The Biogeography of Polyploid Ferns Across Space and Time

Abstract. A review of work on the origin and phylogenetic relationships of six systems of allopolyploid homosporous ferns and their progenitors serves as the basis for assessing the history and status of ideas relating to the biogeography of polyploid ferns. The likely dates of origin for the polyploids from recent analyses provide a historical context not previously available, making possible rigorous tests of hypotheses about the history of polyploids, especially the origin, distribution, and duration of polyploid ferns as lineages. Insularity figures prominently in developing an understanding of polyploid fern biogeography. The origin and persistence of polyploid ferns is enhanced by the empty niche space and high disturbance on Darwinian islands. In addition, the greater precinctiveness of polyploid ferns related to larger spore size is likely a selective advantage on islands, consistent with the number of high polyploids encountered among ferns on oceanic islands. Though recently originated polyploid ferns usually have identifiable progenitors with which they are partially sympatric, the rate of expansion of these ferns beyond the range of their progenitors varies. In two cases from Austral regions, long-term persistence and divergent speciation have been demonstrated in widespread polyploids and polyploid lineages with unknown diploid progenitors, though the general consensus is that polyploids do not persist in the long term.