four species in this genus: R. strobilacea, R. grallata, and R. sp.‘Cooper Ck’ from Australia, and R. papuana from New Guinea. The Australian species have restricted distributions and high conservation status. Allozymes were used to study the genetic variation and distinctiveness of all four species. Genetic diversity varied significantly amongst the four species. The species in the centre of the genus distribution contained the highest genetic diversity, regardless of rarity. The undescribed R. sp.‘Cooper Ck’ was identified as a clearly distinct species with morphological affinities to R. papuana, but genetic affinities to R. grallata. The study showed that, where the distributions of R. grallata and R. sp.‘Cooper Ck’ overlapped, there was evidence of hybridization. Reproductive participation within populations was typically low with limited flowering synchrony. Populations of all four species were inbred, but higher levels of inbreeding were not correlated with lower genetic diversity. The timing of flowering appeared to be determined by climate. Altitudinal variation in phenological timing in R. sp.‘Cooper Ck’ has led to genetic isolation within the species, but has also limited its genetic introgression with the co-occurring R. grallata. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157, 455–474.